9 CT Coronary Angiography

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9

CT Coronary Angiography

Stephan Achenbach

are acquired with electrocardiographic (ECG) gating;

therefore, a regular sinus rhythm is necessary. Several studies have shown that a low heart rate during data acquisition – with consequently longer diastolic phases of relatively little coronary motion – substantially improves image quality [1–5]. Furthermore, when using ECG- correlated tube current modulation, full X-ray output is only activated during a ﬁxed time period in diastole, which reduces the radiation dose. A low heart rate with a longer cardiac cycle allows for greater reductions in radiation exposure than faster heart rates with a shorter cardiac cycle [6]. Even though it is possible to obtain high-quality images by MDCT in patients with faster heart rates, slower heart rates will thus improve image quality and reduce radiation exposure. Most experts advocate the use of beta-blocking agents in preparation for the scan in order to achieve heart rates of 60 beats per minute or less during inspiration. At our institution, we use a protocol that combines administration of 100 mg atenolol one hour before the scan to patients with heart rates above 60 beats/min, followed with intravenous injection of up to four doses of 5 mg metoprolol when the patient is on the scanner table until the heart rate is below 60 beats/min. This has proven to be a very safe and effective method in our institution.

For EBT, the overall scan time decreases with faster heart rates. Thus, it is favorable to avoid EBT scanning with low heart rates. Many authors suggest the use of intravenous atropine for patients with slow heart rates in order to decrease scan time, thus reducing the necessary amount of contrast agent and breath-hold duration.

Image quality for both EBT and MDCT is further improved through coronary vasodilation. Nitrates are rec- ommended immediately prior to the scan. At our institution, we administer 0.8 mg glyceryl trinitrate sublingually immediately before starting the scan. Other institutions have reported use of 0.4 mg.

Further preparations include placement of ECG leads (making sure contact is not lost during deep inspiration), and instructions to the patient about the importance of breath-holding and the sequence of the examination.

123 Computed tomography (CT) produces high-resolution

cross-sectional imaging. Injection of contrast agent raises the density in the blood pool well above that of the vessel wall and surrounding tissue. Thus, CT “angiography”

(CTA) can be performed. CT angiography of the coronary arteries poses some unique challenges. Given the small lumen of the coronary vessels (typically 1 to 4 mm) and their constant and rapid motion, CT angiography of the coronary artery lumen requires maximal spatial and temporal resolution. Sufficient temporal resolution was first achieved with electron beam tomography (EBT), with the first successful attempts at angiography of the coronary arteries in the mid 1990s. The recent development of multidetector CT scanners (MDCT) with faster imaging speed has allowed improved coronary artery visualization. This was initially performed with simultaneous acquisition of 4 slices. The technology then advanced to 8, 16, and now up to 64 narrowly collimated slices. This provides high spatial resolution and sufficient temporal resolution to permit adequate and reliable visualization of the coronary artery lumen. Comparison to invasive coronary angiography demonstrates increasing accuracy for stenosis detection.

This is expected to bring coronary CTA into the main- stream of cardiology practice.

Imaging Protocol

Preparation

Patient preparation is required to achieve optimal image quality in CT coronary angiography (Table 9.1). Data acquisition protocols have to be tailored to maximize spatial and temporal resolution. The patient must not have contraindications to the administration of iodinated contrast material. Patients must be able to hold their breath in inspiration for 8 to 20 seconds, depending on the CT scanner used (this can be up to 40 seconds for electron beam tomography). Image acquisition and reconstruction

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Image Acquisition

Image acquisition consists of three steps (Table 9.2). First, a projection image is used for localizing the position of the heart (Figure 9.1). It is important to prescribe a scan volume that is as small as possible, yet covers the entire heart. This assures complete visualization of the coronary vessels with the least possible contrast dose and radiation exposure. To prescribe the scan area, the tracheal bifurcation is frequently used as a reference point. However, we prefer to place the cranial border of the scan volume at the mid left pulmonary artery, which has a less variable rela- tionship with the position of the heart.

