British Journal of Radiology (2004) 77, S87-S97
© 2004 British Institute of Radiology
doi: 10.1259/bjr/30634988
Multislice CT in cardiac and coronary angiography
A F Kopp, MD1,
A Küttner, MD1,
T Trabold, MD1,
M Heuschmid, MD1,
S Schröder, MD2 and
C D Claussen, MD1
Departments of 1 Diagnostic Radiology and 2 Internal Medicine, Division of Cardiology, Eberhard-Karls-University Tuebingen, Germany
Correspondence: Andreas F Kopp, MD, Department of Diagnostic Radiology, Eberhard-Karls-University Tuebingen, Hopp-Seyler-Strasse 3, 72076 Tuebingen, Germany
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Abstract
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In the last 2 years, mechanical multidetector-row CT (MDCT) systems with simultaneous acquisition of four slices and a half second scanner rotation time have become widely available. Data acquisition with these scanners allows for considerably faster coverage of the heart volume compared with single slice scanning. This increased scan speed can be used for retrospective gating together with 1 mm collimated slice widths and allows coverage of the entire cardiac volume in one breath-hold. First results from studies in correlation with intracoronary ultrasound suggest that MDCT technology not only offers the possibility to visualize intracoronary stenoses non-invasively, but also to differentiate plaque morphology. This is especially the case with the next generation of 16-row MDCT systems. An increased number of simultaneously acquired slices and submillimetre collimation for cardiac applications allows true isotropic scanning with high temporal resolution. Contrast-enhanced MDCT is a promising non-invasive technique for the detection, visualization and characterization of stenotic artery disease. It could act as a gatekeeper prior to cardiac catherization and finally replace conventional diagnostic modalities.
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Introduction
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Examination of the function, perfusion and viability of the heart muscle as well as of the morphology and function of the coronary arteries is of utmost importance in the diagnostic assessment of coronary artery disease. The current gold standard to assess the degree of stenotic artery disease is coronary angiography. In Germany alone, the total number of angiographic procedures rose by 45% from 1995 to 2000, while the fraction of interventional procedures remained almost constantly low at approximately 30% [1]. Although coronary angiography has become a safe procedure with only a small associated risk [2], the inconvenience for the patient as well as the economic burden have fuelled the quest to find an alternative, non-invasive method to visualize and assess coronary arteries. In the last 2 years, mechanical multidetector-row CT (MDCT) systems with simultaneous acquisition of four slices and a half second scanner rotation time have become widely available [3–5]. Current recommendations to perform non-invasive coronary CT angiography rely on studies performed with this type of system [5–10]. With the recent introduction of 16-row technology, spatial and temporal resolution is significantly improved. This leap in technology will redefine the role of coronary CT angiography in clinical cardiology.
The present article focuses on the technology principles and clinical applications of multislice CT in cardiac imaging and will discuss the technological advances and improved clinical performance of state-of-the-art 16-slice CT equipment.
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Data acquisition with MDCT
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CT acquisition of the heart should be performed in a single, short breath-hold scan with high temporal resolution to eliminate cardiac motion and high isotropic spatial resolution (i.e. submillimetre) at the same time to adequately visualize complex cardiothoracic anatomy and the coronary arteries. In 1984, electron beam CT (EBCT) was introduced as the first cross-sectional non-invasive imaging modality that could visualize the cardiac anatomy and coronary arteries. In 1998, mechanical spiral CT systems with simultaneous acquisition of four detector slices and a minimum rotation time of 500 ms were introduced that provided a substantial performance increase over the single- and dual-slice spiral CT systems that had been available until then. These multislice CT scanners can cover larger scan volumes with slice widths down to 1.0 mm and thus provide higher spatial resolution for improved visualization of small vessels and the complex anatomy. Different detector configurations are in use that enable simultaneous collimation of four slices with different slice widths. In the "fixed array" detector design, detector rows with equal spacing are used; the so-called "adaptive array" detector design consists of less detector rows with different sizes that become wider towards the outer area of the detector. In both concepts the thinnest slices results from collimation of the inner four detector rows and thicker slices are generated by electronic combination of adjacent detector rows. Higher temporal resolution compared with older mechanical spiral CT scanners is provided by faster rotation speed with rotation times down to 500 ms combined with specialized reconstruction algorithms.
