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Eur J Cardiothorac Surg 2006;30:443-450
© 2006 Elsevier Science NL

Review

Multi-slice computed tomography in coronary artery disease

^a Department of Radiology, City Hospital, Dudley Road, Birmingham, UK
^b Department of Biosurgery and Surgical Technology, Imperial College London, 10th Floor QEQM building, St. Mary's Hospital, London W2 1NY, UK
^c Department of Cardiothoracic Surgery, Imperial College London, 10th Floor QEQM building, St. Mary's Hospital, London W2 1NY, UK
^d Royal Berkshire and Battle NHS Trust, London Road, Reading RG1 5AN, UK
^e Birmingham Heartlands Hospital, Bordesley Green East, Birmingham, UK

Received 15 March 2006; received in revised form 20 May 2006; accepted 1 June 2006.

^_* Corresponding author. Tel.: +44 207 886 1147; fax: +44 207 886 6777. (Email: tathan5253{at}aol.com).

	Abstract

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
Multi-slice computed tomography technology is emerging as a realistic investigation in patients with suspected disease in native, stented or grafted coronary arteries. A non-invasive diagnostic tool is desirable as these patients are at high risk for complications of invasive angiography. A 64-slice CT may achieve the desired diagnostic accuracy, and overcome the limitations of spatial resolution, respiratory motion, artifacts from calcification and stents, and radiation dose considerations to produce reliable image quality. These advances, as well as the capacity for integrated functional cardiac assessment, may change the referral patterns in patients who have had previous bypass surgery or percutaneous intervention. This review outlines the debated issues about 64-slice cardiac CT in patients before and after coronary artery bypass surgery, as well as coronary stenting and functional assessment. A review of the recent literature on native coronary artery and bypass graft assessment by multi-slice CT is also performed.

Abbreviations: CA = coronary angiography • CABG = coronary artery bypass graft • CT = computed tomography • CTA = computed tomography angiography • EBT = electron beam tomography • ECG = electrocardiogram • LVWF = left ventricular wall function • MSCT = multi-slice CT • MRI = magnetic resonance imaging

Key Words: CABG new technology • Computed tomography • Multi-slice

	1. Introduction

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
Coronary artery disease is a major cause of morbidity and mortality across the world. The diagnosis of coronary artery disease in patients with ischemic symptoms is a prerequisite for stenting, angioplasty or bypass grafting. According to the British Heart Foundation, each year in the UK over 28,000 patients with confirmed native coronary artery disease undergo coronary artery bypass grafting (CABG). Recurrence of ischemic symptoms after CABG is due to native coronary artery disease progression, and/or new atherosclerotic disease in the conduit vessels [1,2]. Patients with recurrent ischemic symptoms after CABG are more likely to have occluded or significantly stenosed grafts at 5 years postoperatively [3]. There is therefore a need to evaluate the coronary artery anatomy, ventricular function and severity of disease in order to facilitate medical decision making.

Selective coronary angiography (CA) is the traditional gold standard for the evaluation of both native coronary arteries and bypass grafts. Complications following CA include trauma to the arterial cannulation site with pseudo-aneurysm formation (1.2%), cardiac arrhythmia (1%), stroke (0.3%), myocardial infarction (0.2%), angina (0.2%), coronary artery or graft dissection, and renal failure secondary to embolic disease or contrast [4]. These associated morbidities, and the need for hospital admission in specialized cardiac units, restrict its use in patients with suspected graft disease.

The emergence of multi-slice computed tomography (MSCT) over the past 5 years as an investigation for cardiac artery disease has prompted numerous studies on its accuracy in patients with ischemic symptoms, with and without coronary artery bypass grafts. The rapid development of this technology means that the latest information is constantly changing. The development of 64-slice CT scanners has led to further promising results. There is limited literature available on this new technology in CABG patients but the preceding literature on 4-, 8- and 16-slice scanners is invaluable in understanding the current situation. This article reviews the available literature on MSCT in patients with suspected native coronary artery and bypass graft disease, with the aim of introducing the latest 64-slice scanner technology. Increasing the number of detectors has improved on many of the suboptimal image parameters that plagued conventional and spiral CT in cardiac imaging. The 64-slice scanners have shown the most promise in overcoming the technical problems preventing reliable, accurate diagnosis of native and bypass graft disease. The factors in overcoming these image quality issues, as well as the clinical applications of 64-slice MSCT, are discussed in this review.

