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Artifacts in ECG-Synchronized MDCT Coronary Angiography

¹ All authors: Department of Radiology, C2S, Leiden University Medical Center, Albinusdreef 2, 2333 ZA, Leiden, The Netherlands.

Received December 11, 2006; accepted after revision March 8, 2007.

 
Address correspondence to L. J. M. Kroft (l.j.m.kroft{at}lumc.nl).

CME This article is available for CME credit. See www.arrs.org for more information.

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This article is available for CME credit. See www.arrs.org for more information.

Abstract

Top Abstract Introduction Impact of Artifacts on... MDCT and Artifacts Spatial Resolution, Temporal... Conclusion References

 
OBJECTIVE. In MDCT coronary angiography, image artifacts are the major cause of false-positive and false-negative interpretations regarding the presence of coronary artery stenoses. Hence, it is important that observers reporting these investigations are aware of the potential presence of image artifacts and that these artifacts are recognized.

CONCLUSION. The article explores the technical causes for various artifacts in MDCT coronary angiography imaging and clinical examples are given.

Keywords: artifacts • cardiac imaging • computed tomography • coronary angiography • CT • diagnostic imaging

Introduction

Top Abstract Introduction Impact of Artifacts on... MDCT and Artifacts Spatial Resolution, Temporal... Conclusion References

 
From the first 4-MDCT feasibility studies to the current clinically applied 64-MDCT investigations, MDCT coronary angiography has evolved into a reliable 3D imaging technique for detecting and excluding coronary artery stenoses with high accuracy [1-8]; it is now considered an appropriate imaging tool for detecting coronary artery disease in certain clinical contexts [9]. However, 2D conventional invasive X-ray coronary angiography is still considered the standard of reference for evaluating the coronary arteries because of its superior spatial and temporal resolutions compared with MDCT. Parts of the coronary arteries cannot be evaluated with MDCT because of image artifacts, and image artifacts are the major cause of false-positive and false-negative interpretations. Hence, it is important that observers reporting MDCT coronary artery angiography investigations are aware of the potential presence of image artifacts and that these artifacts are recognized on the images.

In this article, the impact of artifacts on MDCT coronary angiography will be explained. The causes of artifacts will be discussed in detail, with special attention to the effect of coronary artery size and motion. Examples of artifacts will be shown that may help the reader recognize these artifacts when reporting MDCT coronary angiography.

Impact of Artifacts on MDCT Coronary Angiography

Top Abstract Introduction Impact of Artifacts on... MDCT and Artifacts Spatial Resolution, Temporal... Conclusion References

 
MDCT coronary angiography image quality and diagnostic performance have greatly improved after recent technical developments. With 4-MDCT, 29% of the coronary arteries could not be evaluated because of artifacts [1]. With 16-MDCT, 22-29% of the coronary artery segments could not be evaluated [10, 11]. One study stated that if these segments that could not be evaluated were excluded or considered negative, 25% of patients with a significant stenosis would have been missed [11]. With 64-MDCT, 3-11% of coronary artery segments still cannot be evaluated [8, 12-15].

Sensitivities and specificities for detecting significant (³ 50%) coronary artery stenoses based on segmental analysis with 64-MDCT (conventional X-ray coronary angiography as the standard of reference) have been found to be good to excellent, in the range of 76-99% and 95-97%, respectively [2-8, 16]. However, these study outcomes are difficult to compare because the study methods vary substantially—for example, in the selection of patients. Moreover, these study results should be interpreted with care because coronary artery segments that could not be evaluated (3-27%) were excluded from analysis beforehand [2-5, 7, 8].

Interestingly, of the coronary artery segments that were included in 64-MDCT studies, the accuracy for detecting stenoses depended highly on image artifacts. False-positive and false-negative interpretations were attributed to image artifacts in 91% [6] to 100% [5] of cases, where the major cause was the presence of calcifications. Other authors support these findings [2-4, 6]. Less frequent causes were motion artifacts [4, 5] and obesity resulting in a poor contrast-to-noise ratio [4]. It can be concluded that artifacts are the cause of coronary arteries that cannot be evaluated and for false-positive and false-negative diagnoses as well.

MDCT and Artifacts

Top Abstract Introduction Impact of Artifacts on... MDCT and Artifacts Spatial Resolution, Temporal... Conclusion References

 
Artifact Definition
In CT, the term "image artifact" can be defined as any discrepancy between the reconstructed Hounsfield values in the image and the true attenuation coefficients of the object in such a way that these discrepancies are clinically significant or relevant as judged by the radiologist [17]. In the definition, it is assumed that the radiologist will recognize relevant artifacts when they are present.