Motion artefacts caused by cardiac pulsation can be minimized in high resolution CT studies of the heart via scanning or reconstruction scan projection data at a time point with the least cardiac motion, i.e. in the diastolic phase of the heart cycle. The heart phases can be determined from a simultaneously recorded electrocardiogram (ECG) signal. Two different ECG synchronization techniques are most commonly employed for cardiac CT scanning: prospective ECG triggering and retrospective ECG gating.
Prospective ECG triggering has long been used in conjunction with EBCT and single slice spiral CT [11]. A trigger signal is derived from the patient's ECG based on a prospective estimation of the present RR interval and the scan is started at a defined time point after a detected R wave, usually during diastole. Multislice CT allows simultaneous acquisition of several slices in one heartbeat with a cycle time that usually allows scanning in every other heartbeat [12]. Thus, shorter breath-hold times are present compared with single slice scanners, and respiratory artefacts can be widely eliminated. Prospective ECG triggering is the most dose efficient way of ECG-synchronized scanning as only the very minimum scan data needed for image reconstruction is acquired. However, usually only rather thick slice collimation (3 mm with EBCT, 2.5–3 mm with 4-, 8- and 16-slice CT) is being used for prospectively triggered acquisition within a reasonably short single breath-hold. Thus, resulting data sets are often not suitable for three-dimensional reconstruction of small cardiac anatomy. Also, the prospectively ECG-triggered technique greatly depends on a regular heart rate of the patient and is bound to result in misregistration in the presence of an arrhythmia.
Retrospective ECG gating overcomes the limitations of prospective ECG triggering with regard to scan time and spatial resolution and can provide higher consistency of image quality for examination of patients with changing heart rate during the scan [13, 14]. This approach requires multislice spiral scanning with slow table motion and simultaneous recording of the ECG trace that is used for retrospective assignment of scan data and heart motion. Phase-consistent coverage of the heart requires a highly overlapping spiral scan with a spiral table feed adapted to the heart rate to avoid gaps between image stacks that are reconstructed in consecutive heart cycles. These image stacks are reconstructed at the exact same phase of the heart cycle and cover the entire heart and adjacent anatomy in the considered scan range. Images are reconstructed from every heart beat and faster scan coverage is possible compared with prospective ECG triggering. Moreover, the continuous spiral acquisition enables reconstruction of overlapping image slices and thus a longitudinal spatial resolution approximately 20% below the slice width can be achieved (e.g. 2.5 mm for 3.0 mm slices, 1.0 mm for 1.25 mm slices, 0.8 mm for 1.0 mm slices, and 0.6 mm for 0.75 mm slices). For these reasons, retrospective ECG gating is the preferred method for imaging small cardiac anatomy and the coronary arteries with thin slices and high spatial resolution in short single breath-hold times.
In every heart beat, fan-beam data of a partial rotation (usually 240–260°) are utilized for image reconstruction that provides a temporal resolution equivalent to half of the rotation time in a centred region of interest (250 ms for 500 ms rotation time and 210 ms for 420 ms rotation time). A multislice spiral interpolation between the projections of adjacent detector rows is used to compensate for table movement and to provide a well defined slice sensitivity profile and images free of spiral movement artefacts. The temporal resolution can be improved by using scan data from more than one heart cycle for reconstruction of an image ("segmented reconstruction"). The partial scan data set for reconstruction of one image then consists of projection sectors from multiple consecutive heart cycles. Depending on the relationship of rotation time and patient heart rate, a temporal resolution between rotation time/2 and rotation time/2M is present, where M equals the number of projection sectors and the number of used heart cycles. Despite theoretically better temporal resolution, "segmented" reconstruction algorithms do not regularly provide superior image quality for display of small cardiac anatomy, as the algorithms are very sensitive to changing heart rates. Therefore, segmented reconstruction and in particular the use of data from more than two heart beats per image is often not practical.
