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MDCT Evaluation of the Coronary Arteries, 2004: How We Do It—Data Acquisition, Postprocessing, Display, and Interpretation

¹ All authors: Department of Radiology and Radiological Science, Johns Hopkins University, 601 N Caroline St., Rm. 3254, Baltimore, MD 21287-0801.

Received February 12, 2004; accepted after revision September 15, 2004.

 
Address correspondence to L. P. Lawler.

L. P. Lawler and H. K. Pannu contributed equally to this article.

Abstract

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
OBJECTIVE. Cardiac CT is rapidly becoming part of clinical practice. The objective of this article is to discuss and illustrate the current practice of coronary artery MDCT, including data acquisition, postprocessing, image display, and interpretation. The practice described reflects our experince with a series of patients referred in routine clinical practice.

CONCLUSION. The reader should gain an insight into the current clinical application of coronary artery CT.

Introduction

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
With the improving temporal and spatial resolution of mechanical MDCT scanners, coronary artery imaging is entering the clinical realm [1–5]. Although 16-MDCT scanning diminishes motion artifact and the limits of single-detector technology, it remains an emerging, imperfect technology that requires careful attention to technique for optimal performance and clinically useful imaging. Future larger detector arrays, changing tube technology, and simplified postprocessing platforms will enhance and change coronary CT angiography protocols. There is not yet any universal agreement on the method of performance or interpretation of such studies or their role in the clinical algorithm, although the demand for coronary CT angiography continues unabated.

In this article, we share our state-of-the-art approach to CT angiography with the goal of providing a framework for those initiating coronary MDCT angiography in clinical practice. We cover data acquisition, processing, and postprocessing and suggest an approach to image review, interpretation, and reporting. Although the emphasis is on native vessel imaging, the principles also apply to bypass graft study. Please note that the article is based on the evidence currently available and although the practice reflects a single approach at our institution, it is of course not the only way to perform coronary CT angiography.

Patient Selection

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
In part because of the lack of large outcome study results, there is not yet a consensus statement on the role of CT angiography in the current cardiac evaluation algorithm. There are no universally accepted absolute indications for cardiac CT angiography, and the patient population and principle of practice at present appropriately reflect those with a low pretest probability of coronary artery disease for which the high negative predictive value of the test can be applied.

Patients are referred for coronary CT angiography if their doctor believes the level of suspicion does not justify an invasive test but there is concern for unsuspected disease that may change management. This group usually reflects asymptomatic patients with a low pretest probability of coronary artery disease and often no cardiac history. They do have identifiable Framingham risk factors, the most common of which are family history and raised cholesterol. Select patients will be referred for coronary CT angiography when they are deemed high risk for conventional invasive angiography or when invasive imaging has failed. Younger patients are also referred if there is suspicion of aberrant origin of the coronary arteries and if findings from catheter angiography or echocardiography are nondiagnostic.

We do not perform coronary CT angiography in those suspected of having acute coronary syndromes, for which conventional angiography remains the standard and intervention is anticipated. Significant disease can be excluded when normal vessels are seen, although confidence in predicting unstable plaque and coronary events falls with increasing atherosclerotic disease burden. We do not perform coronary CT angiography in those with a heavy burden of calcified plaque because estimating the degree of stenosis at the site of calcified plaque will be difficult due to blooming artifact and because a catheter angiogram will still be necessary. We do not proceed with studies in patients with a heart rate of more than 70 beats per minute (bpm) or with significant arrhythmia because the resulting images are poor. Breath-hold difficulties and the inability to remain supine and motionless are relative contraindications. We do not do plaque characterization studies in routine practice, although it is applied in a research capacity.

At present there is no specific reimbursable code for dedicated coronary CT angiography, but there are number of working groups engaged in this issue as the use becomes more widespread. Some patients pay out of pocket for the study, although the examination can be coded under a contrast study of the chest with a charge for 3D reconstruction.

Patient Preparation

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
The main preparation involves slowing the patient's heart rate to approximately 60 bpm because this lower heart rate increases the relative proportion of the cardiac cycle spent in diastole and limits motion artifact [6, 7]. During the patient assessment, one should note that the heart rate frequently rises with the anxiety of being placed on the scanner and with the injection of contrast material. A decrease in heart rate has also been noted during the initial seconds of breath-holding. At our institution, generally oral ß-blockers are prescribed by the referring doctor 2–3 days before scanning and organized at the time of scheduling. Some sites also give ß-blockers at the time of the study as well.