Technical Considerations for Coronary Artery Imaging and Artifacts
Coronary artery MDCT is technically complex and requires high spatial resolution, high temporal resolution, good low-contrast resolution, intravascular contrast enhancement, and a short scanning time. The key acquisition parameters in cardiac MDCT are section thickness, the rotation time of the X-ray tube, and the pitch factor. The acquisition section thickness is measured along the z-axis and determines the minimal voxel height in a reconstructed image. The rotation time (in milliseconds) is the time needed for a 360° rotation of the X-ray tube. The pitch factor is the ratio of patient displacement along the z-axis direction per tube rotation divided by the total thickness of all simultaneously acquired sections [18].

Basically, almost all MDCT image artifacts can be explained by limitations relating to spatial resolution, temporal resolution, noise (Table 1), and the reconstruction algorithms used. In the images, artifacts are mainly observed as blurring, blooming, streaks, missing data, discontinuities, and poor contrast enhancement. Artifacts may be grouped in technical- (physics-based, scanner-based, and reconstruction-based), operator-, and patient-related causes [19]. Defining the different artifact categories seems rather arbitrary. For example, poor patient instruction that results in breathing artifacts may be categorized as an operator-dependent artifact, whereas breathing artifacts that occur despite adequate breath-holding instructions may be considered patient-dependent.

Spatial Resolution, Temporal Resolution, Noise, and Reconstruction Algorithm

Top Abstract Introduction Impact of Artifacts on... MDCT and Artifacts Spatial Resolution, Temporal... Conclusion References

 
Spatial Resolution
Spatial resolution, the ability to visualize small structures in the scanned volume, must be considered in three dimensions. A voxel is the volume element that is represented by a 2D pixel in the axial xy-plane. The third dimension is the z-axis and the corresponding voxel height. A reconstructed field of view of 200 mm and a characteristic 512 x 512 pixel matrix result in a pixel size of 0.4 x 0.4 mm² in the axial xy-plane. These values are typical for single-detector helical CT acquisitions and for the current MDCT scanners. Reconstructions with a smaller field of view can substantially decrease the pixel size in the axial plane.

The great improvement in spatial resolution with the current MDCT scanners is due to the feasibility of acquiring volumes with thinner sections, which is especially important for reduction of the partial volume effect. With 4-MDCT, 4 x 1 mm or 2 mm collimation scans were obtained in 20- to 45-second breath-holds [1, 20-22]. With 16-MDCT, 16 x 0.5 mm or 0.75 mm collimation scans were obtained in 16- to 30-second breath-holds [23-25]. With 64-slice and 64-row MDCT scanners, 64 x 0.6 mm and 64 x 0.5 mm collimations are achieved within 11- to 15-second breath-holds [2, 5, 6]. Thinner collimation causes smaller voxel height, and shorter breath-holds allow more patients to hold their breath during image acquisition. Also, the scanned volume is obtained in substantially fewer heart beats, thereby decreasing the amount of image artifacts due to variations between heart beats.

View larger version (10K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —Principle of full width at half maximum (FWHM) of response of very small object for describing spatial resolution that can be achieved. Visualization of two ideal points separated by distance of less than one FWHM (A) and separated more than one FWHM (B). Response of ideal point is represented by gray area; composite response of two ideal points is represented by curving black line. At separation distance of less than one FWHM, two points cannot be distinguished separately; at distance of more than one FWHM, two points can be observed individually. Note that this criterion assumes static condition, or, in other words, that no motion artifacts are present.

View larger version (11K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —Principle of full width at half maximum (FWHM) of response of very small object for describing spatial resolution that can be achieved. Visualization of two ideal points separated by distance of less than one FWHM (A) and separated more than one FWHM (B). Response of ideal point is represented by gray area; composite response of two ideal points is represented by curving black line. At separation distance of less than one FWHM, two points cannot be distinguished separately; at distance of more than one FWHM, two points can be observed individually. Note that this criterion assumes static condition, or, in other words, that no motion artifacts are present.

