quality requirements for a noninvasive coronary angiogram may be less stringent than those for invasive coronary angiography.
Noninvasive coronary angiography using electron beam tomography was initiated in 19951 and since then many studies using this technique have been published.2–10 Multislice computed tomography (MS-CT) is a rapidly emerging technique, which permits noninvasive visualiza- tion of the coronary arteries.11–17 This chapter provides the basics of MS-CT technology, gives an update on the current diagnostic perform- ance of MS-CT coronary angiography, and dis- cusses the potential role of MS-CT in clinical cardiology.
1. Basics of Computed Tomography
Computed tomography (CT) imaging is a cross- sectional imaging method using diagnostic X- ray radiation as the penetrating radiation source to delineate the anatomy of a slice or a section of a patient (tomogram). The cross-sectional image is reconstructed from data obtained from the specific linear X-ray attenuation of the penetrat- ing X-ray beam. Tissues of different structures within the body have specific X-ray attenuation qualities depending on the atomic density of each tissue.
The basic design of a CT scanner is illustrated in Figure 6b.1. A rotating X-ray source produces an X-ray beam that passes through the patient.
1. Basics of Computed Tomography . . . 99
2. Spiral CT . . . 101
3. Multislice CT: Speed of Performance . . . 102
4. MS-CT Coronary Scan Technique . . . 102
5. Heart Rate and MS-CT Coronary Angiography . . . 102
6. Tissue Contrast and MS-CT Coronary Angiography . . . 102
7. Diagnostic Performance of 64-Slice MS-CT Scanner . . . 102
8. Limitations of MS-CT Coronary Angiography . . . 104
9. Clinical Implementation of MS-CT Coronary Angiography . . . 104
10. Conclusion . . . 105
Invasive coronary angiography is at present the only universally accepted, widely available method for coronary artery imaging. It is highly reliable to assess the presence of coronary obstructions, but its main limitation is related to the invasive nature of the technique, requiring a short monitored hospitalization and a dedicated team consisting of a technician and cardiologist to perform the investigation, thereby making it costly. The procedure also has a small (occa- sionally fatal) complication rate.
A noninvasive imaging technique would be highly desirable, but to become an alternative to diagnostic invasive coronary angiography, this technique should have sufficiently high quality that it can approximate the versatility and relia- bility of traditional coronary angiography. This is almost impossible to achieve, but the high-
6
Computed Tomography Techniques and Principles.
Part b. Multislice Computed Tomography
P.J. de Feyter, F. Cademartiri, N.R. Mollet, and K. Nieman
99
The X-ray beam is collimated as a fan beam and the width of the collimators controls the thick- ness of the cross-section image. The fan beam is subtended by an array of detectors at the oppo- site side of the X-ray source. The detectors measure the attenuation from the projection of the X-ray beam when passing through the anatomy within the slice.
An exact cross-sectional image can be con- structed from the attenuation measurements obtained from a series of projections covering the full 360° round the patient. The Hounsfield unit (HU) or CT number is the attenuation value of various tissues related to water which is cali- brated to zero (HU = 0). The various tissues in the body have different attenuation values (Table 6b.1). In CT imaging, tissues with a very high attenuation (such as bone) are represented on the display image as white, tissues with very low
attenuation values (such as air in the lungs) are represented as dark, and tissues with moderate attenuation values are represented by various shades of gray (Figure 6b.2).
Fan Beam Projection Measurement X-Ray Source
Object Slice
Scan Field
X-Ray Detector Array (500-1000 Channels)
Filtered Backprojection Reconstruction
Parallel Beam Projections Projection Angles
Projections 500-1000 Channels
500-1000 Projections
Covering Views in 180°
Angle Interval
Filtered Backprojection
Resonstructed Scan Field
Image Matrix 512¥512 Pixels
a b
c
Figure 6b.1. Basic design of the CT scanner. Schematic diagram of CT data acquisition and image reconstruction. A A fan-shaped X-ray beam passes through the cross-section of the patient, located within the scan’s field of view. The patient’s mor- phology attenuates these X-rays after which the residual rays are registered by a detec- tor array on the opposite side.The electrical intensity signals from the detector are con- verted by analog digital converters into digital intensity signals, which are trans- formed by computer into an attenuation projection profile.Each angular projection or attenuation profile consists of typically 500–1000 individual data points that belong to individual ray projections. A large number of attenuation profiles, from con- secutive angular fan beam projections are generated and serve as basis for computa- tion of the X-ray attenuation distribution of the patient’s cross-sectional morphology that represents the image slice. B For most reconstruction algorithms, the measured fan beam projections are rearranged to par- allel beam projections via two-dimensional interpolation algorithms. To reconstruct a slice,the minimally required amount of data is acquired during a half rotation of the X- ray source and detector array,which consists of 500–1000 parallel beam projections. C The final computation of the slice is per- formed via a so-called backprojection of the filtered parallel beam projections.
