British Journal of Radiology (2006) 79, 9-16
© 2006 British Institute of Radiology
doi: 10.1259/bjr/67045628
President's conference papers |
Cardiac applications of multislice computed tomography
A de Roos, MD1,
L J M Kroft, MD1,
J J Bax, MD2,
H J Lamb, MD1 and
J Geleijns, PhD1
Departments of 1 Radiology and 2 Cardiology, Leiden University Medical Center, Albinusdreef 2, 2333 ZA Leiden, The Netherlands
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Abstract
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Introduction
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Multislice CT (MSCT) is gaining clinical acceptance for cardiac imaging owing to improved temporal and spatial resolution of the latest 16-slice and 64-slice technology. Although the cardiac MSCT applications are promising, there is still room for further technical improvements and optimization of post-processing techniques for cardiac evaluation.
Interestingly, the data acquired for CT angiography of the coronary arteries can also be used to create volumetric cine loops of cardiac function. The functional data are available without the need for repeat scanning or for administration of additional contrast material [1]. Furthermore, MSCT allows assessment of first-pass perfusion and delayed enhancement imaging in patients with subacute myocardial infarction. Recently, it has been reported that MSCT reveals microvascular obstruction or the so-called no-reflow phenomenon as a late perfusion defect in patients with re-perfused acute infarctions, similar to observations made by other techniques like MRI [2]. With further development MSCT may allow combined assessment of the presence and extent of coronary atherosclerosis, the percent diameter stenosis, plaque characterization and the effect of the lesion on perfusion and myocardial function.
In this review, the technical requirements of cardiac MSCT and some frequent clinical applications are discussed.
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MSCT imaging requirements
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Requirements for cardiac MSCT image acquisition depend strongly on the clinical problem. For example, CT coronary angiography requires excellent spatial and temporal resolution, whereas only modest spatial and temporal resolution is sufficient for the assessment of the anatomy of pulmonary veins and the left atrium. In general, the higher the requirements for image quality become, the more complex the acquisition, the longer scan time and the higher patient dose. Main aspects with regard to imaging performance are low-contrast and spatial resolution, temporal resolution, and scan time. Patient dose and radiation risk should always be considered as the counterpart of image acquisition and image quality.
Low-contrast resolution and spatial resolution
Low-contrast resolution is the ability to visualize structures that demonstrate only a small difference in Hounsfield units compared with their direct environment. In cardiac applications of CT, native tissue contrasts are in general not sufficient to differentiate between, for example, the vessel wall and its unenhanced lumen, or the heart and the inner chambers. Contrast enhancement is thus mandatory for visualizing the lumen of coronary arteries, the heart chambers, pathology of the myocardium or anatomy of pulmonary veins. Low-contrast resolution depends on tube current (mA), the reconstructed slice thickness, tube voltage, beam filtration and the reconstruction algorithm, and is strongly correlated to radiation exposure. In general, low-contrast resolution performance of CT scanners is not a limitation for the application of cardiac CT.
Spatial resolution, or high-contrast resolution, determines the ability to visualize contours of small structures within the scanned volume. Small objects can only be resolved when there is a rather large contrast with the direct environment. Considerable improvement of spatial resolution in clinical acquisitions was achieved with the latest generations of multislice CT scanners. This is of importance, particularly for the application of CT coronary calcification scoring and CT coronary angiography. The actual diameters of the lumen of normal coronary artery segments range from 5 mm in the proximal segments to less than 1 mm in the distal segments [3]. This means that spatial resolution of 1.0 mm in all three dimensions should be sufficient for imaging of the coronary arteries, except for distal segments that would require a spatial resolution of at least 0.5 mm. Bypass graft diameter typically ranges from 4 mm to 6 mm. A spatial resolution of 2 mm3 (voxel size) might thus be sufficient for imaging the lumen of bypass grafts. For imaging of small structures within the coronary arteries, such as atherosclerotic plaque and stents, excellent spatial resolution, even better than 0.5 mm3, might be required. Voxel size is often used as an indicator of spatial resolution. However, voxel size should be interpreted with care since smaller voxel size does not necessarily imply better spatial resolution. Spatial resolution is preferably expressed as the response of a delta-function; in CT, this response is either called a point-spread-function (spatial resolution in the axial plane) or a slice sensitivity profile (spatial resolution along the z-axis). Spatial resolution is limited by the acquisition geometry of the CT scanner, the reconstruction algorithm and the reconstructed slice thickness. The performance of current 64-slice scanners with regard to spatial resolution, expressed as the full-width half-maximum of the response of a delta-function, is within the range 0.6–1.0 mm in all three dimensions.