	2. Image quality considerations

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
2.1 Cardiac motion
The success of 64-slice MSCT in imaging coronary arteries and bypass grafts depends on its ability to overcome problems with cardiac motion, cardiac arrhythmias and heart rate. With the heart in constant motion, MSCT angiography uses electrocardiogram (ECG) gating to capture data at points during the cardiac cycle when cardiac motion is minimized. Retrospective reconstructions have been used, in which the data is acquired as a block, and is retrospectively reconstructed to achieve optimal imaging. Dose considerations, however, are making tube modulation algorithms more common, as described below.

The R–R interval is conventionally chosen as the ECG interval for estimation. The choice of reconstruction time may be made as an absolute time delay (in milliseconds after or before the R wave), or more commonly, the relative delay expressed as a percentage of the R–R interval [5].

Choice of reconstruction point depends on heart rate, regularity, vessel selection and previous graft surgery. Native right coronary and left circumflex arteries exhibit more motion than the left anterior (LAD) artery due to their position in the coronary groove, and greater effect of atrial contraction at the end of diastole [6]. Whilst grafts exhibit less motion than the native coronary arteries, due to their position away from the myocardium, lateral and inferior wall grafts have greater motion than the anterior grafts [6]. Using 64-slice technology gated at 40–60% of the R–R interval (corresponding to late diastole), gives artifact-free images of most grafts [7] and native arteries [8]. In patients with sinus tachycardia [9], arrhythmias [7,9] or extra-systolic beats [9,10], manual reconstructions of native or grafted vessels may be performed if artifacts prevent confident diagnosis. In native artery imaging, different reconstruction intervals are favored for different vessels [6]. Some centers make multiple reconstructions at different percentages of the R–R interval and select the best image dataset for each vessel.

Heart rate affects the optimal reconstruction interval. In coronary artery imaging, mid-diastolic reconstruction appears better in patients with low heart rates and early diastolic reconstruction in patients with high heart rates [11]. Motion artifacts are known to increase with heart rate, and recent 64-slice literature suggests improved accuracy in diagnosing stenosis in patients with heart rate below 70 beats per minute for both native [12] and bypass vessels [7].

Beta blockers are given in some studies to reduce heart rate and lengthen diastole, the period of the cardiac cycle of least cardiac motion [13]. Increasing the proportion of time in diastole therefore helps improve image quality. The optimal threshold heart rate for beta blocker administration is unclear in native artery and graft imaging, although 65 [7,14], 70 [15–17], 80 [18] and 90 [9] beats per minute have been suggested. The improved temporal resolution of 64-slice MSCT scanners may soon make the need for beta blocker administration obsolete, as in studies without beta blocker administration up to 90% of scans are of diagnostic quality [10]. In patients with chronic obstructive pulmonary disease, there are recognized risks of airways hyper-responsiveness, reduced FEV1 and bronchodilatation [19] preventing routine use of beta blockers. There is, as yet, no consensus as to whether the diagnostic benefit of beta blocker administration outweighs its risks in patients undergoing 64-slice MSCT of native or grafted arteries.

2.2 Temporal resolution
The temporal resolution of MSCT, or the time required to acquire the data for one image, determines the overall scan time. Scan sequences are acquired during a single breath hold, and image quality depends on the patient's ability to suspend respiration. Respiration artifact is more likely as the scan time increases. Heart rate increases after approximately 20 s [13] of breath holding and may further affect image quality due to problems with ECG gating.

Scan time has been reduced to 8 s with 64-slice technology [8] which reduces respiration motion and improves image quality compared to previous MSCT scanners which had longer scan times. The faster rotation speeds and greater number of detectors mean that scan times can be reduced without having to increase the slice intervals. A recent 64-slice study [20] of patients with underlying bronchopulmonary or pulmonary vascular disease demonstrated diagnostic image quality in 90% of patients. The origin of the internal mammary artery may need to be imaged if a left internal mammary artery (LIMA) or right internal mammary artery (RIMA) graft is present, extending the length of the scan window and increasing the respiration suspension time. Reducing temporal resolution as much as possible limits the additional motion artifact in these wider scan windows.

Another option to decrease scan time is to use data from more than one cardiac cycle to reconstruct the image. This is particularly useful in patients with high heart rates [21]. This approach depends on constant heart rate and is vulnerable to gaps in data if the table speed is too great for the gantry rotation. Typically, one cardiac cycle is used for patients with heart rate of 65 or less, and some centers employ more than one cardiac cycle for reconstructing images for heart rate over 65 [5,8].