Table 6b.1. Attenuation values of various tissues in the body A. Structures with high density (HU range:>150)
Metal objects Bones
Iodinated contrast in blood
B. Structures with mid-density (HU range:<150 to -50) Fibrotic tissue
Water Soft tissues
C. Structures with low density (HU range:-50 to -1000) Fat tissue
Air
2. Spiral CT
The basic concept of spiral CT is that the data necessary for the reconstruction of a cross- sectional image are acquired as the X-ray source rotates around the patient while the table moves continuously with a predefined speed. These two motions create a spiral volume of data. From this spiral volume of data, cross-sectional images are reconstructed using helical interpolation algo- rithms (Figure 6b.3).
Figure 6b.2. Cross-sectional image of thorax.White = sternum and spine; dark
= lung tissue; gray = myocardium.
Figure 6b.3. Multislice spiral CT data acquisition and image reconstruction.
Rotating around the patient are 12–16 parallel detector rows with a submillime- ter slice width each. While the patient continuously moves through the scanner, the tube current is altered according to the ECG. During systole, the radiation output is lowered to 20%, whereas, during diastole, triggered at the estimated 50% RR interval time point, the nominal exposure (100%) is used during a 400-ms interval to acquire high-quality images. This way, the total dose can be reduced by 50%. From the acquired scan data, ECG synchronized cross-sectional image series are reconstructed with an overlapping increment via spiral interpo- lation algorithms to improve the volumetric quality of the data set. Retrospective ECG gating allows reconstruction timing related to the upcoming R wave or selec- tive repositioning of reconstruction intervals during irregular RR intervals. By varying the position of the 210-ms reconstruction window within the 400-ms interval of nominal tube current,image data sets are retrospectively reconstructed during slightly different phases (at 350, 400, and 450 ms before the following R wave), and the most optimal result is selected for further analysis. The standard length of the reconstruction interval and temporal resolution at 0.42-s rotation time is 210 ms. Depending on the heart rate, the effective reconstruction interval can be between 105 and 210 ms by using “segmented”reconstruction algorithms, that combine data from two consecutive heart beats.
3. Multislice CT: Speed of Performance
Cardiac imaging requires fast acquisition of data, to reduce the inherent cardiac motion arti- facts. The speed of MS-CT is mainly determined by the number of detector rings and the X-ray tube rotation time. The higher the number of detection rings and the faster the X-ray tube rotation, the shorter the scan time, resulting in less artifacts. MS-CT is the recent technical improvement of spiral CT. The latest generation features 64 detector arrays, which is extremely important for coronary imaging because it con- siderably reduces the scan time. For instance, a 1-detector ring CT would require more than 160 seconds’ scan time to cover the heart, whereas this is reduced to 40 seconds with a 4-detector ring MS-CT and to 5 seconds with the 64- detector ring MS-CT. The use of the latter tech- nique would translate into the acquisition of a MS-CT coronary angiogram within a breathhold of just a few seconds, which can be achieved in almost all patients.
The X-ray tube rotation time, which deter- mines the speed of acquisition of a cross-section, is a significant parameter for coronary imaging.
The faster the rotation, the shorter the acquisi- tion time and thus the shorter the temporal resolution, which would result in better
“motion-free” images. Technical improvements over time have significantly reduced the X-ray tube rotation time, decreasing it from 1000 ms in the early 1990s to 420 ms in 2002 to 375 ms in 2003 and 330 ms in 2004.
4. MS-CT Coronary Scan Technique
In spiral CT scanning, the data are acquired during the entire cardiac cycle. To obtain motion-free coronary images, only the data acquired in the relative motion-free diastolic phase of the cardiac cycle are used for the recon- struction of the axial cross-sections.18–21This is achieved by prospective or retrospective electro- cardiogram (ECG) gating (Figure 6b.3), although for coronary angiography, the latter is preferable.