Sample ß-blocker protocols include giving 50 mg of metoprolol orally 1 hr before scanning or 5 mg of metoprolol IV a few minutes before the study. Calcium channel blockers are used if ß-blockers are contraindicated. Contraindications for ß-blocker therapy include asthma, atrioventricular conduction block, heart failure, diabetes, and Raynaud syndrome. Three ECG leads are placed over the patient as specified by the manufacturer of the scanner to obtain an ECG tracing on the scanner console.

Contrast Injection and Scan Delay

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
The patient's history is reviewed to exclude the typical contraindications for contrast administration such as renal insufficiency and allergic reaction. Informed consent is obtained at our institution. The cardiovascular effects of contrast media are in part related to their osmolality and include abnormalities of conduction and contractility, although changes in hemodynamic parameters are generally not significantly different among the various nonionic contrast media [8, 9].

We use a dual-head power injector (Stellant, Medrad) to inject 100 mL of isosmolar nonionic contrast material through an 18- to 20-gauge needle into an antecubital vein at a rate of 3.5–4 mL/sec; the injection is followed immediately by a 25-mL saline flush. The purpose of a saline flush is to diminish beam-hardening contrast artifact within the right ventricle that obscures the right coronary artery. It also facilitates delivery of the entire contrast volume in a short bolus [10].

At our institution, the scan delay is empirically timed to coincide with the end of the contrast injection and the beginning of the saline flush. This delay is approximately 25 sec. An alternative method of determining the scan delay is to use bolus-tracking or test bolus techniques. With the former technique, a region of interest is drawn at the ascending aorta and scan acquisition begins when a predefined threshold Hounsfield unit of contrast is reached after power injection. With the test bolus technique, a small contrast bolus is administered while scanning a fixed level in the ascending aorta. From this, a curve of the contrast density rise and fall is generated. Assuming this test bolus will reflect the behavior of the main contrast volume, the scan timing may be deduced from the peak density value on the curve. A good technique shows the highest contrast in the left ventricle and coronary arteries with less density in the right ventricle and pulmonary arteries.

Scanning Protocol

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
The scan coverage is from the carina through the heart base in a cranial-to-caudal direction. The aortic arch is the superior extent if bypass grafts or internal mammary arteries are also assessed. Some advocate an initial unenhanced study of the heart either by routine scanning or as per a coronary calcification scoring protocol. This study is performed to identify and exclude those patients whose burden of calcified plaque will significantly compromise any coronary CT angiography.

The thinnest detector collimation possible is selected, typically in the range of 0.75–1 mm on a 16-MDCT scanner (Sensation 16, Siemens Medical Solutions). The fastest gantry rotation time possible is selected, a typical value being 420 msec. Typical table feed is 2.8 mm per gantry rotation, which gives a lower pitch than routine scanning. The peak kilovoltage is 120 kVp, but it can be lowered to 100 kVp for thin patients. The effective milliampere-second setting selected is higher (500 mAs) than for routine non-ECG-gated CT studies to decrease image noise from partial scan reconstruction and small slice width. ECG pulsing can be used whereby the tube output is decreased during systole and increased during diastole when target images will be reconstructed, reducing radiation dose by approximately 50%. The effective dose will range from 7 to 10 mSv and is higher in women [11]. The dose is higher than routine thoracic CT angiography and higher than conventional angiography.

Retrospective ECG gating is used for helical coronary CT angiography studies performed on an MDCT scanner. With this method, the scanning data and ECG tracing are recorded simultaneously but independently. It allows the scanning data to be acquired throughout the cardiac cycle for subsequent reconstruction during specified periods of the cycle. The entire heart is imaged as a volume for subsequent 3D manipulation. With ECG gating, there is motion artifact if the heart rate is fast or the rhythm is irregular. In select cases of patients with high heart rates, a non-ECG-gated routine CT of the heart with thin collimation and fast gantry rotation speed may be sufficient to answer the clinical question (e.g., aberrant vessel origin or high-grade proximal stenosis). However, ECG-gated studies are the optimal method for clinical practice in most patients.