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Calcium scoring
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EBCT has been established as a non-invasive imaging modality for the detection and quantification of coronary calcium using the Agatston scoring algorithm [15, 16]. Current EBCT protocols are used for measuring coronary arterial calcification by acquiring a stack of contiguous 3 mm thick sections [17]. The calcium score, as originally proposed by Agatston, is determined on the basis of the product of the total area of a calcified plaque and an arbitrary scoring system for those pixels with an attenuation greater than 130 Hounsfield units (HU). Theoretically, this multisection data set should give a clear representation of the amount of calcification in the major coronary arterial tree, yet high interscan variability up to 60% has impaired the ability to measure coronary arterial calcification precisely and repeatedly [18]. Spiral multislice CT holds promise to overcome this limitation: coupling the technique of retrospective gating with nearly isotropic volumetric imaging, the reliability of coronary calcium quantification especially for small plaques was found to significantly improve [19]. Using ECG-gated volume coverage with multislice spiral CT and overlapping image reconstruction (2.5 mm collimation, 1 mm increment), an interscan variability of approximately 5–8% can be achieved [20]. With the advent of multislice CT with significantly reduced interstudy variability, we can now begin to define the effects of treatment regimens on coronary arterial calcification and to determine whether changes in coronary arterial calcification in individual patients have a predictive value for future coronary events. If these differences in calcium score over time result in a difference in event rates, it is conceivable that serial measurements of calcium score by MDCT will provide a powerful and much needed predictive tool [21].
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CT coronary angiography for detection of stenoses
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For non-invasive coronary CT angiography, high requirements for spatial resolution, low-contrast detectability and temporal resolution have to be fulfilled at the same time. Image quality depends on various patient and scanner parameters, and optimization of examination protocols is critical for best balance of imaging parameters and best examination results. The imaging protocol for MDCT angiography of the coronary arteries on a 4-row scanner is relatively straightforward. To establish the scan delay time, a test bolus of 15 ml of contrast medium and 20 ml of saline chaser bolus is used. The circulation time is determined by measurements of CT density values in the ascending aorta. Imaging commences at the circulation time plus 3 s [22]. A bolus of 120 ml of non-ionic contrast medium (400 mg I ml–1) is injected through an 18 G catheter into an antecubital vein [23]. Usually, the craniocaudal size of the heart to be covered by the scan is in the range 10–12 cm. 4-slice CT scanners with 500 ms rotation time and an individual detector width of 1.0 mm cover the entire heart during a 30–40 s breath-hold with reconstructed slice width of 1.3 mm [24].
The overall diagnostic quality of non-invasive cardiac and coronary CT angiography largely depends on choice of the appropriate reconstruction time point within the cardiac cycle, patient heart rate during the examination and contrast enhancement. The motion pattern of the left heart and the left anterior descending (LAD) and circumflex (CRX) coronary arteries follows the left ventricular contraction, whereas the right coronary artery (RCA) moves synchronously with the right heart, i.e. the right atrium. Because of these different motion patterns, different reconstruction time points over the cardiac cycle can result in optimal display of different cardiac anatomy and different coronary arteries. For optimal image quality, the reconstruction window within the cardiac cycle should be selected individually for each of the three major coronary arteries [14, 25, 26].
The results of MDCT coronary angiography in the detection and quantification of coronary lesions with 4-row technology obtained so far from different centres are encouraging [27, 28] (Figure 1
). CT angiography of the coronary arteries yielded a sensitivity of 75–90%, a specificity of 90–95%, a positive predictive value of 0.7–0.9 and a negative predictive value of 0.8–0.9 for detection of haemodynamically significant stenoses in the major segments of the coronary arteries [23] (Table 1). However, in these studies 20–30% of the proximal arteries could not be adequately evaluated.
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Figure 1. Multidetector-row CT angiography with 4-row technology (collimation 4 x 1 mm, pitch 1.5, 120 ml Imeron® 400). (a) Anterior view of left coronary artery with left anterior descending (LAD) artery in volume rendering technique. (b) Lateral view of left coronary artery with LAD artery and circumflex branch. (c) Maximum intensity projection of right coronary artery (RCA) with calcified plaques (arrow). (d) Diaphragmatic surface with posterolateral and interventricular branches of RCA (arrows).
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Table 1. Results for 4-row multidetector-row CT angiography for detection of haemodynamically relevant stenoses (n=number of patients included in the study)
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Plaque imaging
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Owing to remodelling of the vessel wall in the presence of plaques occupying less than 40% of the area within the internal elastic membrane, such plaques escape detection by conventional coronary angiography. This fact must be borne in mind because it may lead to potential underestimation of the extent of coronary artery disease when conventional coronary angiography is used. However, CT coronary angiography allows for the detection of early changes before narrowing of the luminal diameter. This is an absorbing potential of the cross-sectional imaging modalities.