Partial Volume Effect
Partial volume effect or artifact is caused by limited spatial resolution and is the result of averaging the attenuation coefficient in a voxel that is heterogeneous in composition, where the average numeric value (in Hounsfield units) is assigned to the corresponding pixel. Partial volume effect is especially present at borders of two tissues or structures with a large difference in Hounsfield units, particularly at the margin of the coronary artery lumen and calcified plaque. The larger the voxels (i.e., pixel size and reconstructed image thickness), the larger the partial volume effect [18]. Partial volume artifacts are most often due to the presence of calcifications and are a major concern in MDCT coronary angiography because they cause false-positive and false-negative interpretations in coronary arteries that could otherwise be evaluated [2-6]. Partial volume artifacts, including blurring and blooming, are best avoided by using thin collimation and a small reconstructed field of view [19].

Temporal Resolution
Temporal resolution is the ability to resolve fast-moving objects in the displayed CT image [18]. Temporal resolution remains the major challenge in MDCT coronary angiography. Limitations in temporal resolution are strongly related to coronary artery size and motion. Motion in general causes degradation of contrast and spatial resolution and introduces artifacts [28]. Cardiac motion presents as blurring and is the major reason for nondiagnostic coronary artery image quality [11-13]. The three major approaches to limit cardiac motion artifacts were already postulated in the 1970s by Harell et al. [29]: reducing the data collection period by faster rotating X-ray tubes, synchronizing CT data acquisition with the cardiac cycle (prospective gating), and reconstructions synchronized to the ECG cycle (retrospective reconstructions) after data acquisition. These measures are currently routinely applied in MDCT coronary angiography.

View larger version (21K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A —59-year-old man imaged for suspected coronary artery disease. Stairstep artifact due to premature atrial contraction with extra systole, followed by compensatory long R-R interval (between sixth and seventh R-R peaks) Note that premature beat is approximately in middle of acquisition (A), which is also true in images (B-D). Note stairstep in right coronary artery (RCA) (arrows) at 3D reconstructions and central luminal line projections (B) and in two long-axis perpendicular curved multiplanar reconstructions (C, D). In these perpendicular curved multiplanar reconstructions, coronary artery is usually more affected in one direction than in other. Step had virtually no effect on left anterior descending coronary artery (LAD in B). Mean heart rate was 59 beats per minute. R-R interval during acquisition varied between 644 and 1,281 milliseconds.

View larger version (63K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B —59-year-old man imaged for suspected coronary artery disease. Stairstep artifact due to premature atrial contraction with extra systole, followed by compensatory long R-R interval (between sixth and seventh R-R peaks) Note that premature beat is approximately in middle of acquisition (A), which is also true in images (B-D). Note stairstep in right coronary artery (RCA) (arrows) at 3D reconstructions and central luminal line projections (B) and in two long-axis perpendicular curved multiplanar reconstructions (C, D). In these perpendicular curved multiplanar reconstructions, coronary artery is usually more affected in one direction than in other. Step had virtually no effect on left anterior descending coronary artery (LAD in B). Mean heart rate was 59 beats per minute. R-R interval during acquisition varied between 644 and 1,281 milliseconds.

View larger version (95K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C —59-year-old man imaged for suspected coronary artery disease. Stairstep artifact due to premature atrial contraction with extra systole, followed by compensatory long R-R interval (between sixth and seventh R-R peaks) Note that premature beat is approximately in middle of acquisition (A), which is also true in images (B-D). Note stairstep in right coronary artery (RCA) (arrows) at 3D reconstructions and central luminal line projections (B) and in two long-axis perpendicular curved multiplanar reconstructions (C, D). In these perpendicular curved multiplanar reconstructions, coronary artery is usually more affected in one direction than in other. Step had virtually no effect on left anterior descending coronary artery (LAD in B). Mean heart rate was 59 beats per minute. R-R interval during acquisition varied between 644 and 1,281 milliseconds.

View larger version (95K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2D —59-year-old man imaged for suspected coronary artery disease. Stairstep artifact due to premature atrial contraction with extra systole, followed by compensatory long R-R interval (between sixth and seventh R-R peaks) Note that premature beat is approximately in middle of acquisition (A), which is also true in images (B-D). Note stairstep in right coronary artery (RCA) (arrows) at 3D reconstructions and central luminal line projections (B) and in two long-axis perpendicular curved multiplanar reconstructions (C, D). In these perpendicular curved multiplanar reconstructions, coronary artery is usually more affected in one direction than in other. Step had virtually no effect on left anterior descending coronary artery (LAD in B). Mean heart rate was 59 beats per minute. R-R interval during acquisition varied between 644 and 1,281 milliseconds.