Scan Reconstruction

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
Slice Width and Field of View
The scan is reconstructed with a slice thickness of 1 mm with 25–50% overlap as is the convention for optimal 3D imaging. A medium-smooth reconstruction algorithm is used, similar to that for most routine chest and abdominal studies. If a coronary artery stent is present, a second reconstruction with a slightly sharper algorithm (higher spatial frequency) can be done through the area of the stent. The field of view is coned down to the heart to improve spatial resolution.

Temporal Resolution and Reconstruction Window
Images are reconstructed for each position in the z-axis during the time of least cardiac motion or diastole. The scan is reconstructed using only a small portion of the cardiac cycle, the duration of which represents the time needed to acquire the necessary data—that is, the temporal resolution. The temporal resolution of currently available MDCT scanners for coronary artery imaging is approximately 105–210 msec. The ECG tracing is reviewed, and the reconstruction window is defined by the operator using either a relative (percentage) or fixed (absolute) time delay from the R wave. The relative delay divides the R-R interval into percentage increments from 0% to 100% (Fig. 1). Image reconstruction is started with a certain delay from the prior R wave. The delay is defined as a percentage of the R-R interval—for example, 60%. These percentage numbers specify the point in the cardiac cycle at which image reconstruction is started. The length of time used to reconstruct the data set depends on the temporal resolution of the scanner. If multiple reconstructions are done with varying delays, there is overlap in the image sets when the reconstruction increment is less than the temporal resolution of the scanner.

View larger version (8K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —ECG trace shows relative delay method for scan reconstruction. R-R interval is divided into percentage increments from 0% to 100%: 0% at first R wave and 100% at second R wave. Image reconstruction is started with a certain delay from the prior R wave. The delay is defined as a percentage of the R-R interval—for example, 60%.

A fixed or absolute time from the R wave can also be used to reconstruct images (Fig. 2). With the absolute delay method, a fixed time delay after the R wave is used to start reconstruction—for example, 400 msec from the R wave. With the absolute reverse method, a fixed time before the next R wave is used to start reconstruction—for example, 400 msec before the R wave. The relative delay or absolute reverse method is usually used for reconstruction. We typically use the relative delay method.

View larger version (8K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —ECG trace shows absolute reverse method for scan reconstruction. Image reconstruction starts at fixed time (in milliseconds) before R wave.

One may reconstruct at different percentage increments a single slice through the mid right coronary artery to find the image of least motion. This is the most diastolic phase of the cardiac cycle and may be partly deduced from the state of wall thickness, chamber volume, and the state of atrioventricular valve closure. Once the image with the least artifact is found, that percentage delay may be used to reconstruct the entire cardiac volume. If there is motion artifact involving one particular vessel, additional reconstructions with different delays are performed. Kopp et al. [5, 12] found that the right coronary artery was best seen early in diastole at 40% of the R-R interval, the left circumflex artery was best seen in mid cycle, and the left anterior descending artery was best seen at 60–70% of the R-R interval. For coronary CT angiography studies, reconstructions at 10% increments from 40–80% will likely provide optimal images to evaluate all the vessels.

This process of creating multiple reconstructions is usually done on the scanner itself and therefore can delay scanning of other patients. It can be done after hours or, to help with workflow, the technologist can be instructed to perform multiple reconstructions for all patients so that all are available for the radiologist to review. Newer scanners will allow processing to be done on a separate workstation so that scanning may be uninterrupted. In our practice, standardized incremental 10% reconstructions are done by the technologist for all studies throughout the cardiac cycle. For example, reconstructions are done at 10% increments from 0–90% of the cardiac cycle. This helps with workflow but does create large amounts of data that require a separate workstation for storage until analysis is completed. The advantage of this approach is that one also obtains systolic and diastolic sampling of the cardiac cycle for functional imaging, such as calculation of ventricular ejection fraction and assessment of wall motion.

Some scanners allow a series of interval reconstructions at a single level to be reviewed at once so the cardiac phase of interest may be reconstructed. Newer tools with cine 3D (sometimes called 4D) of all cardiac phases also allow one to pick the most diastolic or systolic images of interest. It is helpful if the raw data of the study are not deleted until the study has been interpreted and no additional information is needed. Having access to the raw data is helpful for cases in which poor image quality is secondary to poor ECG gating—for example, when there is a poor ECG trace or arrhythmia. Review of the raw data is done at the workstation before the raw data are erased. ECG editing allows you to review where the data reconstruction was relative to the cycle of the ECG trace. If you think the portion of the cycle used is incorrect, you can manually change it. Any additional beats that are used for image reconstruction may be deleted.