The R wave of the ECG is used to retrospectively
“gate” the scan so that only the data acquired during a predefined window in the diastolic cardiac phase are selected for reconstruction of the cross-sectional image.18–21
5. Heart Rate and MS-CT Coronary Angiography
The relative motion-free period in the diastolic phase of the cardiac cycle is shorter during faster heart rates, which causes blurring of the coronary image at moderately fast heart rates (±70–80 bpm) and which often precludes reconstruction of high-quality images with heart rates higher than 80 bpm.17,22 Heart rate control by using beta-blockers before MS-CT scanning to reduce the heart rate to less than 70 bpm is a key prerequisite to produce high- quality MS-CT coronary images. New bi- segmental reconstruction algorithms using data from two or more cardiac cycles may reduce the “effective” temporal resolution to less than 100 ms.
6. Tissue Contrast and MS-CT Coronary Angiography
The attenuation values of the coronary lumen filled with blood, the coronary wall, or coronary atherosclerotic plaques have only small differences and are therefore hardly visible with MS-CT. To enhance the tissue contrast difference, an X-ray contrast intravenous injec- tion of 80–100 mL is administered, which allows differentiating the coronary lumen from the coronary wall and atherosclerotic plaques and permits detection of coronary lumen obstruc- tions and nonobstructive coronary plaques (Figure 6b.4).
7. Diagnostic Performance of 64-Slice MS-CT Scanner
The highest diagnostic performance is nowadays achieved with the 64-detector MS-CT scanner in combination with heart rate control using beta- blockers before scanning in case of heart rates higher than 70 bpm. The better results are obtained because of the advantages offered by the new scanner including a faster X-ray tube rotation time of 330 ms and extended number (32–64) of thinner detector rows (0.75 mm) resulting in a shorter total scan time of approx- imately 6–12 seconds (Figure 6b.5).
The accuracy of MS-CT for noninvasive detec- tion of coronary stenosis is an area of active research. Most published series have reported sensitivity values for detection of significant stenosis in proximal segments in the region of 85%–95%. Four recent studies have compared conventional coronary angiography with 64- slice MS-CT coronary angiography (Table 6b.2).23,24 Raff et al. excluded 12% of the coronary segments from evaluation because of insufficient image quality, whereas the other investigators did not exclude coronary segments from evaluation. The sensitivity, specificity, and predictive accuracy were high in both studies.
Compared with other branches of the coronary tree, the right coronary artery with its high degree of motion was still most vulnerable to image-quality degradation.
Figure 6b.4. Left anterior coronary artery (LAD) without contrast – with contrast. cMPR, curved multiplanar reconstruction.
Figure 6b.5. MS-CT and invasive coronary angiography. The arrow indicates a significant obstructive coronary lesion located at the proximal LAD. CCA, conven- tional coronary angiography; MSCT-CA, multislice CT coronary angiography; D1, first diagonal branch; D2, second diagonal branch; LAD, left anterior descending coronary artery; arrow, left anterior descending coronary artery (LAD) with and without contrast.
Table 6b.2. Diagnostic performance of 64-slice MS-CT to detect a significant coronary stenosis (50% diameter)
Excluded vessel Sensitivity Specificity Positive predictive Negative predictive
NP segments (%) (%) (%) value (%) value (%)
Leschka23 67 0 94 95 88 98
Leber24 55 0 76 97 75 97
Raff25 70 12 86 95 66 99
Mollet26 51 0 99 95 76 99
8. Limitations of MS-CT Coronary Angiography
As indicated in the previous section, fast heart rates adversely affect the image quality and consequently the diagnostic reliability of the images. Reduction of the heart rate with beta- blockers before the scan will resolve the problem in the majority of the cases but, unfortunately, it is not possible in all cases, particularly in patients with diabetes mellitus or with high anxiety levels, to sufficiently reduce the heart rate.
Persistent heart rhythm disturbances such as atrial fibrillation preclude MS-CT coronary angiography because of inappropriate electro- cardiogram (ECG) synchronization resulting in interslice discontinuity, and differences in the volume of the ventricles caused by differ- ences in diastolic filling times with attendant differences in three-dimensional localization within the thorax are an additional contributing factor. An occasional heart rhythm abnormality (such as an atrial or ventricular extra systole) can be corrected by retrospective editing of the ECG.
Calcium deposits cause a strong attenuation of the X-ray, causing high-density artifacts by beam hardening and partial volume effect. The conse- quence is that calcified plaques appear larger than they actually are, thereby increasing the apparent severity of a narrowing. In addition, severe calcification of the coronary wall obscures the coronary lumen and causes misinterpreta- tion of the presence and severity of a coronary stenosis.