The second step of image acquisition is the determination of contrast agent transit time from injection into the peripheral vein to peak enhancement in the ascending aorta, using the smallest amount of contrast possible.

Determination of contrast agent transit time can be achieved either through bolus injection and acquisition of repeated low-dose images at the level of the ascending aorta (“test bolus”), or through “bolus tracking” algorithms that automatically start image acquisition when the arrival of contrast agent increases the CT density to a particular value within a predeﬁned region of interest in the ascending aorta [7]. We prefer “test bolus” measurement of the transit time for several reasons: it ensures correct placement of the intravenous line, conﬁrms appropriate starting level of the scan volume, and gives the patient one more opportunity to become familiar with the breath-hold commands.

The scan parameters used for acquisition of data to visu- alize the coronary arteries vary with scanner speciﬁcations.

Typical image acquisition protocols are listed in Table 9.3.

In any case, protocols should be optimized to maximize spatial and temporal resolution, and precautions should be taken to avoid excessive radiation exposure. Contrast agent is typically injected at a rate of 5 mL/s for the duration of the scan (approximately 18 seconds for 16-slice CT, 10 seconds for 64-slice CT). Using a saline bolus to follow the injection of contrast agent is very useful to keep the contrast bolus compact, achieve optimal enhancement in the coronary arteries, and avoid enhancement of right atrium and ventricle (Figure 9.2).

Image Reconstruction

In MDCT, images are reconstructed with retrospective ECG gating. Detailed explanation of reconstruction parameters is beyond the scope of this chapter. In brief, image reconstruction is usually performed in diastole (e.g. around 65%

of the cardiac cycle), but may need to be repeated at other periods if motion artifacts are present. A medium soft kernel is used to achieve good balance between sharpness and image noise. The ﬁeld of view should be chosen as small as possible to completely cover the volume of the heart. Reconstructed slice thickness equals collimation or is a little bit larger (e.g. reconstruction of 0.75 mm slice thickness for data acquired with 0.6 mm collimation). The reconstruction interval is less than the slice thickness.

Therefore, overlapping slices are created which again improve the image noise and facilitate image post- processing. The EBT scanner provides axial images with a

Table 9.1. Preparations for CT imaging of the coronary artery lumen Contraindications Non-sinus rhythm

Inability to follow breath-hold commands Contraindications for iodinated contrast Lower heart rate (MDCT) Target: heart rate <60 beats/min

e.g. atenolol 100 mg 1 hour before scan, metoprolol 5 mg, up to 4 doses (20 mg) immediately before scan Increase heart rate (EBT) Target: heart rate >70 beats/min

e.g. atropine Coronary vasodilation Sublingual nitrates

Table 9.2. Sequence of scanning for visualization of the coronary artery lumen 1. Localization of heart position Example: low-dose projectional anteroposterior

view of the chest

2. Determination of contrast agent Example: injection of test bolus followed by transit time repeated acquisition of low-dose images at the

level of the ascending aorta

3. High-resolution volume dataset With intravenous injection of contrast and scan delay according to contrast agent transit time (“Bolus tracking” techniques allow combination of steps 2 and 3 in one acquisition)

Figure 9.1. A low-dose frontal projectional image of the chest is usually acquired in order to pre- scribe the scan volume. We typically place the cranial start of the scan volume (white box) at the level of the mid left pulmonary artery (arrow).

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slice thickness equal to the collimation. A medium sharp reconstruction kernel is usually used.

Data Evaluation

The tortuous course, branching, and small diameters of the coronary arteries can make evaluation challenging and evaluation of the reconstructed axial images usually is not sufﬁcient. For evaluation, coronary CTA datasets are therefore routinely transferred to dedicated workstations.