It is widely accepted that only one-third of myocardial infarctions arise directly from significant coronary stenosis. Non-stenotic (<75%) plaques cause approximately 80% of fatal myocardial infarctions. Approximately 90% of all patients with acute myocardial infarction had no haemodynamically relevant lesion. Much more frequently, rupture of a vulnerable plaque with subsequent thrombus formation is the reason for occlusion of a coronary artery [29]. Preliminary data indicate that MDCT angiography might allow detection and assessment of non-calcified, lipid-rich plaques (Figure 2) [28, 30]. Schröder et al [30] investigated non-invasive detection of coronary plaques and plaque composition by MDCT compared with intracoronary ultrasound (ICUS) as the gold standard. MDCT and ICUS yielded identical results with regard to plaque composition and quantification of lesions [31]. Becker et al also investigated the criteria that allow for morphological characterization of atherosclerotic coronary lesions based on MDCT imaging in human cadaver heart specimens. They compared MDCT findings with histopathology. Becker found a high sensitivity for detection of atherosclerotic lesions type IV, Va, Vb and Vc according to the American Heart Association (AHA) classification [32]. Based on mean CT attenuation, he could reliably differentiate predominantly lipid-rich plaques from predominantly fibrous-rich plaques. Thus, this new technology holds promise to allow for the non-invasive detection of rupture-prone soft coronary lesions and may have the option to lead to early onset of therapy [30, 33].
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Figure 2. Non-calcified, soft, lipid-rich plaque in left anterior descending artery (arrow) (Somatom Sensation 4, 120 ml Imeron® 400). The plaque was confirmed at intracoronary ultrasound.
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Function
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With ECG-gated MDCT, spiral scanning two- or three-dimensional images can be reconstructed in incrementally shifted heart phases with a temporal resolution of up to 125 ms. With multiplanar reformation, the heart can be displayed in any desired plane, such as the short and long axis. This allows functional analysis in a one-stop shop approach for every patient undergoing CT angiography of the coronary arteries. The ability to obtain functional information from routine contrast-enhanced cardiac examinations on a conventional whole-body CT scanner could obviate the need for an additional study with a second imaging modality. Halliburton et al [34] evaluated MDCT as a method for volume determination of the left ventricle compared with the gold standard, cine magnetic resonance imaging, in 15 patients with chronic ischaemic heart disease. Measurement of left ventricular volume during end-diastole and end-systole with MDCT compared with MRI on a fast gradient system was significantly less for both volumes. However, values for ejection fraction with MDCT and MRI were not statistically different. Similar results were reported by several authors [35, 36]. The exact determination of left ventricular volume during end-systole seems to be the most critical issue, with a temporal resolution of 
125 ms. Further improvement in temporal resolution will facilitate functional analysis.
When multiple cardiac phases are extracted, animated movies of the beating heart can be available. However, only limited data are available for the usefulness of functional assessment of wall motion using MDCT. Mochizuki et al [37] evaluated post-processing interactive multiplanar animation for the evaluation of wall motion in 15 patients compared with conventional left ventriculography. By extracting multiple cardiac phases, interactive animated movies were generated. Extracted cardiac phases ranged from 8 to 11, depending on the patient's heart rate. The interactive animated movies were displayed in six planes and the left ventricle was divided into seven segments according to the AHA classification. Wall motion was visually scored into three grades: normal, hypokinesis and akinesis (severe hypokinesis to dyskinesis). The scores of MDCT and biplane ventriculography agreed in 99 (94%) of 105 segments [37].
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Myocardial perfusion
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Myocardial perfusion defects are often observed as low density in the risk area of acute myocardial infarction on contrast-enhanced helical CT [38]. However, the clinical meaning of this perfusion defect has not yet been elucidated. Koyama et al [39] presented the first data on the potential role of CT in 45 patients with acute myocardial infarction with regard to the clinical outcome after successful reperfusion therapy. They visually assessed myocardial perfusion with regard to the depth of the perfusion defect. When compared with SPECT (single photon emission CT), these data corresponded closely to the non-viable infarct area and its depth predicted the outcome in the chronic phase. Even the volume of the infarcted area could be reliably assessed. In addition to these early perfusion defects, Koyama et al [40] found that in some patients the perfusion defects disappeared when the CT scan was repeated several minutes later (late enhancement). With regard to the existence of early perfusion effect and late enhancement, they classified patients with acute myocardial infarction into three groups: Group 1, showing no perfusion abnormalities; Group 2, showing early perfusion defect and late enhancement; and Group 3, showing persistent perfusion defect in early and late phase. Koyama et al [40] concluded that this myocardial perfusion pattern on contrast-enhanced CT might predict clinical outcome of acute myocardial infarction after reperfusion therapy.