2.3 Spatial resolution
Spatial resolution is a measure of the number of individual pixels of information that make up a digital CT image. As the number of pixels increases the detail increases. All MSCT has superior spatial resolution to spiral and conventional CT. As gantry rotation speeds increase, the minimum slice thickness decreases, with sub-millimeter sections throughout the heart obtainable during a single breath hold. Isotropic voxel resolution (consistent three-dimensional image quality in any reconstruction plane) is being achieved with 64-slice scanners at 0.4 mm x 0.4 mm x 0.4 mm [8]. The best spatial resolution achieved with 16 detector scanners is 0.5 mm x 0.5 mm x 0.6 mm, whilst for CA 0.2 mm x 0.2 mm two-dimensional resolution is standard [5].

2.4 Radiation Dose
The radiation dose to the patient during constant-output retrospectively gated 64-slice MSCT is estimated at 6.7–18 mSv [12,22] which is higher than for CA (2.1 mSv), electron beam (EB) CT (1.7 mSv) [23] or for prospective ECG gating [24]. Radiation dose can be reduced by 30–50% by lowering the tube output during the parts of the cardiac cycle not involved in data reconstruction [24]. An average effective dose of 4.95 mSv was achieved by 64-slice MSCT using tube current modulation whilst maintaining image quality in both functional and arterial stenosis scanning [10]. Alternative methods to reduce radiation dose include reducing tube kilo-voltage, which leads to dose reductions of up to 55% in experimental models of cardiac CT imaging [25]. Small field filters are being considered to minimize the dose to tissues outside the field of interest [26]. In general terms, the radiation dose will increase as image quality and number of acquired images increases and so protocols to reduce dose will become paramount as CT technology advances.

	3. Clinical applications of 64-slice MSCT

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
3.1 Detection of native coronary artery stenosis
The non-invasive detection of stenosed or occluded coronary arteries and grafts by MSCT has become more reliable as the number of detectors has increased. The sensitivity and specificity of 16-slice MSCT for detection of significant native artery stenosis vary between studies [27–30] (Table 1 ), partly due to different temporal resolution, cutoff for significant stenosis, and reader protocols. The inability to evaluate all vessels because of motion artifact, partial voluming and poor contrast opacification (due to heart failure) plagued 16-slice MSCT. However the improved image quality of 64-slice MSCT has led to promising results in stenosis detection.

However, the specificity is high across the 64-slice studies (94–99%), indicating that 64-slice MSCT is an excellent investigation to exclude coronary artery stenosis. The varying selection of patients, technical parameters and interpretation protocols between studies makes direct comparison difficult. The sensitivity in patients with coronary calcification, arrhythmia, high heart rate and coronary stents is adequate to prompt more routine use of 64-slice MSCT in investigation of native coronary artery stenosis. Undoubtedly there will be further literature available in the near future on the promising diagnostic accuracy of 64-slice MSCT.

	4. Coronary artery calcification (CAC) measurement

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
The amount of atherosclerotic plaque has been shown to be highly correlated with CAC scoring [33]. CAC scores in conjunction with EBCT imaging give prognostic indications of future ischemic events in asymptomatic patients [34], smokers [35] and diabetics [36]. Calcium scoring is as reproducible when calculated from MSCT as with EBCT [37]. Various scores for overall CAC burden, such as the Agatston score, are used to predict ischemic events. The score ranges from 0 to over 1000, with all cause 5-year survival reducing as the score increases. In native artery imaging with 64-slice scanning, patients with Agatston scores under 100 show 94% sensitivity and 95% specificity [12] for luminal stenosis of 50% or more. Overall sensitivity of 95% and specificity of 90% for detecting stenoses over 50% has been shown in non-selected, consecutive populations with high Agatston scores [12]. Patients with Agatston scores over 400 have reduced rates of assessable vessels, and diagnostic accuracy for stenosis detection is reduced, even with 64-slice scanners [12].

Serial CAC measurements on EBCT have been used in conjunction with statin therapy to predict future myocardial infarction [38], monitor statin therapy [39], and more recently, individual plaque assessment has been put forward as an alternative measure of prognosis [40]. Despite this, up to 15% of individuals without CAC on MSCT may have non-calcified atherosclerotic plaques [41], making the detection of CAC an inadequate screening tool for atherosclerotic disease.