View larger version (75K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3 —54-year-old woman with suspected coronary artery disease. Image shows blurring due to motion caused by premature atrial contraction. At short R-R interval, rest phase was too short for motion-free imaging of coronary artery segment that presumably had large motion range at this time, causing blurring. This segment of right coronary artery is frequently affected by motion artifacts. Mean heart rate was 66 beats per minute. R-R interval during acquisition varied between 641 and 1,194 milliseconds.

The coronary arteries move substantially during the cardiac cycle with considerable intra- and interpatient variation regarding motion patterns and ranges [32]. The right coronary artery has the greatest velocity and range [33, 34]. Transverse (axial xy-plane) displacement is the major part of motion and ranges between 6 and 42 mm for the right and between 3 and 20 mm for the left coronary artery [32]. Motion velocity increases with heart rate [33, 34], but motion range does not [32]. The right coronary artery is affected most by motion artifacts [12].

Excessive coronary artery motion of an in-plane distance greater than its diameter corresponds to visible motion on transverse images [33]. This amount of "own diameter motion" seems a rather comfortable criterion, particularly when the limitations in temporal resolution affect CT of small structures (small-diameter coronary artery segments). With the current MDCT scanners, coronary artery motion exceeds the velocities needed for motion-free imaging when variations in cardiac motion during the cardiac cycle are not taken into account. However, motion velocity and speed change during the cardiac cycle, and we must use the "rest phase" for imaging with the fewest motion artifacts (Fig. 4). But even then, the cardiac rest period, defined as the time with a displacement of the coronary artery of less than 1 mm, has a mean duration of 120 milliseconds but ranges from 66 to 333 milliseconds among patients [32]. Consequently, the temporal resolution of 165 milliseconds that is currently achieved with the fastest 330-millisecond rotation times when using half-scan reconstructions is longer than the mean rest period of 120 milliseconds and is not fast enough to image the general patient free of motion artifacts, notwithstanding optimal phase selection for image reconstruction in the cardiac cycle.

View larger version (99K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4 —51-year-old woman with suspected coronary artery disease. Image shows motion range for right coronary artery (RCA) during cardiac cycle. Image reconstructions were performed at 0%, 40%, and 80% of R-R interval and show identical orientation of 3D images in upper row and identical levels of images in middle row. In lower row, level that best displayed origin of RCA is displayed. Note large amount of motion of RCA during cardiac cycle. Note that RCA is displayed sharply at 80% of R-R interval, but not at 0% and 40% time phases. Mean heart rate was 52 beats per minute. R-R interval during acquisition varied between 1,095 and 1,189 milliseconds.

To overcome limitations in temporal resolution and motion artifacts, increased gantry rotation speed is favored. However, a rotation time of less than 200 milliseconds to provide a temporal resolution of less than 100 milliseconds, regardless of cardiac frequency (half-scan reconstruction), would already result in an increase in mechanical G-forces that is beyond mechanical engineering limits [36]. Other strategies are as follows:

Lowering the cardiac frequency by using ß-blockers or dilating the coronary arteries by using nitroglycerin—Coronary artery image quality is inversely dependent on heart rate [1, 4, 12, 20, 21, 37]. Administering ß-blockers for coronary artery MDCT is widely used for reducing the heart rate, preferably to a frequency of less than 60-65 bpm [2-5, 7, 8, 38]. Despite these publications, it has recently been found that the effect of heart rate on image quality is limited and mainly affects the visualization of the left circumflex coronary artery. Instead, the variability in heart rate during acquisition was found to be important. The stabilizing effect of ß-blockers was shown to be the major determinant that resulted in superior image quality in patients receiving ß-blockers as compared with those who did not [39]. Some authors use vasodilating medication as well for maximizing coronary artery size [40, 41]. In MDCT coronary angiography, the use of nitroglycerin has been found to increase proximal coronary artery diameters by 12-21% [41]. However, the added value on diagnostic accuracy is not clear yet.

Using segmental reconstruction instead of half-scan reconstruction—The single-segment approach (i.e., half-scan reconstruction) and the two or more segment approach (i.e., multisegment reconstruction) are the two image reconstruction algorithms used for low and higher heart rates, respectively. With half-scan reconstruction, data obtained from a single 180° gantry rotation are used for image reconstruction [36]. With multisegment reconstruction, data of two or more successive cardiac cycles are combined that cover, as separate segments, 180° acquisition. Multisegment reconstruction algorithms require a stable and predictable heart rate during image acquisition. The temporal resolution is improved by a factor 2n (n = number of cycles and segments) of the rotation time [36]. For 64-MDCT with 330 milliseconds rotation time, it was found that image quality achieved with two-segment reconstruction was not significantly improved compared with half-scan reconstruction for heart rates exceeding 65 bmp, although in 65% of patients the best overall image quality was achieved by two-segment reconstruction [13].