Single- or Multisegment Data Reconstruction
Temporal resolution is determined largely by gantry rotation time but may be enhanced by segmental reconstruction [13]. Single reconstruction is the preferred method and is successful in patients with low heart rates. A set of images, equal to the total detector width, is reconstructed from data acquired during one gantry rotation and one cardiac cycle (Fig. 3). The image is reconstructed using data acquired during approximately 240° of the gantry rotation (180° + a portion of the detector fan angle) rather than the entire 360°. The temporal resolution is therefore improved to half the gantry rotation time [14]. If the rotation time is 420 msec, the temporal resolution is approximately 210 msec, although it may vary between 210 and 280 msec.

View larger version (34K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3. —Illustration with ECG trace shows how entire volume is reconstructed using single-segment reconstruction. (Courtesy of Corl FM, Baltimore, MD)

In patients with low heart rates ( 50 bpm), single-sector reconstruction in mid-diastole is preferred. In those with a high heart rate ( 70 bpm), image reconstruction cannot be completed in the short diastolic time available without giving artifacts [13]. Therefore, multisegment reconstruction is done using small segments of data acquired during two or more cardiac cycles [13, 15] (Fig. 4). Multiple segments of data are combined to make the final image at end-systole. The resulting temporal resolution equals the gantry rotation time divided by 2N, where N is the number of cycles. If the rotation time is 420 msec, the temporal resolution is 105 msec. However, the z-axis spatial resolution can decrease if there are gaps in the acquired data [13]. The maximum number of cycles typically used is two or three, beyond which image quality falls. To avoid loss of z-axis resolution, we prefer single-segment reconstruction over multisegment reconstruction and have found that prescan optimization of the patient's heart rate is more helpful than postscan processing to get a good quality scan.

View larger version (8K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —ECG trace shows how a set of images is reconstructed using data acquired during two cardiac cycles for multisegment reconstruction.

Problem Solving

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
Occasionally, the study is not optimal because of cardiac motion artifacts, contrast artifact, and poor contrast bolus. The optimal study is obtained when the patient has a regular sinus rhythm.

In cases in which the cardiac rhythm is irregular and there is artifact due to cardiac motion, postprocessing ECG editing can be attempted to improve the quality of the study. Such editing is limited to one or two beats because greater manipulation will lead to gaps in the data. The technique of editing varies among scanner vendors and can be obtained from the applications specialist of your scanner's vendor. Cardiac motion is identified when there is stairstep artifact in the cardiac or vessel contour on reconstructed images (Fig. 5A, 5B). This stepoff between two sets of reconstructed images occurs when the heart is not in the same position from the time the first set is acquired to the time the second set is acquired. The ECG trace is reviewed to ensure that the bars specifying the part of the cardiac cycle during which the images are being reconstructed are positioned in diastole. An aberrant R wave that occurs sooner than expected—shortening the anticipated R-R interval—can result in the reconstruction bar being positioned in systole (Figs. 5A, 5B and 6A, 6B, 6C, 6D). The image slices with artifact are identified, and the corresponding reconstruction bar on the ECG trace is deleted.

View larger version (72K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —Images of 52-year-old man show stairstep artifact. Coronal multiplanar reconstruction CT image shows stairstep artifact (arrows) in cardiac contour.

View larger version (5K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —Images of 52-year-old man show stairstep artifact. ECG trace shows early R wave(arrowhead). Relative delay method has been used to reconstruct images. Reconstruction is in diastole for first cardiac cycle. Reconstruction overlaps systole for next cycle because of early R wave. Arrow shows start of reconstruction of second image set. ECG trace can be edited to delete this reconstruction. Double-headed arrows show relative delay.

View larger version (32K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A. —Images of 60-year-old woman show stairstep artifact and effect of editing ECG trace. Coronal multiplanar reconstruction CT image shows stairstep artifact (arrow).

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B. —Images of 60-year-old woman show stairstep artifact and effect of editing ECG trace. ECG trace from scan shown in A reveals second reconstruction set for images 13–24 (arrow) overlapping systole. Second and third reconstruction sets are in different points of cardiac cycle. Third reconstruction set is for images 25–31.