Temporal resolution
Temporal resolution determines whether fast moving objects can be resolved in the CT image. Good temporal resolution limits motion artefacts and blurring of the image. Principally, good temporal resolution can be achieved by a short reconstruction window providing snap shots of the beating heart and coronary arteries. Good temporal resolution in cardiac CT is realised by fast data acquisition (fast rotation of the X-ray tube), but even more importantly by a dedicated reconstruction algorithm.
A recent paper [4] provides information on the rest period of the heart, which is a measure for the required reconstruction window. The rest period is defined as the time during which the 3D motion of a coronary artery is less than 1 mm. It was reported that, for patients with a heart rate of 64±9 beats per minute (BPM), the end-systolic rest period duration was 76±34 ms; and the mid-diastolic rest period duration was 65±42 ms for the proximal to middle segments of the right coronary artery. For the left coronary artery tree, the end-systolic rest period duration was 80±25 ms; the mid-diastolic rest period duration 112±42 ms. From these data it is concluded that the duration of a "snap shot" of the coronary arteries, or in other words the reconstruction window, should be shorter than 65–110 ms. This is in good agreement with earlier papers; in one paper it is suggested that the reconstruction window should be lower than 100 ms for coronary angiography in mid-diastole at 62±10 BPM [5], and in another paper it is stated that a 100 ms reconstruction window is relatively optimal for most patients at heart rates up to 90 BPM [6]. All of these considerations assume image reconstruction at the cardiac phase point that is associated with least motion, e.g. a reconstruction window starting between 60% and 80% of the interval between two consecutive R-waves. More strict criteria for the reconstruction window apply if the heart should be assessed at more than one cardiac phase point, including those that are associated with rapid movement of the heart wall, e.g. for studying the dynamics of the myocardium. More strict criteria apply as well when a 1 mm displacement of a coronary artery within the duration of the snap shot becomes unacceptable. This may happen, for example, when imaging small distal parts of the coronary arteries, quantifying coronary stenoses and assessment of coronary atherosclerotic plaque.
General reconstruction algorithms that are used for general CT applications provide, in principle, a temporal resolution equal to the rotation time (360° rotation, full reconstruction), the best achievable temporal resolution with general reconstruction algorithms is slightly longer than 50% of the rotation time (180° rotation, half reconstruction). Current 64-slice scanners that are used for cardiac applications provide a rotation time of 330–400 ms. These typical rotation times are not short enough for achieving a 100 ms or shorter snap-shot of the heart, even if a 180° rotation half-reconstruction is applied. Therefore, dedicated reconstruction algorithms are used in cardiac CT that allow for reconstruction of synchronized images from transmission data acquired during two or more successive heart cycles according to a method described already in 1977 [7]. These so-called segmented (multicycle) reconstruction algorithms allow for merging synchronized transmission data from successive heart cycles. The more heart cycles that can be included in the reconstruction, the better the temporal resolution. A low pitch factor, which is typical for cardiac CT acquisition, is required to acquire data from more than one heart cycle. A pitch factor as low as 0.2 is required to record at least two heart cycles and to achieve a temporal resolution in the order of magnitude of 100 ms for typical heart rates between 60–80 BPM. Figure 1
shows, as an example, the temporal resolution that is achievable with a reconstruction algorithm that can merge transmission data from an unlimited number of heart cycles. The figure illustrates the dependence of the reconstruction window on rotation time and heart rate and was calculated for a pitch factor of 0.2. From Figure 1 it can be concluded that, for achieving the shortest reconstruction window, rotation time should be adapted to the heart rate.
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Figure 1. Temporal resolution of CT coronary angiography. The temporal resolution depends strongly on the rotation time and the reconstruction algorithm. In segmented (multiphase) reconstructions, temporal resolution depends also strongly on the pitch factor. The lower the pitch factor, the more cardiac phases are captured during the acquisition and the better temporal resolution. The graphs are calculated for a pitch factor of 0.2. The graphs clearly show the dependence of temporal resolution on heart rate and rotation time.
