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Original Research |
1 Department of Radiology, Northwestern University Feinberg School of Medicine,
676 N St. Clair St., Ste. 800, Chicago, IL 60611.
2 Department of Biomedical Engineering, Northwestern University Feinberg School
of Medicine, Chicago, IL 60611.
Received August 20, 2005;
accepted after revision February 28, 2006.
Address correspondence to J. C. Carr
(jcarr{at}radiology.northwestern.edu).
Abstract
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MATERIALS AND METHODS. Fifty patients underwent high-resolution contrast-enhanced MR angiography on a 1.5-T whole-body system. The 50 patients were composed of two groups of 25 consecutive patients; one group underwent MR angiography with ECG gating and the other group underwent MR angiography without ECG gating. A sagittal (3D) gradient-echo fast low-angle shot (FLASH) sequence (TR/TE, 2.8/1.4; flip angle, 25°; readout, 512; voxel size, 1.4 x 0.8 x 1.3 mm) with an asymmetric k-space scanning scheme in all three gradient axes was used, and 0.2 mmol/kg of gadopentetate dimeglumine was injected at 2 mL/s. Sharpness of the thoracic aorta was evaluated at three levels by generating a signal intensity profile across the aortic vessel wall and calculating the distance between two points along a line representing the slope of the signal intensity profile. Both sides of the intensity profile were analyzed and averaged and then used to calculate sharpness. An additional group of six patients was included who had undergone both a gated and an ungated sequence; results of this group were analyzed independently.
RESULTS. Quantitative analysis of the sharpness of the ascending thoracic aorta showed a significant increase in sharpness in both the 50-patient and six-patient groups (p < 0.005) with the addition of cardiac gating.
CONCLUSION. Cardiac gating significantly improves the sharpness of the ascending aorta, a portion of the aorta that is subject to a great deal of blurring caused by cardiac motion. High-resolution contrast-enhanced MR angiography with cardiac gating can produce high quality images of the thoracic vasculature, thus enabling accurate diagnosis of vessel disease.
Keywords: cardiac gating • cardiovascular imaging • MR angiography
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CT angiography (CTA) is increasingly being used as the first-line tool for assessing the thoracic aorta. CTA has the advantage of producing detailed images of the vessel wall and lumen with high spatial resolution. When ECG gating is used, cardiac motion artifact in the aortic root can be negated, making it possible to accurately measure aneurysm dimensions throughout the thoracic aorta. As a result, CTA is currently the preferred method for follow-up of patients with thoracic aortic aneurysms [6]. However, CTA has drawbacks surrounding its use as well, such as potentially nephrotoxic contrast agents and exposure to large amounts of radiation with repeated imaging [6].
In recent years, contrast-enhanced MR angiography (MRA) has emerged as an alternative to diagnostic catheter angiography [7-13]. Contrast-enhanced MRA is minimally invasive when compared with X-ray angiography and provides images that rival those obtained with conventional angiographic techniques. MRA has been associated with 2D and 3D time-of-flight techniques for evaluation of the carotid vasculature, and excellent results have been associated with the use of MRA for the detection of arterial disease [14-21]. These results provided the indication that a more expansive use of the technique was possible.
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Recently, contrast-enhanced MRA has been improved by ECG gating, which was made possible by the addition of acceleration techniques such as iPAT and TREAT (time-resolved, echosharing, angiographic technique) [25, 26]. When acceleration techniques are used in combination with time-resolved subsecond contrast-enhanced MRA, acquisition times can be reduced to 300 milliseconds; and with durations this short, ECG-triggering can be used to gate the acquisition period to diastole (Carr JC et al., presented at the 2005 annual meeting of the International Society for Magnetic Resonance in Medicine). Figure 1 depicts the period of acquisition during the cardiac cycle. ECG gating is intended to improve vessel sharpness when imaging the vasculature, including the thoracic aorta; however, whether this addition to the contrast-enhanced MRA protocol has significantly improved vessel sharpness has not been evaluated. The purpose of this retrospective study was to quantitatively evaluate vessel sharpness in patients before and after the implementation of ECG gating to determine if the technique of gating did indeed improve vessel sharpness and thus the clinical value of contrast-enhanced MRA images in evaluating disease of the thoracic aorta.
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Patients
This retrospective study included images from 50 patients (22 men, 28
women; age range, 19-80 years; mean age, 52 ± 18 years) who were
referred for investigation of thoracic aorta disease. The 50 patients were
divided into two groups of 25 consecutive patients for comparison; one group
underwent MRA with ECG gating and the other group underwent MRA without ECG
gating. The patients were equally distributed around the time at which our
institution began to incorporate the ECG-gating sequence into the standard
imaging protocol. Both the group of patients without a gated scan (10 men, 15
women; age range, 19-80 years; mean age, 54 ± 19 years) and the group
with a gated scan (12 men, 13 women; age range, 21-76 years; mean age, 51
± 17 years) were imaged under identical circumstances and were
differentiated only by the presence or absence of gating. Six of the patients
are included in both groups because they received multiple imaging for
follow-up and thus provide a powerful comparison of the effectiveness of
gating. Thus, these patients are included in the total analysis and in an
independent analysis. All studies were performed in accordance with the
institutional review board guidelines.