View larger version (38K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6C. —Images of 60-year-old woman show stairstep artifact and effect of editing ECG trace. ECG trace shown in B was edited to delete second reconstruction (arrow). Images 13–31 (arrowhead) are now reconstructed from next cardiac cycle.

View larger version (47K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6D. —Images of 60-year-old woman show stairstep artifact and effect of editing ECG trace. Coronal multiplanar reconstruction CT image shows stairstep artifact (arrow) is gone as a result of editing of ECG trace (C).

View larger version (25K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8B. —Images of coronary artery in 56-year-old man obtained using postprocessing techniques. Multiplanar reconstruction CT image shows centerline vessel straightening of right coronary artery (arrowhead).

If there is increased beam-hardening artifact from contrast material in the right heart that affects the right coronary artery in particular, a saline bolus after contrast injection or slightly increasing the scan delay may be helpful. If the contrast bolus is poor, the injection speed can be increased. Artifact may occur because of respiratory motion or image noise. Limiting the scanning time to the minimum necessary can help reduce the likelihood of respiratory motion. For scans with increased image noise, a smoother reconstruction algorithm and slightly thicker slice reconstruction of 1.5 mm can decrease the amount of noise. However, the spatial resolution also decreases.

Data for Postprocessing

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
A large MDCT data set is obtained from sampling the entire cardiac cycle, from which selective postprocessed image sets must be generated for greatest diagnostic yield. Although an infinite number of data sets of the cardiac cycle may be generated from the raw data, the needs of coronary CT angiography are limited to the diastolic frames.

We use a workstation (Leonardo, Siemens Medical Systems) that permits real-time axial, multiplanar reconstruction, maximum-intensity-projection, and volume-rendering techniques [16]. However, these tools are available on the workstations of most scanner vendors and are present on many third-party stand-alone workstations. To this workstation, we typically send a range of reconstructions—10%, 20%,... 90%—representing the systolic and diastolic sampling of the entire R-R interval of the cardiac cycle. At present, we do this because there is independent motion of the right and left coronary arteries. The right coronary artery may show greater motion in late diastole, whereas the left coronary artery branches may show greater motion in early diastole. Therefore, more than one reconstruction may be necessary to optimally show both vessels.

There is also interpatient variation, so that the reconstruction window in the cardiac cycle that is optimal for the right coronary artery in one patient may not be suitable for that vessel in another patient. Therefore, the point in the cardiac cycle chosen for image reconstruction has to be customized for each individual patient and for each coronary artery [5, 12]. Although the 40–70% reconstructions suffice in most patients, the full range of data also allows for supplemental volume measurement, functional imaging, and 4D review in select cases where attendant pathology makes this of clinical value. Future software upgrades promise to make data reconstruction and selection of optimal postprocessing phase simpler and even semiautomated.

The workflow varies among institutions. At the outset, it is good for the radiologist to be involved in all aspects of data acquisition and postprocessing to learn and to help technologists understand and develop the imaging protocol. In most established practices, the technologist does most of the initial data processing that requires reconstruction of image sets throughout the cardiac cycle. Newer technologies may automate much of this handling of the raw data. Postprocessing is done by those available and willing to do it.

At our institution, the radiologists perform all the 3D postprocessing, in part because of the complexity of the conditions and clinical questions but also to allow direct real-time consultation with referring doctors. However, perhaps in most institutions, it is the technologists who are trained in the use of the workstations, although the feasibility of this approach requires a clear consensus among radiologists about the types of multidimensional images and perspectives required in individual patients.

Viewing Perspectives

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
Unlike conventional angiography, there are countless multidimensional viewing angles available with MDCT angiography. For efficiency, we limit these to the standard and patient-specific views. The standard views are those favored by most clinicians and similar to conventional angiography: the right anterior oblique view of the right coronary artery, the left anterior oblique view of the left coronary artery, and the left lateral oblique view of the left anterior descending and circumflex arteries (Fig. 7A, 7B, 7C).

View larger version (158K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A. —Coronary artery anatomy in 56-year-old man on 3D MDCT images obtained with volume-rendering technique. Left lateral view shows left anterior descending (arrow) and circumflex (arrowhead) arteries.

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B. —Coronary artery anatomy in 56-year-old man on 3D MDCT images obtained with volume-rendering technique. Superior view shows origins of right (arrow) and left (arrowhead) main coronary arteries.