Dense calcifications of vessels are a major cause of false positive and overestimation of significant stenosis in both native and grafted arteries [8,9,12,42], although the improved spatial resolution of 64-slice scanners has reduced the effect of partial voluming [8,42]. Partial volume effects are due to averaging of all tissue densities within a voxel volume, with a speck of calcium (metal density) causing an entire voxel to appear dense. Smaller voxel sizes reduce this effect through greater detail and improved resolution of the lumen edge. A 64-slice MSCT can predict the grade of stenosis in native coronary arteries to within 25% with 90% probability compared to invasive CA [12]. MSCT coronary population screening is not routinely performed because of this tendency for false positive results and the potential for unnecessary invasive CA which has associated morbidity and mortality [4].

	5. Coronary stents

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
In patients with coronary stents, blooming artifact, partial voluming and limited spatial resolution prevented 4- and 16-slice MSCT from reliably assessing stent patency [43]. Blooming artifact leads to overestimation of the size of calcified lesions and underestimation of luminal diameter due to overflow of optical stimulus from one pixel to adjacent pixels. Coronary stents are commonly associated with surrounding calcification [9], which contributes to the significant proportion of stents which are not assessable on 16-slice MSCT. In one study, 18% were not evaluated due to poor image quality [43]. In a recent 64-slice study of 67 stented lesions [9], 9 (13%) were not evaluated due to poor image quality.

Coronary stents are small in diameter and more difficult to image than either native coronary arteries or the larger diameter graft conduits [43]. A 16-slice MSCT achieved better sensitivity and specificity (86% and 100%, respectively) for stents with luminal diameter over 3 mm, than in narrower stents (54% and 100%) [43]. In the 64-slice study mentioned above, 13 of 14 restenoses of 50% or more were identified, with the missed stenosis attributed to motion artifact. Two false positive diagnoses were attributed to the effect of surrounding calcification [9]. Overall sensitivity and specificity were 93% and 96%, respectively. This study used smooth and sharp kernel reconstruction algorithms during image processing. Another 64-slice MSCT study of 13 stented lesions [8] demonstrated 2 missed stenoses of less than 75% narrowing and 4 false positive diagnoses of stenosis in non-diseased stents. The low accuracy in this study was attributed to high-density stent material causing artifacts, and to not using the sharp kernel reconstruction algorithm.

	6. Functional assessment

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
Left ventricular wall function (LVWF) has traditionally been assessed by echocardiography. The improved temporal and spatial resolution of 16-slice MSCT made assessment of cardiac function possible. Cine mode slice sequences have been found to have up to 89% overall accuracy compared with echocardiography assessment of wall segment function as either normal or abnormal, and sensitivity of 66% and specificity of 96% for segmental wall dysfunction [44]. Ejection fraction evaluation by 16-slice MSCT had a correlation of 96% with two-dimensional echocardiography [45]. The introduction of 64-slice MSCT suggests that functional as well as morphological information can be gained from MSCT angiography.

For functional scan protocols, two-dimensional images are obtained in the short axis of the heart instead of the conventional axial plane. The ventricular lumen is delineated either manually or by using automated software protocols. Images are obtained at set points through the cardiac cycle. The ventricular volume within each slice is calculated from the area multiplied by slice thickness. The overall end diastolic volume (EDV) and end systolic volume (ESV) is calculated by summing the ventricular volumes for set points during the cardiac cycle corresponding to end diastole and end systole. Stroke volume is calculated as EDV minus ESV. Ejection fraction (EF) is calculated as stroke volume divided by EDV [10]. By combining data from more than one cardiac cycle, the temporal resolution is improved and the variability in EF results is reduced [46] especially in patients with high heart rates.

A 64-slice functional assessment is possible in the majority of patients [10]. In a recent study, 93% of patients in sinus rhythm and underlying bronchopulmonary disease were successfully evaluated for both left and right ventricular function [10]. The remaining 7% were not assessable, due to frequent extra-systolic beats or high-contrast streak artifact within a ventricular cavity which prevented confident delineation of the endocardial contours. Beta blocker administration may reduce the incidence of extra-systolic beats and may be advisable in some patients. Streak artifact may be reduced by using a saline flush of the contrast column, which was not used in that study. Additionally, the use of tube modulation to reduce radiation dose does not appear to affect image quality [10].

The facility to compare both LVWF and vessel stenosis makes MSCT a very useful clinical investigation in patients with suspected coronary disease. Ventricular function is an important prognostic indicator of coronary atherosclerotic disease. In patients with coronary bypass grafts or stents, 64-slice MSCT can be used to monitor cardiac function and response to intervention, in addition to assessing graft patency and stenosis. In patients with bypass grafts and recurrent ischemic symptoms, the left ventricular function is an important consideration in planning redo-surgery.