Imaging Technique
All patients underwent breath-hold high-resolution aortic MRA on a 1.5-T
whole-body system (Sonata or Avanto, Siemens Medical Solutions). A
four-channel phased-array body coil was placed on the patient's chest for
signal reception. A 3D gradient-echo FLASH pulse sequence was used for
high-resolution MRA. An asymmetric k-space scanning scheme in all three
gradient axes was used (partial echo, 6/8; phase encoding direction, 63% in
partition direction). The remainder of the k-space was filled in with zero
padding. Parallel imaging with a generalized autocalibrating partially
parallel acquisitions (GRAPPA) factor of 2 was used. The following scanning
parameters were used: TR/TE, 2.8/1.4; flip angle, 25°; readout, 512; voxel
size, 1.4 x 0.8 x 1.3 mm; field of view, 300 x 400 cm;
matrix size, 250 x 512. Breath-hold time for both the gated and ungated
groups was 20 seconds, and each group had two acquisitions: unenhanced and
contrast-enhanced. Gadolinium (0.2 mmol/kg) (gadopentetate dimeglumine,
[Magnevist, Berlex Laboratories]) was injected at 2 mL/s through a peripheral
IV cannula during image acquisition, and the contrast transit time was
calculated using a standard timing bolus acquisition. Digital subtraction of
3D data sets was used and there was automatic maximum-intensity-projection
(MIP) postprocessing. Examinations were viewed with PACS workstations
(PathSpeed 8.0, GE Healthcare).
Quantitative Analysis
All contrast-enhanced MRA images were subjected to quantitative analysis at
three different locations along the thoracic aorta
(Fig. 2). Quantitative analysis
was performed to determine the sharpness at various points along the thoracic
aorta to compare the images acquired with and without gating. The locations
chosen to represent and calculate the sharpness of the images were the
ascending thoracic aorta, apex of the arch of the aorta, and descending
thoracic aorta.
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Quantitative analysis was performed using the ImageJ software package, version 1.34 (National Institutes of Health). Sagittal source images (2D MRA images) were used for the quantitative analysis for each of the regions of interest. Once the images were obtained, the segment of the aorta that was of interest was magnified to 200% of the original image size. The source image was then rotated if necessary such that the region of interest was in a horizontal plane. Evaluation of vessel sharpness was based on a previously published technique [27]. A point spread or signal intensity profile was generated perpendicular to the major axis of the vessel using a field height of 3 pixels and a width that would cover the diameter of the vessel region of interest. A field height of 3 pixels signifies three consecutive signal intensity profiles of 1 pixel in height that are averaged to form one signal intensity profile for each patient. The purpose of using a field height of 3 pixels was to avoid any image artifacts that might disrupt the signal intensity profile, thus ensuring that the profile obtained was a true representation of the signal intensity profile. Signal intensity profiles were then normalized by the maximum value for pixel signal intensity value such that all values fell into the range 0.0-1.0.
For evaluation of the vessel sharpness, the maximum value for each side of the profile was noted. Using these values, the 20% maximal intensity, A, and 80% maximal intensity, B, were calculated. The signal intensity profile was then used to determine the slope of the line that represents the increase of signal intensity (Fig. 3). Once the slope of each side of the signal intensity profile was determined, the distances between A and B (d1 and d2) were calculated (in pixels) for each side of the profile, and the average value, d, was calculated [27]. Sharpness was calculated as the reciprocal of d, or 1 / d; thus, the higher the value of 1 / d, the better the sharpness of the image [27].
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As stated previously, additional levels of sharpness were superior in the entire study population for the gated scans despite not showing a statistically significant difference. There was an increase of 0.02 sharpness units (6%) at the level of the aortic arch and an increase of 0.04 sharpness units (13%) in the descending thoracic aorta; both increases in sharpness were greater than the SE. Although not statistically significant differences, these increases in sharpness do provide a noticeable increase in image sharpness and thus a certain level of significance with regard to the usefulness of contrast-enhanced MRA in diagnosis. The results of the quantitative analysis for the entire study population are summarized in Figure 5 and Table 1.