Assessment of more than one or two time points of enhancement and the calculation of classic myocardial perfusion parameters with MDCT is even more challenging. In a first attempt, Wintersberger et al [41] analysed myocardial contrast dynamics using ECG-triggered MDCT in nine patients. A prospectively ECG-triggered transaxial dynamic scan (4 mm x 5 mm) over 35 heart beats was applied to analyse myocardial enhancement patterns with subsequent assessment of perfusion parameters. Quantitative flow calculations revealed values close to those within normal myocardium (0.73±0.20 ml g–1 min–1). In regions of impaired blood supply, amplitudes and upslopes of myocardial enhancement tended to be lower. They concluded that assessment of myocardial contrast dynamics is possible using MDCT, however ventricular coverage and injection protocols need to be improved [41].
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Limitations
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With 4-row technology a number of factors are known to decrease image quality of MDCT angiography and to make image interpretation difficult [42]. The two factors held mostly responsible are higher heart rates and heavy calcification. Becker was one of the first to describe the negative effect of higher heart rates on image quality. These data have been confirmed by others [43]: excellent diagnostic image quality can only be obtained at heart rates <65 bpm. The reason for this heart rate limitation lies in the temporal resolution of the CT image acquisition and reconstruction system. To obtain heart rates below 65 bpm for optimal image quality, either 80 mg Esmolol intravenously or 50–100 mg metoprololtartrate orally can be administered prior to the scan.
Assessment of luminal diameter in the presence of extensive calcification yields unsatisfactory results. Even if non-high grade coronary lesions are known, it can be difficult to determine the progress of that specific lesion. However, there are only limited published data available that quantify the amount of calcification critical for image interpretation. In a recent study we included a total of 66 patients with a history of coronary artery for MDCT angiography. Total calcium score as well as all coronary arteries including distal segments and side branches were assessed with respect to evaluability and the presence of coronary artery lesions or occlusions. Results were then compared with quantitative coronary angiography. Of all patients, only 24 (36%) were diagnosed correctly. In the other 42 patients clinical diagnosis was either not possible or it was incorrect. Artefacts due to elevated heart rates or severe coronary artery calcification were the main cause of degraded image quality inhibiting correct diagnosis. Analysis of the data suggested a threshold for maximum heart rate and maximum calcification (63 bpm and Agatston score 300, respectively). A second analysis was made using these thresholds. Now 22 (92%) of 24 patients were correctly diagnosed. This indicates that MDCT angiography can also be performed in patients with manifest coronary artery disease when selected properly within certain thresholds. Reasonable thresholds might be heart rates >63 bmp and extensive calcification with a total Agatston score >300.
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From 4 to 16 rows
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True isotropic resolution has not yet been reached with 4-slice CT systems. Consequently, an increased number of simultaneously acquired slices and submillimetre collimation for cardiac applications was the next step on the way towards true isotropic scanning with multislice CT. the 16-row multislice CT scanner Siemens SOMATOM Sensation 16 was introduced in 2002, offering simultaneous acquisition of 16 slices with 0.75 mm or 1.5 mm collimated slice width each. Similar to the 4-slice CT scanner, the SOMATOM Sensation 16 has an adaptive array detector (Figure 3). It consists of 24 detector rows, the 16 central ones being 0.75 mm wide in the centre of rotation, the 4 outer ones on both sides being 1.5 mm wide. The total z-coverage in the isocentre is 24 mm. For CT angiography of the coronary arteries, a collimation of 0.75 mm (13.2 mm s–1 feed) with a gantry rotation time of 420 ms is used. Spiral scanning with 16 submillimetre slices represents a breakthrough on the way towards true isotropic resolution for routine clinical applications. As a consequence, the distinction between longitudinal and in-plane resolution will gradually become history and the traditional axial slice will loose its clinical predominance.