These workstations permit interactive scrolling through the dataset, interactive rendering of oblique multiplanar reconstructions and maximum intensity projections, as well as three-dimensional (3-D) volume rendering. While two-dimensional methods of image post-processing and evaluation will usually be sufficient to evaluate the data, 3- D rendering can provide a useful overview of the anatomy and can effectively convey the findings. Interactive maneu- vering through the dataset has been proven to be more effective and accurate than evaluation of pre-rendered reconstructions [8]. Evaluation of the coronary arteries concerning the presence of stenoses will typically be limited to reporting absence of stenosis, presence of non- significant luminal narrowing, presence of a hemodynamically significant luminal narrowing (e.g. more than 50%

diameter reduction), or total occlusion. Accurate percent grading of the degree of stenosis is not possible to our current knowledge.

Figure 9.2. Axial image obtained by 64-slice MDCT showing optimal contrast enhancement: Full enhancement within the left ventricular cavity (LV) and coronary arteries (large arrow: right coronary artery, small arrow: left anterior descending coronary artery), but the contrast bolus has already passed the right ventricle, which does not show enhancement.

Table 9.3. Typical scan parameters for high-resolution imaging of the coronary arteries

Electron beam tomography Collimation 2¥ 1.5mm

Image acquisition time 100 ms

Table increment Stepwise, 3 mm after each cardiac cycle

Tube voltage 135 kV

Tube current 900 mAs

Scan length Approximately 120 mm

Scan duration Approximately 40 heartbeats (30 seconds)

Contrast agent 4 mL/s for the duration of the scan (e.g. 120 mL), followed by saline 50 mL at 4 mL/s

16-slice MDCT^a Collimation 16¥ 1.0mm

Rotation time 370 ms

Table increment Continuous, 3.0 mm/rotation

Tube voltage 120 kV

Tube current 500 mAs maximum, reduction in systole

Scan duration Approximately 18 seconds

64-slice CT^b Collimation 64¥ 0.6mm

Rotation time 330 ms

Table increment Continuous, 3.8 mm/rotation

Tube voltage 120 Kv

Tube current 700 mAs maximum, reduction in systole

Scan duration Approximately 10 seconds

aSuggested scan parameters for Siemens Sensation Cardiac, parameters vary for other scanners.

bSuggested scan parameters for Siemens Sensation 64, parameters vary for other scanners.

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Clinical Applications

Accuracy for Stenosis Detection

Development of the EBT scanner initially provided the necessary temporal resolution for visualization of the coronary arteries and stenosis detection (Figure 9.3). First comparisons to invasive coronary angiography in the mid 1990s demonstrated the feasibility of stenosis detection [9–20]. However, the limited spatial resolution and long scan time (requiring breath-hold up to 40 seconds) led to image artifacts. Thus, a substantial number of coronary arteries were unevaluable for detection of stenoses [9–20].

Also, the large amount of contrast agent that was necessary to achieve arterial enhancement throughout the long image acquisition interval, and the limited availability of EBT scanners, prevented widespread clinical applications.

The introduction of 4-slice multidetector spiral CT scanners around the year 2000 permitted further advances in coronary artery visualization. Again, comparisons to invasive angiography demonstrated high accuracy for stenosis detection in datasets with high image quality, but artifacts caused mainly by the limited temporal resolution were frequent and the breath-hold time and necessary amount of contrast were similar to electron beam tomography [21–32]. Substantial progress was made with the introduction of scanners with 16 slices starting approximately in the year 2002. These systems provided for data acquisition with collimations of 1.0 mm or less and gantry rotation times of 500 ms or less [33,34]. Sixteen-slice systems are now widely available and are currently regarded as a prerequisite for

CT coronary angiography (Figures 9.4–9.7). However, 16- slice technology has not been able to solve all the problems of the older scanners. Severe calcifications, for example, remain a problem because they can impair image quality and reduce diagnostic accuracy. In a comparison of patients with calcifications of varying degree, Kuettner et al. convincingly showed that both sensitivity and specificity for stenosis detection were 98% in a group of 46 patients with an “Agatston score” equivalent of less than 1000, while only 25 of 35 stenoses were detected correctly in patients with an “Agatston score” equivalent of 1000 or more (sensitivity of 71%) [35].

When comparing studies which evaluate the accuracy of cardiac computed tomography for the detection of coronary stenoses by means of validation against invasive coronary angiography, it has to be considered that the results of different investigations are not immediately comparable.