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As previously mentioned, an additional group of patients was identified who had undergone contrast-enhanced MRA both before the addition of the ECG gating and after the addition of the ECG gating. Again, similar results were obtained in the smaller subset of patients—that is, a statistically significant difference was found in the sharpness of the ascending aorta, and a difference was found in the other thoracic aorta segments that was not statistically significant but did provide a noticeable difference in image sharpness (Fig. 6). The gated images had a sharpness of 0.219 ± 0.021 and the images without gating, a sharpness value of 0.113 ± 0.018 (p < 0.005). There was an increase of 0.012 sharpness units at the level of the aortic arch and an increase of 0.018 sharpness units in the descending thoracic aorta. The most common aortic abnormality was aneurysm (30 patients) and, when compared with those without aneurysm, the images showed no difference in sharpness. Thus, there was an improvement in sharpness with gating irrespective of size. Results from the study of the patients receiving imaging both with and without ECG gating are summarized in Figure 6 and Table 1.
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MRA has emerged and advanced as an excellent noninvasive technique for evaluating the vasculature [28]. The technique is robust and accurate when used to evaluate the thorax; however, conventional MRA is sensitive to cardiac and respiratory motion artifacts [28]. Such artifacts could potentially lead to a misdiagnosis, such as an aortic aneurysm, that could lead to an unnecessary procedure with the risk of harm to the patient. Technical advances such as parallel imaging have allowed cardiac or ECG gating to offset the effects of cardiac motion [29].
Digital subtraction angiography is regarded as the gold standard for imaging of the vasculature. Digital subtraction angiography and CTA are high-resolution techniques that provide rapid images that are regarded as having high diagnostic value [30]. Even the smallest blood vessels can be imaged with high spatial resolution on digital subtraction angiography, as can the dynamic filling and draining of vessels [30]. CTA provides the ability to image large areas of the vasculature quickly and, with the advent of 64-MDCT scanners, can provide spatial resolution as low as 0.4 mm3 [6]. However, despite the speed and high resolution of these techniques, there are significant drawbacks that call for another imaging technique to replace them.
Digital subtraction angiography is invasive and presents a level of morbidity [30]. In addition, there is usually an admission to the hospital associated with the procedure, which adds to the already high cost. Digital subtraction angiography and CTA both use nephrotoxic iodinated contrast agents, the use of which could create problems that might be worse than the initially suspected ailment [30]. Both techniques also use ionizing radiation that can potentially be harmful [30].
With the advent of 64-MDCT scanners, the radiation exposure from CTA can be substantial, particularly when ECG gating is used [6, 30]. Exacerbating this issue are the particular implications for patients with aneurysmal disease, who require regular follow-up at 6- to 12-month intervals to determine the correct timing of surgical repair. The radiation doses would be cumulative in this group of patients, potentially resulting in hazardous consequences.
Repeated use of CTA or digital subtraction angiography could lead to serious complications in the kidneys or systemically from the large amount of radiation exposure to the patient. Contrast-enhanced MRA has emerged as a successful and accurate technique for the imaging of numerous blood vessels [27]. Contrast-enhanced MRA, because of the drawbacks associated with digital subtraction angiography and CTA, has become the initial imaging technique used for evaluation of numerous vascular conditions. T1-shortening, gadolinium, and rapid imaging with a 3D gradient-echo pulse sequence are used in contrast-enhanced MRA, which provides a safe contrast agent, no ionizing radiation, and rapid imaging times [27]. The combination of the speed of the technique, the accuracy of diagnosis, and the noninvasive nature of the technique shows that contrast-enhanced MRA is a powerful tool in the diagnosis of complications in the vasculature.
Cardiac or ECG gating of MRA images is a development that enhances the ability of MRA as the main diagnostic tool for the diagnosis of vascular disease. The previous use of contrast-enhanced MRA without ECG gating has shown low vessel sharpness and blurring due to cardiac motion artifact in the ascending thoracic aorta, which lowers the diagnostic power of the images. Parallel imaging strategies, in which the coil sensitivity profile is used to encode part of the spatial information, are used to increase acquisition speed, thus allowing the addition of gating [31, 32]. A significant amount of hardware is needed to implement contrast-enhanced MRA with cardiac gating; thus, it has not been a widely studied procedure. However, if the gating does provide a significant increase in vessel sharpness or an increase in the diagnostic quality of the images or both, then gating needs to be investigated to provide validation of this noninvasive technique that can provide a much safer alternative to digital subtraction angiography and CTA.
Thoracic aortic aneurysms are extremely serious and can lead to almost instant death if misdiagnosed. Therefore, it is important to determine with a high degree of accuracy whether the aneurysm reaches the threshold for surgery or if it simply requires monitoring. Low vessel sharpness could lead to the placement of an aneurysm in an improper category and thus place the patient at significant risk, either because unnecessary surgery is performed or because necessary surgery is not performed. Thus, the addition of ECG gating, with its potential to increase vessel sharpness and diagnostic quality, needs to be evaluated.