View larger version (159K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7C. —Coronary artery anatomy in 56-year-old man on 3D MDCT images obtained with volume-rendering technique. Anterior view shows right coronary artery (arrow).

A superior perspective on MDCT angiography or a perspective in the plane of the coronary origins provides a clear depiction of the site and patency of the right and left coronary origins and provides a satisfactory view of the left main coronary artery. Two angled right anterior oblique perspectives usually depict the proximal and middle portions of the right coronary artery. A left lateral view depicts the left anterior descending artery, but a superior perspective is required for diagonal and septal perforating branches. The circumflex artery is best seen using a left lateral or posterior oblique projection for the proximal and mid portions. The posterior descending artery in the atrioventricular–interventricular groove from the right coronary artery or circumflex artery is best appreciated with an inferior perspective.

The patient-specific views are those dedicated perspectives that best depict individual pathology to best effect. Because stenotic changes in caliber are frequently noncircular and irregular, one will find that at least two perspectives of each artery segment are required to confidently exclude caliber change.

Postprocessing Techniques

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
Current postprocessing uses a combination of 2D and 3D techniques [4, 17–19]. Two-dimensional tools include routine axial planar imaging and multiplanar reconstruction. Multiplanar reconstruction requires little computer power and reorders the voxels into an arbitrary straight or curved imaging plane (Fig. 8A). Curved multiplanar reconstruction images may be generated through user definition of the reconstruction plane from a series of planar source images. This technique is subject to user error defining the vessel center, however, and only one branch at a time may be depicted. Centerline tools have sought to minimize this error and facilitate semiautomated drawing of curved multiplanar reconstructions. With this tool, the user plants a seed point in the region of the vessel and it automatically defines a plane in the center of the lumen contrast. This approach is limited in its application for small-caliber vessels and has difficulty discriminating contrast material and calcification to define the vessel center. Curved vessel images obtained with centerline tools may be straightened to show the relative caliber at different anatomic points (Fig. 8C).

View larger version (75K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8A. —Images of coronary artery in 56-year-old man obtained using postprocessing techniques. Coronal multiplanar reconstruction CT image shows right coronary artery (arrowhead).

View larger version (110K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8C. —Images of coronary artery in 56-year-old man obtained using postprocessing techniques. Anterior maximum-intensity-projection image shows right coronary artery (arrowhead).

Three-dimensional tools include maximum-intensity-projection and volume-rendering techniques [4, 17–20]. Maximum intensity projection is a projection technique akin to conventional angiography that preferentially displays density values from a visual ray above a chosen threshold value. Slab maximum intensity projection with editing planes in front and behind a vessel are an efficient means by which to produce high-contrast images over long vessel segments (Fig. 8C). Volume rendering is a computer-intensive technique with high fidelity to the acquired data that displays the range of density values within a voxel and confers opacity and depth to the image. We try not to vary segmentation thresholds or transfer functions because this can lead to diminished intra- and interscan consistency by obscuring or creating stenoses [19, 21, 22]. We do not use shaded-surface display or virtual angioscopy [15], although the latter may have some value in evaluating patency in the setting of heavily calcified vessels or stents.

Gray-scale images suffice, although color images have an esthetic appeal and can better separate vessels from underlying cardiac chamber contrast material. The center and width are adjusted to limit the blooming artifact of large calcifications or coronary stents.

Vessel Review Sequence

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
It is important to note that though a small field of view around the heart may be generated for the CT angiography, the raw data contain the entire thorax, which must be reviewed in the axial plane with standard window settings to evaluate for noncardiac abnormalities or disease. Second, we assess for non–coronary-artery cardiac findings of significance—for example, valvular or pericardiac abnormalities. Myocardial thinning, contour change, or attenuation change may reflect prior infarct and is noted. We also comment on gross abnormalities of chamber size and shape.

An initial brief 2D axial planar and volume-rendered 3D review of the coronary arteries will greatly facilitate subsequent dedicated vessel interpretation by giving the radiologist an overview of vessel course so that segmentation editing may be more correctly applied. American Heart Association nomenclature is applied [2, 5, 23], which acknowledges three main coronary arteries: left anterior descending, circumflex, and right coronary with right, left, or codominant systems (Fig. 9). The coronary anatomy is divided into 29 segments, although in practice only 12–15 are routinely used, and disease is classified as one-, two-, three-vessel or left main distribution. We have established a style and language of reporting in consultation with our referring clinicians.