Next to differences in the scanner technology used, the studies differ widely in the selection of included patients and prevalence of signiﬁcant stenoses. Similarly, studies differ in the number and location of coronary segments that were included in the analysis. For example, some inves- tigators excluded coronary segments with reduced image quality from analysis while others did not. Regardless of these differences, the published studies uniformly reported high sensitivities for the detection of coronary artery stenoses, ranging from 82% to 95% (Table 9.4). Similarly, speciﬁcity for stenosis detection ranged from 86% to 98%

[33,34,36–41]. Of note, the negative predictive value was uniformly found to be 97% or higher, but it has to be considered that the low prevalence of signiﬁcant coronary

a b

Figure 9.3. a 3-D reconstruction of the heart and coronary arteries obtained by electron beam tomography in a patient with a stenosis of the left anterior descending coronary artery (large arrow) and occlusion of the right coronary artery (small arrow). b Invasive angiogram of the left anterior descending coronary artery. Corresponding high-grade stenosis in mid-vessel (black arrow).

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e f

c d

a

b

Figure 9.4. Two-dimensional multiplanar reconstructions of the coronary arteries obtained by 16-slice MDCT: left main and left anterior descending coronary artery (a), left circumﬂex coronary artery (b), right coronary artery (c).Three-dimensional visualization of the coronary arteries (d, e, f).

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a b

c

Figure 9.5. An 8 mm thick maximum intensity projection in a slightly oblique para-axial projection (a) and 3-D reconstruction (b) in a patient with left anterior descending coronary artery stenosis (arrow).

Corresponding coronary angiography (c).

a b

Figure 9.6. Stenosis of the right coronary artery (arrow, a – invasive coronary angiogram).Three consecutive axial images (each 1 mm slice thickness) show normal coronary artery lumen proximal to the stenosis (arrow, b), absence of a contrast-enhanced lumen at the level of the stenosis (arrow,

c), and re-establishment of a patent and normal coronary lumen distal to the stenosis (arrow, d).

Same stenosis in curved multiplanar reconstructions (arrows, e and f), and in 3-D volume-rendered reconstruction (g, h).

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c d

e f

g h

Figure 9.6. (Continued)

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stenoses inﬂuences the negative predictive value towards higher levels.

Even within the studies that were performed using 16- slice technology, it can be observed that faster rotation times and lower heart rates positively inﬂuence the diagnostic accuracy. Sixty-four-slice scanners with improved temporal and spatial resolution are expected to improve diagnostic accuracy while lowering the number of unevaluable segments (Figure 9.8), but comparisons to invasive angiography have not been published to date and this assumption remains to be proven in adequate clinical trials.

Clinical Implications

Coronary CT angiography does not have the same spatial or temporal resolution as invasive coronary angiography

(Figure 9.9) and cannot be performed in all patients (e.g.

those with arrhythmias). Furthermore, it is a purely diagnostic tool and does not provide the option for immediate interventional treatment. Thus, clinical application of CTA in patients with a high pretest likelihood for coronary artery disease is of limited value, such as in older individuals with typical chest pain. If the predicted necessity for an intervention is reasonably high, the patient should proceed directly to invasive angiography rather than CT.

Also, routine “screening” of asymptomatic individuals by coronary CT angiography will not be beneﬁcial, since treatment of an asymptomatic stenosis is generally not expected to alter the patient’s prognosis.

However, many patients are symptomatic with atypical chest pain and have positive or equivocal stress test results, so that invasive coronary angiography is deemed necessary to rule out the unlikely presence of stenoses. Such patients with low or intermediate pretest likelihood are, for

a b

Figure 9.7. a Multiplanar reconstruction showing an eccentric stenosis of the left main coronary artery (small arrow) and of the left anterior descending coronary artery (large arrow). b Corresponding lesions in invasive angiography. In addition, a left circumﬂex stenosis is present (not shown as an MDCT image).