A quantitative analysis was performed to obtain an objective and easily comparable data set to determine if the addition of gating provides the postulated increase in vessel sharpness. The first analysis focused on two consecutive groups of 25 patients who had undergone contrast-enhanced MRA either before or after the addition of cardiac gating to the standard contrast-enhanced MRA protocol for aortic evaluation. This analysis showed that ECG gating did provide a significant increase in the sharpness of images of the ascending thoracic aorta. A highly significant increase in vessel sharpness at the level of the ascending thoracic aorta, in which the value more than doubled, provided an image with a highly significant increase in diagnostic value.
At the other levels that were quantitatively analyzed in the gated and ungated groups, an increase in the sharpness of the vessel was found that provided a noticeable increase in the sharpness of the image. Considering the high level of sharpness that was present in contrast-enhanced MRA at the aortic arch and descending thoracic aorta, any increase in sharpness brings the image to a point at which evaluation of aortic disease can be performed with great confidence. Vessel sharpness in the ascending aorta with the addition of gating comes close to reaching the sharpness of the descending thoracic aorta, where cardiac motion artifact should not play a major role in blurring. This fact shows that the gating almost eliminates the artifacts and may, with future technologic advances, mask the effects of motion artifact altogether.
Once the analysis of the two large patient groups had been completed, a subset of six patients was selected who had undergone both ECG-gated imaging and ungated imaging. These patients provided particularly convincing evidence for the role of ECG gating in the improvement of vessel sharpness because they had stable aneurysms or dissections and the imaging characteristics had not changed other than the addition of gating. Thus, on their second scan, the difference in sharpness should only be due to the addition of gating because the patients in each group were identical.
The images obtained for these patients underwent the same quantitative analysis and were then compared. Results obtained for this group mirrored the results of the previous analysis, reflecting a significant increase in sharpness in the ascending thoracic aorta and clinically significant increases in sharpness at the aortic arch and descending thoracic aorta. This subset of patients was important to confirm the results obtained with the earlier groups because by carrying out this analysis, individual factors that may have skewed the results were eliminated. Therefore, the increase in vessel sharpness can be more confidently attributed to the addition of ECG gating and not to any other factors.
Increasing the sharpness of the thoracic aorta, particularly the ascending thoracic aorta where the vessel sharpness was very low, was and is critical due to the need for contrast-enhanced MRA to be as diagnostically powerful as possible. With low vessel sharpness, an accurate assessment of the diameter of an aortic aneurysm is difficult because of the blurring of the wall. Determining the actual position of the wall is difficult and errors can easily be made. The consequences of such errors can be drastic, as mentioned previously; an untreated aneurysm because of underestimation of dilation could lead to serious complications, including death. In addition, an overestimation of the diameter of an aneurysm could lead to a patient being unnecessarily referred for surgery, which carries its own level of morbidity and mortality. Increasing the accuracy and confidence of diagnosis is the key goal of any change to an imaging technique, and the addition of ECG gating to contrast-enhanced MRA does just that. Showing that the gating technique drastically improves vessel sharpness makes it clear that the addition of gating is a necessary step in the improvement of contrast-enhanced MRA in imaging of the thoracic vasculature.
This analysis does have some limitations. Most noticeably, no comparison was made with other imaging techniques such as digital subtraction angiography. The reason for this is twofold. First, the institution at which this study was performed uses contrast-enhanced MRA as a first-line study for investigation of the thoracic vasculature because of the invasive nature and high cost of conventional angiography [30]. Second, numerous studies have compared MRA with digital subtraction angiography and other techniques, with MRA not proving to be significantly different [5]. Another limitation was the lack of a true paired comparison over the entire study population. Because this was a patient study, it was a rare occurrence for a patient to have been imaged by both techniques. However, the addition of the six patients who had undergone imaging by both techniques strengthens the study and somewhat dispels this weakness, as does the large size of both groups for the quantitative analysis. However, it would have been ideal if the entire population had been a truly paired comparison.
In conclusion, the addition of ECG gating to contrast-enhanced MRA of the thoracic aorta significantly improved the sharpness of the ascending aorta and improved the clinical value of the images at all levels of the thoracic aorta. The large groups that underwent quantitative analysis strengthen the study, as does the inclusion of a paired comparison in a subset of six patients. With the fast imaging time, quality of images, noninvasive nature of the imaging, and large anatomic coverage, contrast-enhanced MRA with ECG gating is ideal to investigate the thoracic vasculature for a large variety of suspected diseases. This technique also has the potential to grow and become more powerful and accurate. Thus, future studies assessing novel pulse sequence parameters with the addition of gating for the detection of vessel disease will be performed.
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