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Figure 3. Adaptive array detector used in the Siemens Somatom Sensation 16. Left: schematic drawing. By proper combination of the signals of the 24 detector rows, the basic collimations 16 x 0.75 mm and 16 x 1.5 mm can be realized. Right: picture of a detector module, which consists of 16 x 24 detector elements.
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The simultaneous acquisition of 0.75 mm cross-sections yields higher spatial resolution, and at the same time the overall scan time is substantially shorter than with previous scanners. Improved spatial resolution, through reduction of partial volume effects, improves diagnostic accuracy and potentially reduces the problems caused by calcification, whereas a shorter scan time may improve scan quality through the shorter breath-hold duration and, in addition, requires less contrast agent (Figures 4–7). Finally, the faster rotation (420 ms) compared with previous scanners provides higher temporal resolution.
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Figure 4. 66-year-old male patient with known single vessel coronary artery disease. (a) Multidetector-row CT angiography of the right coronary artery with a 16-row CT scanner (Somatom Sensation 16, 80 ml Imeron® 400). Even subsegmental branches (arrows) can be readily delineated. (b) Multiplanar reconstruction in the long axis depicts aneurysm of left ventricle s/p myocardial infarction.
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Figure 5. 59-year old patient with known wall changes in proximal left anterior descending artery without significant obstruction. (a,b) The entire coronary tree is well visualized. Excellent image quality of the displayed coronary tree with only moderate calcifications present (Agatston score 257). (c) Visualization of the entire right coronary artery up to the apex. Protocol: collimation 12 x 0.75 mm, pitch 0.31.
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Figure 6. 58-year-old male patient with two vessel disease (16-row coronary multidetector-row CT angiography). Note the excellent image quality as well as the absence of calcifications at the site of obstruction. High grade ostial lesion (a) and a tandem lesion in the proximal circumflex artery (b) are to be seen.
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Figure 7. Multiplanar reconstructions along the left anterior descending artery in a patient who presented for stent patency control and refused coronary catheterization, obtained on a SOMATOM Sensation 16. Rotation time 0.42 s, basic collimation 0.75 mm, reconstructed slice width 3 mm (a), 1.5 mm (b) and 0.75 mm (c).
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Ropers et al [44] analysed coronary stenoses on 16-row coronary MDCT angiography with a reference diameter down to 1.5 mm, thus covering all lesions that may be potential targets for revascularization. Both motion and calcification rendered fewer arteries (12%) unevaluable than in most previous studies and a high sensitivity (92%) and specificity (93%) for the detection of coronary stenoses was achieved [44]. Nieman et al [45] demonstrated similar results with a sensitivity, specificity and negative predictive value of 95%, 86% and 97%, respectively, in a comparable patient population using the same 16-slice CT technology. However, their analysis was restricted to the major branches with a vessel size 2 mm. What is striking about this study was that only 7% of all coronary branches were poorly assessable. If further studies confirm these first results, MDCT could be recommended to act as a gatekeeper prior to cardiac catheterizations.
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Patient dose
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Despite its undisputed clinical benefits, multislice scanning is often considered to require increased patient dose [46]. Indeed, a certain dose increase compared with single slice CT is unavoidable owing to the physical principles of multislice CT [47]. During ECG-gated spiral imaging of the heart, data are acquired with overlapping spiral pitch and continuous X-ray exposure. Thus, ECG-gated spiral acquisition requires higher patient dose than ECG-triggered sequential acquisition for comparable signal-to-noise ratio. When performing multiple reconstructions in different cardiac phases, for optimal image quality of individual vessels all spiral data are used for image reconstructions and no data are omitted. To obtain the same diagnostic information, multiple sequential acquisitions would have to be performed with repeated injections of contrast material. This would eventually result in the same or even higher X-ray exposure. However, ECG-gated spiral acquisition by prospectively ECG-controlled online modulation of the tube output allows reduction of X-ray exposure [48]. By reduction of the tube output during heart phases that are not likely to be targeted by the ECG-gated reconstruction, dose savings up to 50% are possible. Dose is further reduced with an increased number of simultaneously acquired slices. The collimated dose profile is in general a trapezoid in the axial direction. In the plateau region of the trapezoid, the entire focal spot is seen by the detector. In the penumbra regions, the focal spot is only partially seen by the detector owing to the limitation of the X-ray beam by the pre-patient collimator. With single slice CT, the entire trapezoidal dose profile can contribute to the detector signal. With multislice CT, only the plateau region of the dose profile may be used to ensure equal signal level for all detector slices. The penumbra region has to be discarded, either by a post-patient collimator or by the intrinsic self-collimation of the multislice detector, and represents "wasted" dose. The relative contribution of the penumbra region increases with decreasing slice width, but it decreases with increasing number of simultaneously acquired slices. This is demonstrated in Figure 8, which compares the "minimum width" dose profiles for a 4-slice CT system and a corresponding 16-slice CT system with equal collimated width of one detector slice. Correspondingly, the relative dose utilization of the 4-slice CT scanner SOMATOM Volume Zoom is 70% for 4 x 1 mm collimation and 85% for 4 x 2.5 mm collimation. The 16-row scanner has an improved dose utilization of 76%/82% for 16 x 0.75 mm collimation and 85%/89% for 16 x 1.5 mm collimation, depending on the size of the focal spot (large/small, respectively).