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Scan time
Scan time is the time interval between the start and the end of one acquisition, sometimes referred to as a sequence. To avoid breathing artefacts and to limit the amount of contrast material in contrast enhanced acquisitions, scan time in cardiac CT should remain at least below 30 s, but preferably below 20 s. The extent of the target volume, as well as acquisition parameters such as rotation time, pitch factor, section thickness and number of simultaneously acquired sections, define scan time. In general a much shorter scan time than 20 s can now be realised with the current generation of 64-slice scanners for typical cardiac CT examinations; for example, a typical scan time for calcium scoring is 2.5 s, for coronary angiography 10 s and for an ungated acquisition of the pulmonary veins 3.0 s.
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Patient dose in MSCT
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Radiation protection of patients is based on justification and optimization. Justification implies that the benefit for the patient outweighs the risk of radiation exposure. Patient dose assessment is required for balancing harm and benefit of the CT examination and to assess the effect of measures for optimization of cardiac CT. Nowadays, most CT scanners provide the user with an indication of patient dose in the form of the CT dose index (CTDI) and dose–length product (DLP). Effective dose can be derived from these dose quantities. Effective dose from cardiac CT coronary angiography is relatively high, mainly due to the need to catch more than one cardiac cycle and the resulting low pitch factor. On the other hand, effective dose from an ungated acquisition, such as in ungated pulmonary vein CT angiography, is relatively low due to the high pitch factor. Effective dose for calcium scoring, assessment of ventricle function or pulmonary veins is in the range 1–3 mSv, effective dose for CT coronary angiography is considerably higher, e.g. in the range 10–15 mSv. Concern about radiation exposure stimulates the development of methods for dose reduction in cardiac CT coronary angiography. The field of view of interest in cardiac CT is rather small and therefore radiation exposure of tissue outside this field of view can be limited by means of a special "small field" beam-shaping filter. Another method for dose reduction is to reduce X-ray output during the systolic phases that are expected to be of less interest for the evaluation of the coronary arteries (ECG triggered modulation of dose). Pitfalls of small field scanning are the occurrence of artefacts and reduced image quality. A pitfall of tube modulation is reduced image quality at certain relevant phases of cardiac cycle, e.g. due to an irregular heart rate.
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Clinical applications
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MSCT provides special opportunities for cardiovascular CT in addition to angiography of the coronary arteries and coronary bypass grafts. These options include assessment of left ventricular (LV) and right ventricular (RV) function, coronary calcification score, myocardial infarction imaging and assessment of the anatomy of pulmonary veins in patients with atrial fibrillation. Each of these applications can be characterized by their specific techniques for acquisition and reconstruction. Table 1 provides information about typical acquisition and reconstruction parameters for some clinically established cardiac CT applications.
Quantitative assessment of coronary artery calcification
Coronary artery calcification is a marker for atherosclerotic lesions in the coronary arteries. The amount of coronary artery calcification is correlated to the risk of coronary events. However absence of coronary artery calcification does not rule out atherosclerosis. Applications of quantitative assessment of coronary artery calcification are screening of asymptomatic individuals with risk factors for coronary artery disease and follow-up of patients who received medication for the treatment of coronary artery disease.
Coronary artery calcification is well visualized with X-ray techniques such as radiography but only CT provides a non-invasive method for detecting and quantifying coronary artery calcification [8]. Coronary calcification is best detected and measured in a plain CT acquisition without contrast enhancement.
Quantification of coronary calcium was introduced in 1990 by Agatston et al [9]. They used electron beam tomography and established the "Agatston score". The Agatston score requires an acquisition with a special protocol (3 mm contiguous slices, 130 kV). The Agatston score is achieved by setting a threshold for the Hounsfield unit (130 HU) and for the size of the lesion (1 mm2). Then a pragmatic weighting of the calcified area is applied depending on the maximum HU in the lesions for each image. The total calcium score is calculated by summing the weighted areas for all images (Figure 2).
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Figure 2. Coronary artery calcification imaging at 64-row multidetector CT (MDCT). 64-row MDCT of a 52-year-old male patient with risk factors for coronary artery disease. Small calcifications in the left anterior descending artery. The total calcium score according to Agatston was 21, and the total volumetric score was 25, indicating mild atherosclerotic plaque with mild or minimal coronary artery narrowings likely. CT-angiography revealed no coronary artery stenoses.