View larger version (73K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A —69-year-old woman with suspected coronary artery disease. Images show poor contrast enhancement. Contrast timing was good because coronary arteries were already enhancing. Note poor enhancement of left ventricle (LV), which should be brightly enhanced (B) (compare with Fig. 4). Also note stent in circumflex coronary artery (A and C), where artery is moderately enhanced. Patient performed Valsalva maneuver during image acquisition that is recognized by contrast column with convex shape toward superior vena cava (SVC on coronal image, D), whereas saline flush should be running through at this time point. High intrathoracic pressure during Valsalva maneuver hampers inflow in right atrium and causes poor contrast enhancement. Mean heart rate was 77 beats per minute. R-R interval during acquisition varied between 776 and 789 milliseconds.

View larger version (144K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B —69-year-old woman with suspected coronary artery disease. Images show poor contrast enhancement. Contrast timing was good because coronary arteries were already enhancing. Note poor enhancement of left ventricle (LV), which should be brightly enhanced (B) (compare with Fig. 4). Also note stent in circumflex coronary artery (A and C), where artery is moderately enhanced. Patient performed Valsalva maneuver during image acquisition that is recognized by contrast column with convex shape toward superior vena cava (SVC on coronal image, D), whereas saline flush should be running through at this time point. High intrathoracic pressure during Valsalva maneuver hampers inflow in right atrium and causes poor contrast enhancement. Mean heart rate was 77 beats per minute. R-R interval during acquisition varied between 776 and 789 milliseconds.

View larger version (133K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5C —69-year-old woman with suspected coronary artery disease. Images show poor contrast enhancement. Contrast timing was good because coronary arteries were already enhancing. Note poor enhancement of left ventricle (LV), which should be brightly enhanced (B) (compare with Fig. 4). Also note stent in circumflex coronary artery (A and C), where artery is moderately enhanced. Patient performed Valsalva maneuver during image acquisition that is recognized by contrast column with convex shape toward superior vena cava (SVC on coronal image, D), whereas saline flush should be running through at this time point. High intrathoracic pressure during Valsalva maneuver hampers inflow in right atrium and causes poor contrast enhancement. Mean heart rate was 77 beats per minute. R-R interval during acquisition varied between 776 and 789 milliseconds.

View larger version (139K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5D —69-year-old woman with suspected coronary artery disease. Images show poor contrast enhancement. Contrast timing was good because coronary arteries were already enhancing. Note poor enhancement of left ventricle (LV), which should be brightly enhanced (B) (compare with Fig. 4). Also note stent in circumflex coronary artery (A and C), where artery is moderately enhanced. Patient performed Valsalva maneuver during image acquisition that is recognized by contrast column with convex shape toward superior vena cava (SVC on coronal image, D), whereas saline flush should be running through at this time point. High intrathoracic pressure during Valsalva maneuver hampers inflow in right atrium and causes poor contrast enhancement. Mean heart rate was 77 beats per minute. R-R interval during acquisition varied between 776 and 789 milliseconds.

Faster scanning with 256-MDCT volumetric scanners—With these next generation volumetric MDCT scanners [43, 44], the entire heart can be covered in only one half rotation. This is expected to eliminate interbeat (stairstep) artifacts, and a short scanning time of 175 milliseconds (half-scan, prospective triggering) minimizes the breath-holding time, allowing almost all patients to breath-hold during image acquisition. Moreover, multisegment reconstruction—for example, two-segment reconstruction—may potentially be used to improve the acquisition time.

Respiratory Motion
Breath-holding exercises and instructions to the patients are particularly important in avoiding motion artifacts by breathing and postural motions that cause blurring. With the current 64-MDCT scanners, most patients can breath-hold for the 10- to 13-second scanning duration. We instruct the patients to hold their breath after breathing in, at approximately three quarters of their maximum, and to lie still without pressing in order to avoid a Valsalva maneuver that may result not only in breathing artifacts but also in poor contrast enhancement (Fig. 5A, 5B, 5C, 5D). Respiratory motion is well recognized at the lung window setting.

Noise
CT noise (quantum mottle) is determined primarily by the number of photons used to make an image. The quantum mottle fraction decreases as the number of photons increases. CT noise is generally reduced by increasing the kVp, mA, or scanning time. CT noise is also reduced by increasing voxel size (i.e., by decreasing the matrix size), increasing reconstructed field of view, by increasing section thickness [45], or by image stacking.