	7. Coronary bypass graft patency assessment on MSCT

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
The complex course of coronary grafts makes three-dimensional imaging a valuable tool, to assess the status of the graft as it travels parallel and perpendicular to the axial images. Bypass conduits typically measure 4–6 mm in luminal diameter. Although wider than most native coronary arteries, the vessel diameter makes good spatial resolution vital to accurate evaluation. Assessment of anastomosis is improved with three-dimensional technologies, offering the ability to view the anastomosis from multiple angles. Evaluation of sequential and Y grafts is also aided with the three-dimensional tool, as two-dimensional axial imaging can be difficult due to multidirectional flow and projection of grafts over other vessels. Grafts, particularly to the LCA, may appear to merge into the myocardium and be lost on axial imaging.

Graft calcification tends to be less than that in native arteries, and correspondingly causes fewer artifacts. Patency assessment is possible by looking for contrast at points both proximal and distal to the area of calcification, but back flow into the graft at the distal anastomosis is possible if the graft is occluded and there is competitive flow from the native artery. Assessment of stenosis is more difficult if there is calcification, with calcification artifact cited as a reason for overestimation of stenosis grade [47].

High-density surgical clips may cause partial volume and beam hardening effects, image quality degradation and unreliability in determining stenosis grade [7]. Clip artifact varies with surgical technique but is generally greater around arterial grafts [15] and may increase when using very small slice thicknesses [47]. Superior spatial resolution of 64-slice scanners has reduced these effects compared to 4- and 16-slice scanners [7].

Knowledge of the operative anatomy is vital in interpretation of vessel patency and stenosis as side branches, congenital variations in coronary artery anatomy and competitive flow from native arteries make blinded interpretation difficult. Good communication between the clinician and the diagnostician is a key part of accurate graft assessment. Ideally a formal operation note should be available during the time that the images are interpreted.

Evaluation of the anastomotic sites is more difficult than the mid segments of the grafts. Competitive back flow from native arteries makes distal anastomotic sites difficult. Proximal anastomotic sites are difficult to assess for stenosis as the graft may have been narrowed during insertion, and not subsequently. Extension of the scan area up to the origin of an internal mammary grafted in situ is not always performed, which may limit the assessment of that graft. The number of non-assessable grafts using 4-slice scanners has limited its use in clinical practice, however 64-slice scanning appears promising.

	8. Coronary bypass graft assessment and MSCT literature review

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
A Medline search was performed to search for articles comparing MSCT with CA in CABG patency and stenosis evaluation. The search keywords were ‘multislice’, ‘coronary bypass graft’ and ‘multidetector’. The related articles function was used to find additional studies. The references of each relevant article were also searched. No language restrictions were made but only articles relating to adult humans were retrieved.

The first articles comparing MSCT with CA in CABG evaluation were published in 2001. Since then, there have been numerous studies evaluating the accuracy of MSCT compared with CA [7,14,15,18,47–63] (Table 2 ), and others assessing various technical aspects of data acquisition and processing. The sensitivity and specificity of 4-, 8-, 16- and now 64-slice MSCT in assessing graft patency has been consistently high compared to coronary angiography, as shown in Table 2. The results for graft stenosis have been more variable, with sensitivity ranging from 67% to 100%. The specificity results show more consistency, ranging from 91% to 100%. The high values indicate that the overall accuracy of MSCT is high compared with the gold standard of CA.

Explanations for false positive occlusion results included graft narrowing, poor intra-luminal contrast enhancement due to severe heart failure, and competitive flow from native coronary arteries. False negative occlusions were partially attributed to failure to cannulate the graft during CA. False positive and negative stenosis results were attributed to either incorrect estimation of the graft narrowing or to distal anastomotic artifact.

	9. Discussion

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
MSCT has developed over the past 5 years as the number of detectors has increased. Spatial and temporal resolutions have dramatically improved, with a corresponding improvement in image quality. A 64-slice MSCT has largely overcome the problems of constant cardiac motion through effective ECG gating techniques, and additional reconstruction options in patients with arrhythmias, tachycardias and extra-systolic beats. This has reduced the proportion of vessels unable to be evaluated due to cardiac motion artifact. A 64-slice MSCT is less vulnerable to respiratory motion artifact than 16-slice MSCT due to its shortened scanning times. Literature has shown a 90% rate of assessable vessels in patients with known significant respiratory disease.