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With the introduction of MSCT, new acquisition protocols came into use; prospective ECG triggering in combination with a half (180°) reconstruction at 120 kV is now generally used for calcium scoring. In prospective ECG triggered MSCT acquisitions, the patient is only exposed within the 170–200 ms acquisition window at diastole and radiation exposure is therefore significantly less compared with retrospective gated MSCT cardiovascular examinations. The application of MSCT for quantification of coronary calcium made it mandatory to switch to new quantification methods that can be compared for different scanners and that are robust with respect to different scanners and acquisition protocols. Alternatives for the Agatston score are the volume score (the volume of all voxels exceeding a certain threshold) and calcium mass (mg) [10]. The latter quantity holds the promise of providing the best physical measure for coronary artery calcification. Unfortunately, there is still a lack of standardization of the MSCT techniques with regard to image acquisition as well as to the methodologies for quantitative coronary calcification scoring. The development of standardized and reproducible protocols is a technical prerequisite for coronary calcification scoring to become a useful clinical tool. In addition, for screening purposes, the coronary calcification score will have to be established as an independent predictor of existing risk factors for cardiovascular disease [11].
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Coronary angiography
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MSCT has rapidly evolved through different stages of technological innovation, allowing high-quality non-invasive 3D imaging of coronary artery morphology (Figures 3 and 4). Recently the diagnostic accuracy of 64-slice MSCT for the identification and quantification of coronary artery stenoses has been reported [12, 13]. The patient-based analysis revealed that 94% of patients who required revascularization were correctly diagnosed by CT. Although excellent accuracy for stenosis detection was noted, technical restrictions for exact quantification of the degree of stenosis and reliable visualization of small vessel segments remain [12]. In an accompanying editorial the authors express the expectation that MSCT will be used in the near future on a routine basis for the identification of patients who do not need revascularization therapy despite the presence of symptoms [14].
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Figure 3. Normal coronary artery anatomy at 64-row multidetector CT (MDCT). 64-row MDCT of a 62-year-old male patient with risk factors for coronary artery stenosis. No stenoses were found at MDCT coronary angiography. Left anterior (a) oblique view and (b) caudal view. LAD, left anterior descending coronary artery; D, diagonal branch of the LAD; IM, intermediate coronary artery branch; Cx, circumflex coronary artery; MO, obtuse marginal branch (of the Cx); DP, descending posterior branch (of the right coronary artery).
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Figure 4. Bypass imaging at 64-row multidetector CT (MDCT). 64-row MDCT of a 78-year-old male patient after coronary artery bypass graft operation (CABG). Occlusion of multiple venous bypass grafts (nr 1 in a). Left internal mammarian artery bypass graft (nr 2 in a,b) with open anastomosis (nr 3 in a,b,c) on the left anterior descending coronary artery (nr 4 in a,c). Poor quality native coronary artery system with multiple stenoses and poor contrast enhancement (nr 4 in a,c). b and c are displayed in two perpendicular longitudinal directions.
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The potential value of MSCT for stenosis quantification is currently under active investigation. Recently, a good correlation between MSCT and quantitative coronary X-ray angiography was shown for stenosis quantification with the use of 16-slice technology, although MSCT revealed a systematic overestimation as compared with the reference standard [15]. Perfusion defects related to previous myocardial infarction or ischaemia may be well visualized with the use of MSCT (Figure 5).
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Figure 5. Multiple perfusion defects imaged with 64-row multidetector CT (MDCT). Same patient (78-year-old male) as in Figure 4 after coronary artery bypass graft operation and multiple venous bypass graft occlusions. Multiple perfusion defects with regional wall thinning.
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In CT coronary angiography, beta-blockers may be used to reduce the heart rate to a lower range, e.g. to 50–60 BPM to increase the cardiac rest period and with this to reduce motion artefacts. The resulting imaging performance is more predictable and of more consistent quality when using such medication. Special reconstruction algorithms for the reconstruction pose an alternative to the use of medication. The segmented reconstruction algorithm yields good temporal resolution even at higher heart rates. Also, when total scanning time is short, e.g. below 10 s, the quality of the scan improves since, due to the reduction of the total amount of heart beats in the scan, less variation can be expected in the heart rate during the acquisition. Hyperventilation and administration of oxygen may be used to stabilize heart rate particularly at scan times of approximately 20 s scanning time or longer.
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Assessment of ventricular function
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With retrospective gated 180° segmented sinogram space reconstruction the data can be reconstructed for evaluation of ventricular function [16]. Diastolic and systolic images can easily be extracted and reconstructed in any orientation for functional evaluation (Figure 6).
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Figure 6. Ventricular function imaging at 64-row multidetector CT (MDCT). 26-year-old male patient after surgery for congenital heart disease. Ventricular function can be assessed after drawing the endocardial ventricular contours in (a) end-diastolic and (b) end-systolic phases at multiple cardiac levels, thereby including the ventricular volumes.