Contrast is the difference in Hounsfield values between tissues and tends to increase as kVp decreases but is less affected by mA or scanning time. CT contrast can be improved by administering an iodinated contrast agent. The displayed image contrast is primarily determined by the CT window-width and window-level settings [45].

Larger chest sizes (obesity) are associated with a higher level of image noise that negatively affects the quality of MDCT coronary angiograms [12]. Inadequate contrast administration (e.g., inadequate volume, injection speed, or timing), inadequate selection of the field of view or region-of-interest placement for bolus tracking, or inadequate breath-holds can result in low contrast-to-noise ratios, resulting in poorly visualized coronary arteries. Contrast media with higher iodine concentrations provide substantially higher attenuation values in the coronary arteries [46], although the added value of these higher-iodine-concentration media on diagnostic accuracy in assessing coronary artery disease has not yet been established. Artifacts on the right coronary artery due to high contrast density in the right atrium can be effectively reduced using a saline flush. However, the diagnostic accuracy in detecting coronary artery stenoses has not been found to be substantially different between uni- or biphasic contrast protocols with or without a saline flush [47]. Probably, the contrast dose and injection speed should be balanced for optimal enhancement. Testing the IV access before contrast administration is advisable and is best performed just before coronary MDCT by flushing a saline bolus with the patient's arms up in the same position as during coronary artery scanning.

View larger version (155K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A —60-year-old woman with suspected coronary artery disease. Geometric distortion due to spiral acquisition, where black "shadow" or "rod" artifact next to contrast-filled right coronary artery is due to miscalculation by reconstruction algorithm. During spiral acquisition, position registered by each view shifts. Miscalculation may cause hypodense artifacts (arrows) that rotate around high-density contrast-filled coronary artery. Note change in artifact position from A to C that is also observed on corresponding levels at coronal reconstruction (D). Mean heart rate was 66 beats per minute. R-R interval during acquisition varied between 916 and 977 milliseconds.

View larger version (155K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B —60-year-old woman with suspected coronary artery disease. Geometric distortion due to spiral acquisition, where black "shadow" or "rod" artifact next to contrast-filled right coronary artery is due to miscalculation by reconstruction algorithm. During spiral acquisition, position registered by each view shifts. Miscalculation may cause hypodense artifacts (arrows) that rotate around high-density contrast-filled coronary artery. Note change in artifact position from A to C that is also observed on corresponding levels at coronal reconstruction (D). Mean heart rate was 66 beats per minute. R-R interval during acquisition varied between 916 and 977 milliseconds.

View larger version (163K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6C —60-year-old woman with suspected coronary artery disease. Geometric distortion due to spiral acquisition, where black "shadow" or "rod" artifact next to contrast-filled right coronary artery is due to miscalculation by reconstruction algorithm. During spiral acquisition, position registered by each view shifts. Miscalculation may cause hypodense artifacts (arrows) that rotate around high-density contrast-filled coronary artery. Note change in artifact position from A to C that is also observed on corresponding levels at coronal reconstruction (D). Mean heart rate was 66 beats per minute. R-R interval during acquisition varied between 916 and 977 milliseconds.

View larger version (152K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6D —60-year-old woman with suspected coronary artery disease. Geometric distortion due to spiral acquisition, where black "shadow" or "rod" artifact next to contrast-filled right coronary artery is due to miscalculation by reconstruction algorithm. During spiral acquisition, position registered by each view shifts. Miscalculation may cause hypodense artifacts (arrows) that rotate around high-density contrast-filled coronary artery. Note change in artifact position from A to C that is also observed on corresponding levels at coronal reconstruction (D). Mean heart rate was 66 beats per minute. R-R interval during acquisition varied between 916 and 977 milliseconds.

To reduce spiral interpolation artifacts, narrow collimation that improves the z-axis resolution should be used [36]. One may expect these spiral CT artifacts to disappear with the introduction of newer generation (256-row) scanners in which acquisition of the entire cardiac volume will be obtained without table movement.

Beam-hardening artifacts are caused by the polychromatic nature of the X-ray beam. As the lower energy photons are preferentially absorbed, the beam becomes more penetrating, which results in lower computed attenuation coefficient Hounsfield values. Beam-hardening artifacts are most prominent at high-contrast interfaces [45]. In cardiac imaging, high-density contrast agent injection can manifest as beam-hardening artifacts by causing dark bands between dense objects in the image [17].