View larger version (140K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9. —Illustration shows coronary arteries and standard nomenclature. (Courtesy of Corl FM, Baltimore, MD).

Dedicated assessment of the four main epicardial vessels is performed in a sequential manner: right coronary artery, left main, left anterior descending, and circumflex arteries. Side-branch vessels are not usually evaluated unless they are of a caliber greater than 2 mm, but they are used for localization. Initially, vessel origin, course, and relative dominance are noted. Distribution and burden of calcification or plaque are evaluated, and main and branch vessel opacification is documented. At this point, many vessels may be defined as assessable or nonassessable. The percentage of assessable vessels will vary between 68% and 94% depending on patient and acquisition factors [24].

Each vessel is initially viewed using volume rendering. Volume-rendering images give an overview of the entire vessel and are useful to depict and correctly name the relative sites of calcification, stenoses, and stents (Fig. 10A, 10B). By seeing the vessel course initially in 3D, one may more efficiently and accurately assess caliber using 2D techniques. The simplest means to view long vessel segments or vessels in their entirety is to place a second parallel plane (slab editing) posterior to the vessel of interest and apply slab multiplanar reconstruction or maximum intensity projection [2, 19, 21] (Fig. 6A, 6B, 6C, 6D).

View larger version (83K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10A. —54-year-old man with coronary artery disease. Volume-rendered left lateral view shows calcified left anterior descending artery with short segment that has proximal stenosis (arrow) of greater than 50%.

View larger version (89K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10B. —54-year-old man with coronary artery disease. Maximum-intensity-projection image shows left lateral view of calcified left anterior descending artery with short segment that has proximal stenosis (arrow-head) of greater than 50%.

It remains controversial which is the most accurate postprocessing technique for caliber quantification [4, 17–20]. We find at present stenoses are frequently detected on the volume-rendering images but are best confirmed and characterized using some form of planar technique (Figs. 10A, 10B and 11A, 11B). Such techniques are closest to the original data acquired and have limited reconstruction artifact. Although absolute caliber measurements can be obtained from 2D and 3D data sets, we do not routinely report them at this time because none of the tools has proven accuracy as yet. Thus, as in most conventional angiography reports, we prefer to provide an impression of percentage of narrowing (in quartiles) relative to the maximum diameter seen.

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11A. —72-year-old woman with coronary artery disease. Coronal volume-rendered view (A) and coronal maximum-intensity-projection view (B) show stenosis (arrowhead) of greater than 70% that is due to focal noncalcified plaque in proximal right coronary artery (arrow).

View larger version (121K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11B. —72-year-old woman with coronary artery disease. Coronal volume-rendered view (A) and coronal maximum-intensity-projection view (B) show stenosis (arrowhead) of greater than 70% that is due to focal noncalcified plaque in proximal right coronary artery (arrow).

Stenoses less than 50% are not considered hemodynamically significant, but they are not necessarily clinically benign: Plaques that underlie them may progress or rupture acutely. Significant stenoses are considered those greater than 50% [2, 5]. High-grade stenoses are generally accepted to be those with more than 70–75% narrowing [23, 25]. We do not attempt any quantification of severely calcified vessels or stented vessels where the lumen cannot be confidently discriminated, and 1- to 1.5-mm vessels cannot be reliably assessed. Although an association of plaque characteristics and sonographic features has been shown [2, 15], there are no data on the ability of clinical CT to detect or quantify noncalcified plaque; thus, plaque characterization is limited to site and calcified or noncalcified descriptors using multiplanar reconstructions. Separate dedicated calcium scoring examinations are performed when specifically requested.

The typical final report describes the pattern of dominance and the burden of atherosclerotic change. Lesions are characterized by location, length, and severity, and associated plaque or calcification is described. Internal mammary arteries, which are invariably patent, are documented. When specifically requested, calcification scores and functional information on wall thickness, motion, and ejection fraction are provided. Any limitations of vessel visualization are acknowledged. Incidental noncardiac and noncoronary cardiac findings are discussed. At present, reports are narrative; however, using a new voice-recognition dictation system, we are formulating a standard reproducible format to cover the technique and findings. We send photographic prints to the referring clinician and digital static JPG or TIFF images and dynamic audio-video inter-leave files when requested.