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Global ventricular function is generally measured as the end-systolic and end-diastolic volume (ESV, EDV). Subsequently, stroke volume (SV) and ejection fraction (EF) can easily be derived from ESV and EDV. Semiautomatic software may be used for ventricular cavity contour detection and for the calculation of global ventricular function. Regional LV wall motion can be assessed by visual scoring of cinematic loops of well described myocardial segments [17].
Integrated CT assessment of the coronary arteries and regional myocardial function allows assessment of the functional consequences of a coronary artery stenosis leading to ischaemia and contraction abnormalities. The usefulness of this combined approach has been reported in patients with hypertension and diabetes mellitus [18, 19]. From the same data set global function and left ventricular mass can also be determined, which have clinical relevance in patients with hypertension for prognosis and guidance of therapy.
Several studies have shown that right ventricular function can also be accurately measured by gated MSCT. The assessment of right ventricular function may have special interest in patients with acute pulmonary embolism. Right ventricular enlargement on chest CT has been shown to be a predictor of early death in patients with acute pulmonary embolism [20, 21]. Even the dimensions of the right ventricle in non-gated CT images may be predictive for mortality in this setting. The potential value of gated MSCT for assessing right ventricular function in patients with pulmonary embolism is now under investigation.
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Assessment of pulmonary veins
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Atrial arrhythmias often originate in the pulmonary veins and can be treated with percutaneous radiofrequency catheter ablation. With this technique, the arrhythmic foci are electrically disconnected from the left atrium by means of catheters placed in the left atrium [22]. Pre-procedural MSCT examination is helpful to depict the anatomy of the pulmonary veins and left atrium and particularly to demonstrate additional pulmonary veins (e.g. middle lobe vein), which is important for planning the interventional procedure. Variations in pulmonary venous anatomy are quite common and comprise variation in the number of veins as well as the occurrence of common ostia and early branching [23]. Three-dimensional surface rendering reconstructions provide a quick overview of the pulmonary venous anatomy, but cross-sectional reconstruction in coronal, sagittal and transverse orientations is necessary for full appreciation of the morphology of the pulmonary veins (Figure 7) [24]. Post-procedural MSCT also offers an opportunity for follow-up of the pulmonary vein after ablation [25].
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Figure 7. Pulmonary vein imaging at 64-row multidetector CT (MDCT). 64-row MDCT, non-ECG-synchronized imaging. 59-year-old male patient. Pre-interventional assessment of pulmonary veins for radiofrequency ablation. Posterior view of the patient's heart. Common ostium for the left pulmonary veins, i.e. the pulmonary veins join before entering the left atrium. Separate ostia for the right pulmonary veins. LS, left superior pulmonary vein; LI, left inferior pulmonary vein; RS, right superior pulmonary vein; RI, right inferior pulmonary vein; LA, left atrium; LPA, left pulmonary artery; RPA, right pulmonary artery; VC, inferior vena cava.
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MSCT pulmonary venography requires a contrast enhanced helical acquisition. To avoid motion artefacts a half reconstruction is generally performed, yielding a reconstruction window of about 165–200 ms. This is sufficiently short for imaging the rather large pulmonary veins with diameters well above 10 mm. Reliable images can be acquired without the use of ECG gating. Breath-hold acquisitions with a high pitch factor and resulting rather low patient dose are routinely obtained. The potential additional value of ECG synchronized MSCT is under investigation.
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Conclusion
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MSCT is a highly accurate tool for the non-invasive detection of coronary artery disease. Further technical advances are expected in acquisition techniques as well as post-processing of the CT data. Detector technology and arrays may be further expanded, allowing shorter imaging times. Improved temporal and spatial resolution will contribute to better stenosis quantification and plaque characterization. Integration of coronary artery imaging and functional data are feasible with current MSCT. Shorter scanning times may allow integration of coronary imaging, first-pass perfusion imaging as well as wall motion analysis from the same data set. Other cardiovascular applications also benefit from the improvements in CT technology. Recently, the value of MSCT for the evaluation of patients with chest pain presenting to the emergency department was reported [26]. It was shown that MSCT is feasible to evaluate chest pain patients comprehensively. During one comprehensive MSCT protocol cardiac and non-cardiac causes of chest pain can accurately be diagnosed. It is expected that MSCT will become a gatekeeper in patients presenting with chest pain from various sources.
Received for publication September 22, 2005.
Accepted for publication October 5, 2005.
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References
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Abstract
Introduction