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Figure 8. Minimum width dose profiles for a 16-slice CT system and a corresponding 4-slice CT system with equal collimated width of one detector slice. The relative contribution of the penumbra region, which represents wasted dose, decreases with increasing number of simultaneously acquired slices.
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Outlook
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The temporal resolution of current multislice CT scanners needs to be improved to provide motion-free and robust coronary imaging, particularly for moderate and high heart rates. Increased temporal resolution can be achieved by segmented reconstruction techniques using two or more segments from consecutive heart cycles for reconstruction. The latest 16-slice scanners achieve 0.42 s rotation time and a better temporal resolution of 105 ms by using two-segment reconstruction (Figure 9). However, segmented reconstruction can improve temporal resolution only in a limited range of heart rates and at the expense of blurring artefacts in the presence of heart rate changes. Further increased rotation speed is the most favourable approach to increase temporal resolution. For example, a 0.3 s rotation time can produce a consistent temporal resolution of 150 ms for all heart rates that may be able to provide motion-free data in patients with low and moderate heart rates (presumably up to 80 min–1), thereby reducing the number of patients who require heart rate controlling medication. Obviously, significant development efforts will be needed to handle the increase of mechanical forces and the increased data transmission rates.
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Figure 9. Temporal resolution (in ms) for 0.42 s and 0.5 s rotation. Temporal resolution as a function of the heart rate for the ACV approach using 0.5 s resp. 0.42 s gantry rotation time. In addition to the absolute improvement, the temporal resolution shows a different dependence on the patient's heart rate for 0.42 s rotation time, reaching its optimum (105 ms) at 81 bpm. This is clinically important, since without administration of beta blockers the majority of heart rates are in the range 75–85 bpm.
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The ultimate CT scanner should cover the entire coronary anatomy in a single heart beat without movement of the table. This can be achieved with area detectors that cover approximately 120 mm scan range with at least 0.5 mm spatial resolution. Area detector technology and related new cone beam reconstruction techniques that can provide in-plane and through-plane spatial resolution of 0.2 mm are in research. With these CT scanners, imaging of high resolution morphology as well as dynamic and functional information via repeated scanning of the same scan range may be possible. The application potential of such technology is being evaluated initially with experimental systems using phantom models and post mortem hearts. Initial experience shows that today's flat panel detector technology is yet too limited in low contrast resolution, and the high radiation dose that is needed to provide adequate signal-to-noise ratio even for high contrast studies is unacceptable for use in human subjects. Owing to the intrinsic slow decay times of flat panel detectors available today, only slow rotation times 10 s are possible at the present time.
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Conclusion
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The emergence of MDCT has had significant impact on cardiac imaging. Cardiac calcium scoring and CT angiography of the coronary arteries as well as functional analysis are no longer limited to a dedicated EBCT scanner. Cardiac imaging can now be performed on a standard-body MDCT scanner. Even with 4-row technology, non-invasive MDCT angiography provides high diagnostic accuracy in the detection of coronary stenoses. First results obtained with the recently introduced 16-row technology showed further improvement in terms of image quality and diagnostic accuracy. In addition, this new technology holds promise for allowing non-invasive detection and characterization of coronary atherosclerotic plaques. It is expected that within the next few years a comprehensive heart examination using MDCT will be developed that has the potential to replace conventional diagnostic modalities.
Received for publication July 14, 2003.
Accepted for publication September 15, 2003.
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Abstract
Introduction