Table 9.4. Diagnostic accuracy of 16-slice MDCT for the detection of coronary artery stenoses

Reference Number of Patients Gantry rotation time Sensitivity (%) Speciﬁcity (%) Non-Interpretable (%) Analysis

Nieman et al. [33] 59 420 ms 95 86 7% Per-artery analysis, all segments >2.0mm

Ropers et al. [34] 77 420 ms 93 92 12% Per-artery analysis, all segments >1.5mm

Mollet et al. [36] 128 420 ms 92 95 – Per-segment analysis, all segments >2.0mm

Martuscelli et al. [37] 64 500 ms 89 98 16% Per-artery analysis, all segments >1.5mm

Hoffmann et al. [38] 33 420 ms 63 96 – Per-segment analysis, all segments

33 420 ms 89 95 – Per-segment analysis, prox. and mid segments

Kuettner et al. [39] 72 375 ms 82 98 7% Per-segment analysis all segments

Mollet et al. [40] 51 375 ms 95 98 – Per-artery analysis, all segments >2.0mm

Schujif et al. [41] 51 400 ms 98 97 6% Per-segment analysis, all segments^a

aThis study includes some patients with bypass grafts and stents which were not included in this evaluation.

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example, younger men and women with atypical chest pain [42]. In these patient groups, non-invasive coronary visualization by CT potentially can be a beneﬁcial tool for further evaluation since it can reliably exclude signiﬁcant

coronary stenoses and avoid the need for invasive angiography. However, appropriately designed clinical studies will have to verify this hypothesis and identify patient groups that will have clinical beneﬁt from coronary CT angiography to rule out the presence of coronary artery stenoses.

CT angiography also has the potential to provide information that is complementary to invasive angiography.

One smaller study has shown that in patients with chronic total occlusion, a challenging subset of patients referred for interventional treatment, occlusion length and degree of calciﬁcation as assessed by CT are more accurate pre- dictors of interventional success than are angiographic parameters [43]. Similarly, it is conceivable that CT angiography may provide information that is valuable prior to interventional treatment of bifurcation lesions or other challenging subsets of coronary stenoses by evaluating plaque burden, extent of calciﬁcation, and by viewing the 3-D anatomy of the vessels. These assumptions, however, need to be proven and would only apply to a relatively small number of patients.

Because of the 3-D nature of the tomographic dataset, CT coronary angiography is the preferred modality to investigate patients with known or suspected congenital coronary artery anomalies (Figure 9.10). Several authors have convincingly shown that MDCT can identify both the origin and the often complex course of anomalous coronary vessels [44–46].

In a number of smaller studies, it has been shown that CT and EBT angiography can provide clinically relevant information in patients with Kawasaki syndrome by demonstration of calciﬁcation, coronary stenoses, and coronary aneurysms [47–49].

Figure 9.8. A 2-D curved multiplanar reconstruction of the right coronary artery (arrows) obtained by 64-slice MDCT, with a rotation time of 330 ms, and 64 ¥ 0.6mm collimation.

a b

Figure 9.9. Typical artifacts which may impair evaluability of coronary artery segments concerning the detection of stenoses. a Motion artifact of the right coronary artery causes a “blurry” appearance of the vessel cross-section (arrow). b Severe calciﬁcation in the proximal left anterior descending coronary artery. Both images acquired by 16-slice MDCT.

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Summary

Non-invasive CT angiography of the coronary arteries is a challenging and rapidly advancing technique. Electron beam tomography initially permitted the non-invasive visualization of coronary arteries and detection of stenoses. Multidetector CT provides very high spatial resolution and allows for excellent visualization of the coronary arteries. Hemodynamically significant stenoses can be detected and excluded with high accuracy. Current limitations of CT angiography include gantry rotation speed, arrhythmias, and the presence of severe coronary calcification. Continued technical progress is expected to overcome some of these limitations. The most likely beneficial clinical application for the use of CT coronary angiography will be to rule out the presence of relevant coronary artery stenoses in symptomatic patients with a low-to-intermediate likelihood of coronary artery stenoses. Sufficiently large outcome studies will ultimately have to define the most beneficial applications of CT coronary angiography and compare CTA to traditional diagnostic methods.

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