What to Expect

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
The left main, proximal left anterior descending, circumflex, and right coronary arteries are easiest to assess, and good quality images are reproducible. Approximately 11 cm of the left anterior descending and the right coronary arteries and 8 cm of the circumflex artery may be displayed and over 70% may be motion-free [2–4]. Successful evaluation is caliber-dependent, and, in particular, interpretation of 3- to 4-mm proximal vessels is superior to distal and branch 1- to 2-mm vessels [3, 4]. The more mobile distal right coronary artery and circumflex artery within the atrioventricular groove can prove more difficult, and 25–32% of arteries will be excluded from analysis because of motion artifact or calcification and poor opacification [2, 5, 25]. An overlapping coronary vein may also limit interpretation for certain portions of the circumflex artery.

Our experience—the ability to detect significant stenoses in 70–95% with specificities ranging from 80% to 95%—reflects that of the literature, and we find a good negative predictive value with high confidence in excluding disease when normal vasculature is depicted. Determining the patency of stents and of heavily calcified coronary arteries remains problematic in small-caliber vessels, although it may be suggested from more distal opacification [24, 26] (Fig. 11A, 11B), and eccentric calcification can be documented on orthogonal vessel imaging (Fig. 12). MDCT angiography performs well in evaluating the less mobile venous and arterial bypass grafts, but these patients frequently have advanced native vessel disease beyond the anastomoses that cannot be fully assessed (Fig. 13A, 13B).

View larger version (112K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12. —63-year-old man with coronary artery disease. Right sagittal maximum-intensity-projection image of left anterior descending artery shows focal inferior eccentric calcification (arrow).

View larger version (99K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13A. —60-year-old man undergoing assessment after coronary artery bypass grafting. Volume-rendered anterior view shows saphenous vein bypass grafts (arrowheads) from aneurysmal aorta (A) and left internal mammary bypass graft (arrow).

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13B. —60-year-old man undergoing assessment after coronary artery bypass grafting. Volume-rendered left lateral view shows saphenous vein (arrowhead) and left internal mammary (arrow) bypass grafts.

We have found CT angiography successful and superior to catheter studies in evaluating aberrant vessel, origin course, and caliber, and our CT angiography findings have shown high concordance with surgical findings (Fig. 14A, 14B). We perform coronary CT angiography in the context of a broad cardiovascular evaluation, and a discordant negative result with a high pretest probability of disease still requires additional cardiovascular evaluation.

View larger version (179K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14A. —30-year-old woman referred for evaluation of aberrant coronary artery origin. Volume-rendered superior oblique image shows common origin of aberrant right (short arrow) and left main (arrowhead) coronary arteries from left aortic sinus. Note left anterior descending coronary artery (long arrow).

View larger version (118K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14B. —30-year-old woman referred for evaluation of aberrant coronary artery origin. Maximum-intensity-projection superior oblique view shows common origin of aberrant right (arrowhead) and left main (arrow) coronary arteries from left aortic sinus.

Conclusion

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 
MDCT angiography and advanced image processing have brought coronary CT angiography into the clinical realm, but it remains the most demanding of CT angiography tests in terms of spatial, contrast, and temporal resolution. This article reflects the current state-of-the-art techniques, for which we do not image the motionless heart, and close attention to acquisition, reconstruction, and postprocessing methodology to refine the data available is required. Although there is no universally accepted approach to vessel assessment, we use a stepwise process to harness the complementary potential of 2D and 3D techniques in an efficient manner for workflow. We provide a combination of images that reflects both the conventional angiography model and novel viewing approaches, and we apply a nomenclature and style of interpretation established with our referring clinicians for consistency and concordance with existing knowledge. It is important to remain cognizant of the limitations of this test and its suitability for individual patients because the current state-of-the-art techniques remain part of a continuously evolving process. Although we acknowledge such limitations, we have found good acceptance among clinicians for its clinical utility for the assessment of specific coronary conditions.

References

Top Abstract Introduction Patient Selection Patient Preparation Contrast Injection and Scan... Scanning Protocol Scan Reconstruction Problem Solving Data for Postprocessing Viewing Perspectives Postprocessing Techniques Vessel Review Sequence What to Expect Conclusion References

 

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