Metal objects can cause complicated artifacts, such as beam hardening and partial volume, that are worsened with object motion, and methods to overcome metal-induced artifacts are particularly difficult to design [17]. In MDCT coronary angiography, these artifacts may occur with stents, pacemakers, or surgical clips. If the density of highly attenuating metal objects is beyond the normal range that can be handled by the computer, severe streaking artifacts occur [19] (Fig. 7A, 7B, 7C, 7D). Sharp high-resolution kernels may be used for improved stent-lumen visualization [49], although sharp kernel filters result in higher image noise and artifacts causing lower in-stent attenuation values [50]. The diagnostic effect of high-resolution kernels on accuracy in evaluating stent patency in patients has not been investigated yet.

View larger version (98K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A —Two patients with suspected coronary artery disease. 34-year-old man with pacemaker lead in right atrium (B) that causes subtle artifacts visible at right ventricular surface in 3D view (arrows, A) and through right coronary artery central luminal line reconstruction (arrow, B). Mean heart rate was 78 beats per minute. R-R interval during acquisition varied between 759 and 790 milliseconds.

View larger version (110K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B —Two patients with suspected coronary artery disease. 34-year-old man with pacemaker lead in right atrium (B) that causes subtle artifacts visible at right ventricular surface in 3D view (arrows, A) and through right coronary artery central luminal line reconstruction (arrow, B). Mean heart rate was 78 beats per minute. R-R interval during acquisition varied between 759 and 790 milliseconds.

View larger version (93K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7C —Two patients with suspected coronary artery disease. 57-year-old man after bypass surgery with metallic sternal wires (C). Severe high-density surgical clip artifacts hamper arterial lumen evaluation at course of left internal mammary artery, which was used for bypassing left anterior descending coronary artery (D). Surgical clips were used for occluding side branches of left internal mammary artery. Mean heart rate was 74 beats per minute. R-R interval during acquisition varied between 760 and 835 milliseconds.

View larger version (103K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7D —Two patients with suspected coronary artery disease. 57-year-old man after bypass surgery with metallic sternal wires (C). Severe high-density surgical clip artifacts hamper arterial lumen evaluation at course of left internal mammary artery, which was used for bypassing left anterior descending coronary artery (D). Surgical clips were used for occluding side branches of left internal mammary artery. Mean heart rate was 74 beats per minute. R-R interval during acquisition varied between 760 and 835 milliseconds.

View larger version (140K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8A —77-year-old woman with suspected coronary artery disease. Curved multiplanar reconstruction of right coronary artery in two perpendicular longitudinal directions. At time point with least motion at 900 milliseconds (at 76% of R-R interval, A and B), right coronary artery is sharply delineated in proximal part but appears interrupted halfway (arrow), whereas further coronary artery segment appears blurred. Severe stenosis cannot be excluded at this time. Additional reconstruction at 500 milliseconds (at 42% of R-R interval, C and D) is of moderate but diagnostic quality and shows that suspected right coronary artery segment is actually open. Mean heart rate was 51 beats per minute. R-R interval during acquisition varied between 1,155 and 1,198 milliseconds.

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8B —77-year-old woman with suspected coronary artery disease. Curved multiplanar reconstruction of right coronary artery in two perpendicular longitudinal directions. At time point with least motion at 900 milliseconds (at 76% of R-R interval, A and B), right coronary artery is sharply delineated in proximal part but appears interrupted halfway (arrow), whereas further coronary artery segment appears blurred. Severe stenosis cannot be excluded at this time. Additional reconstruction at 500 milliseconds (at 42% of R-R interval, C and D) is of moderate but diagnostic quality and shows that suspected right coronary artery segment is actually open. Mean heart rate was 51 beats per minute. R-R interval during acquisition varied between 1,155 and 1,198 milliseconds.

View larger version (128K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8C —77-year-old woman with suspected coronary artery disease. Curved multiplanar reconstruction of right coronary artery in two perpendicular longitudinal directions. At time point with least motion at 900 milliseconds (at 76% of R-R interval, A and B), right coronary artery is sharply delineated in proximal part but appears interrupted halfway (arrow), whereas further coronary artery segment appears blurred. Severe stenosis cannot be excluded at this time. Additional reconstruction at 500 milliseconds (at 42% of R-R interval, C and D) is of moderate but diagnostic quality and shows that suspected right coronary artery segment is actually open. Mean heart rate was 51 beats per minute. R-R interval during acquisition varied between 1,155 and 1,198 milliseconds.