The X-ray exposure during MS-CT coronary angiography using a retrospective image recon- struction protocol is in the region of 10 mSv per investigation.27–29 This high dose of radia- tion limits repeated MS-CT investigations, in particular in younger patients, and technical improvements are required to reduce the radiation exposure. One way to reduce the radiation levels is to use ECG controlled X-ray tube current modulation.27The radiation dose is estimated to be reduced by approximately 40% by using an R wave-triggered modulation X-ray by lowering the radiation dose during systole (data not used for reconstruction) and giving full-dose X-ray radiation only during diastole.
9. Clinical Implementation of MS-CT Coronary Angiography
The advantages and disadvantages of invasive and noninvasive coronary angiography are listed in Table 6b.3. It seems unlikely that MS-CT coro- nary angiography will be equal to the versatility and high resolution of diagnostic invasive coro- nary angiography, which not only is unsurpassed to detect and quantify coronary stenosis (quan- titative coronary angiography) but also serves as an invaluable road map for catheter-based coro- nary diagnostic tools and treatment. Yet, MS-CT coronary angiography may be an acceptable alternative, now or in the foreseeable future, in a number of potential applications (Table 6b.4).
The main strength of MS-CT is its noninvasive
Table 6b.3. Diagnostic coronary angiography and MS-CT Diagnostic coronary angiography MS-CT Procedure Cardiologist/nurse/ Technician
technician
Reading Cardiologist Cardiologist/radiologist
Costs High Low
Procedural time 30 min 10 min
Post-processing time 0 20 min
Throughput Low High
Radiation exposure 6–8 mSv 8–10 mSv
Resolution High Acceptable
Collaterals High Minimal
Obstruction QCA Visual
Nonobstructive plaque – +
Calcified plaque – ++
Motion artifacts None Unacceptable at high HR
Availability Worldwide Limited
QCA, quantitative coronary angiography; HR, heart rate.
Table 6b.4. Potential applications of MS-CT coronary angiography 1. Evaluation of chest pain in patients with a typical chest pain and an
equivocal stress test 2. Evaluation of coronary anomalies
3. Alternative to invasive coronary angiography preceding percutaneous coronary intervention in hospitals without percutaneous coronary intervention (PCI) facilities
4. Early detection of atherosclerosis in high-risk individuals 5. Evaluation of obstructive coronary atherosclerosis in asymptomatic
patients with known coronary artery disease
6. Coronary risk evaluation in patients undergoing major noncardiac surgery
7. Evaluation of coronary artery disease in nonischemic patients scheduled for aortic or mitral valve operations
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14. Knez A, Becker CR, Leber A, et al. Usefulness of multislice spiral computed tomography angiography for determination of coronary artery stenoses. Am J Cardiol 2001;88(10):1191–1194.
15. Vogl TJ, Abolmaali ND, Diebold T, et al. Techniques for the detection of coronary atherosclerosis: multi- detector row CT coronary angiography. Radiology 2002;223:212–220.
16. Kopp AF, Schroeder S, Kuettner A, et al. Non-invasive coronary angiography with high resolution multidetec- tor-row computed tomography. Results in 102 patients.
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18. Mao S, Lu B, Oudiz RJ, et al. Coronary artery motion in electron beam tomography. J Comput Assist Tomogr 2000;24:253–258.
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nature, which may permit assessing early detec- tion of nonobstructive or obstructive athero- sclerotic plaques in asymptomatic individuals with multiple risk factors. (For a calcium score assessment and a comparison between multi- slice CT and electron beam CT, see also Chapter 7.)
10. Conclusion
The ability of CT imaging to visualize and measure disease process in the coronary arteries has increased our knowledge of atherosclerosis and coronary heart disease. Beyond feasibility studies, further well-designed, prospective single or multicenter studies are required to assess the diagnostic performance of MS-CT in various patient populations with different levels of prevalence of coronary artery disease and to establish the diagnostic role of MS-CT in cardi- ology before we embrace this promising tech- nique as a clinically acceptable new diagnostic tool. Moreover, the fundamental characteristics of MS-CT such as the X-Y spatial resolution, slice thickness, and temporal resolution need to be further optimized to consider MS-CT coronary angiography as a reliable clinical diagnostic tool to detect coronary atherosclerotic obstructions.
In particular, the temporal resolution needs improvement to meet the challenges of motion- free imaging also during faster heart rates.
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