The increased radiation exposure of MSCT compared to CA makes its routine use controversial. Dose reduction techniques like tube current modulation have reduced the dose, and future dose-reducing devices are planned to further decrease dose levels. However, the use of MSCT coronary artery screening is not recommended, because of dose considerations and the risk of unnecessary invasive CA and its potential side effects.

A 64-slice MSCT has shown excellent accuracy in excluding significant native coronary artery stenosis, although further literature is required for widespread acceptance of its accuracy in problematic patient groups. The ability to reliably confirm stenosis is impaired by associated calcification artifact, although these patients are likely to be referred for invasive CA under current indication guidelines. At this stage, exclusion of significant stenosis is reliable on 64-slice MSCT, and accuracy of diagnosis of stenosis is also acceptable.

Coronary artery calcification scores have been shown to be as reliable on MSCT as the accepted methods of EBCT measurement. These scores are good prognostic indicators of future disease progression. The reasonably high incidence of non-calcified atherosclerotic plaques, however, makes the routine screening for coronary artery calcification an impractical tool. The use of CAC scoring on MSCT remains a prognostic one in symptomatic patients, particularly in monitoring response to medical treatments.

Coronary stents have previously proven difficult to reliably image on 16-slice MSCT due to the high density of the stent material, as well as the narrow luminal diameter of the stent. The improved spatial resolution has addressed both these issues, with good accuracy and assessability of stented lesions reported on 64-slice scanners. However, the problem with non-assessable grafts remains one to be fully overcome.

Ventricular functional assessment is rapidly proving accurate on 64-slice MSCT compared to echocardiography. The imaging is possible with only a small increase in dose requirements, whilst the additional clinical information proves invaluable in making clinical treatment decisions. Ejection fraction and stroke volume measurements are accurate and the future possibilities include assessment of myocardial perfusion in acute myocardial infarction to assess viable muscle.

A 64-slice MSCT may assist in the preoperative planning of patients undergoing initial or repeat CABG, by offering the cardiac surgeon an accurate assessment of vessel stenosis, and demonstration of the course of native coronary arteries, grafts and internal mammary arteries which may be avoided during sternotomy [64].

The practical considerations of routine 64-slice MSCT angiography include the initial costs of hardware and software, durability, processing time and the appropriate support system of picture archiving and communications system (PACS). The choice of scanning hardware will depend on the available technology, with a view to remaining capable of producing acceptable imaging for at least 5 years [65]. Economic evaluation information from 2005 indicates that despite the initial cost of over one million pounds for a 64-slice scanner, training and service agreements, if MSCT cardiac angiography replaces invasive CA the subsequent savings in cardiologist time, contrast material and examination cost may make the initial outlay a viable economic strategy (NHS Quality Improvement, Evidence note 9, July 2005, http://www.nhshealthquality.org).

The emergence of radiological cardiac investigations may mean a direct diagnostic role for cardiothoracic surgeons in the future. The current referral patterns to cardiologists for invasive angiography to investigate recurrent ischemic symptoms may become a direct referral for non-invasive MSCT coronary angiography. The capacity to avoid invasive angiography in high-risk patients, as well as gain functional information, is driving the push towards MSCT in patients after CABG surgery. This potential development means that cardiothoracic surgeons should understand the benefits as well as limitations of MSCT technology.

	10. Conclusions

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 
It remains to be seen whether the accuracy, cost, interpretation time, availability of trained staff and patient throughput limitations of 64-slice MSCT angiography see it replace CA in assessment of native and grafted coronary vessels. The spatial and temporal resolution limitations of 16-slice MSCT are less significant in 64-slice MSCT, where the effects of respiratory motion, calcification artifact and cardiac motion are reduced. The evolving methods for radiation dose reduction promise to decrease the dose requirements whilst the information to be gained by cardiac MSCT continues to grow. The additional information available on 64-slice MSCT regarding vessel wall calcification, ventricular function and surrounding structures makes it a valuable tool for the cardiothoracic surgeon. The role for the cardiothoracic surgeon in investigating recurrent symptoms in post-CABG patients may become a direct diagnostic one.

	References

Top Abstract 1. Introduction 2. Image quality considerations 3. Clinical applications of... 4. Coronary artery calcification... 5. Coronary stents 6. Functional assessment 7. Coronary bypass graft... 8. Coronary bypass graft... 9. Discussion 10. Conclusions References

 

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