View larger version (130K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8D —77-year-old woman with suspected coronary artery disease. Curved multiplanar reconstruction of right coronary artery in two perpendicular longitudinal directions. At time point with least motion at 900 milliseconds (at 76% of R-R interval, A and B), right coronary artery is sharply delineated in proximal part but appears interrupted halfway (arrow), whereas further coronary artery segment appears blurred. Severe stenosis cannot be excluded at this time. Additional reconstruction at 500 milliseconds (at 42% of R-R interval, C and D) is of moderate but diagnostic quality and shows that suspected right coronary artery segment is actually open. Mean heart rate was 51 beats per minute. R-R interval during acquisition varied between 1,155 and 1,198 milliseconds.

View larger version (90K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9A —51-year-old man with suspected coronary artery disease. Automatic segmentation reconstruction artifact with interruption and apparent stenosis of right coronary artery (RCA) at crux where it diverges into RCA continuation in atrioventricular groove and in posterior descending branch (PD). Point of interruption is where central luminal line (dotted line, A) has lost its way at crux and is not in center of coronary artery (A). This is easily repaired by manually replacing erroneous point (A) correctly in central lumen at curved multiplanar reconstruction. After replacement, artery is continuous; compare B and C. Mean heart rate was 52 beats per minute. R-R interval during acquisition varied between 1,124 and 1,172 milliseconds.

View larger version (116K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9B —51-year-old man with suspected coronary artery disease. Automatic segmentation reconstruction artifact with interruption and apparent stenosis of right coronary artery (RCA) at crux where it diverges into RCA continuation in atrioventricular groove and in posterior descending branch (PD). Point of interruption is where central luminal line (dotted line, A) has lost its way at crux and is not in center of coronary artery (A). This is easily repaired by manually replacing erroneous point (A) correctly in central lumen at curved multiplanar reconstruction. After replacement, artery is continuous; compare B and C. Mean heart rate was 52 beats per minute. R-R interval during acquisition varied between 1,124 and 1,172 milliseconds.

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9C —51-year-old man with suspected coronary artery disease. Automatic segmentation reconstruction artifact with interruption and apparent stenosis of right coronary artery (RCA) at crux where it diverges into RCA continuation in atrioventricular groove and in posterior descending branch (PD). Point of interruption is where central luminal line (dotted line, A) has lost its way at crux and is not in center of coronary artery (A). This is easily repaired by manually replacing erroneous point (A) correctly in central lumen at curved multiplanar reconstruction. After replacement, artery is continuous; compare B and C. Mean heart rate was 52 beats per minute. R-R interval during acquisition varied between 1,124 and 1,172 milliseconds.

Diagnostic Interpretation Errors
For 64-MDCT, false-positive and false-negative coronary artery interpretations have been explicitly explained by obvious technical limitations—that is, image artifacts due to calcifications, motion, and obesity [2-6]. This was also the case for 4-MDCT [21] and 16-MDCT [10, 11] coronary angiography. Therefore, missing coronary artery abnormalities seems to be most commonly related to artifacts and less related to lack of diagnostic perception in the case of well-trained observers.

Technical errors that can be avoided are often related to patient handling and postprocessing handling. Suggestions for avoiding MDCT coronary angiography artifacts are presented in Table 1. Global measures are the following:

Patient preparation—Lower the heart rate in patients with heart rates exceeding 65 bpm by using ß-blockers (if allowed). Obtain a good ECG signal. Prepare contrast injection and prepare the patient with breath-holding instructions.

Acquisition—Take care for adequate timing of contrast injection, and use an adequate dose and injection speed. Provide good breath-holding instructions.

Postprocessing—Choose the best phase for coronary artery reconstruction. Review the source images to confirm findings at advanced workstation reconstructions.

Diagnostic interpretation—Recognize artifacts and report diagnostic limitations.

Conclusion

Top Abstract Introduction Impact of Artifacts on... MDCT and Artifacts Spatial Resolution, Temporal... Conclusion References

 
The main artifacts that hamper MDCT coronary angiography image interpretation are motion artifacts that cause blurring and incorrect diagnoses due to coronary artery calcifications. This article has explored artifact causes and provided examples. Recognizing artifacts is important in the diagnostic process.

Acknowledgments
 
We thank Raoul M.S. Joemai for producing Figure 1A, 1B.

References

Top Abstract Introduction Impact of Artifacts on... MDCT and Artifacts Spatial Resolution, Temporal... Conclusion References

 

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