Image quality and diagnostic accuracy of 16-slice multidetector computed tomography for the detection of coronary artery disease in obese patients

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1 (2006) 30, & 2006 Nature Publishing Group All rights reserved /06 $ ORIGINAL ARTICLE Image quality and diagnostic accuracy of 16-slice multidetector computed tomography for the detection of coronary artery disease in obese patients C Burgstahler 1,3, T Beck 1,3, A Kuettner 2, A Reimann 2, AF Kopp 2, M Heuschmid 2, CD Claussen 2, S Schroeder 1 1 Department of Internal Medicine, Division of Cardiology, Eberhard-Karls-University Tuebingen, Tuebingen, Germany and 2 Department of Diagnostic Radiology, Eberhard-Karls-University Tuebingen, Tuebingen, Germany Background: Cardiac multislice spiral computed tomography (MSCT) scanners permit visualization of the coronary arteries with an overall good sensitivity (sens) and specificity (spec). However, in obese patients (pts), who are at higher risk to develop coronary artery disease (CAD), image quality of MSCT is supposed to be limited. At present, there are no data whether the accuracy of MSCT depends on the body mass index (BMI). Thus, we compared the catheter-controlled MSCT results from normal weight and obese pts in a cohort of 117 pts with regard to sens, spec, positive predictive value (PPV), negative predictive value (NPV) and image quality. Methods and material: In all, 21 normal weight pts (group I: BMIo25, years, number of risk factors ), 60 pts with mild overweight (group II: BMI 25 30, years, number of risk factors ) and 36 obese pts (group III: BMI 430, years, number of risk factors ) were examined by MSCT (Sensation 16 Speed 4 D s, Siemens, Germany, gantry rotation time 375 ms) and invasive coronary angiography. MSCT results were compared blinded to the results of the coronary angiography with regard to the presence or absence of a significant stenosis (450%) in a modified AHA 13 segment (sgt) model. Image quality was assessed on a qualitative scale between 1 (very good) and 5 (insufficient image quality) for each sgt. Results: Sens, spec, PPV and NPV were statistically not different in all three groups (I: 0.88/0.97/0.91/0.96, II: 0.83/0.97/0.88/ 0.95, III: 0.87/0.99/0.96/0.96). 3 pts (group I 1, group II 2) had to be excluded from analysis due to technical problems. Group I had significantly less risk factors (Po0.001) and image quality was significantly better than in group II and III (Po0.05). Group II and III did not differ with regard to risk factors or image quality. Conclusions: Overweight and obesity have an impact on MSCT image quality but did not hamper the diagnostic accuracy. Thus, MSCT is a noninvasive method to detect or rule out CAD also in pts with higher BMI. These retrospective data have to be confirmed in larger prospective trials. (2006) 30, doi: /sj.ijo ; published online 8 November 2005 Keywords: multislice spiral computed tomography; 16 row; coronary artery disease; image quality Introduction Coronary atherosclerosis is known to be the major cause for morbidity and mortality in the industrial world. Worldwide Correspondence: Dr S Schroeder, Department of Internal Medicine, Division of Cardiology, Eberhard-Karls-University Tuebingen, Otfried-Mueller-Str. 10, Tuebingen, Germany. Stephen.Schroeder@med.uni-tuebingen.de 3 These two authors contributed equally to this work. Received 27 February 2005; revised 15 August 2005; accepted 29 August 2005; published online 8 November 2005 almost 17 million persons are dying annually as a result of coronary artery disease (CAD). 1 Obesity is one of several known risk factor to develop CAD. 2 For some years, multislice spiral computed tomography (MSCT) is a tool for noninvasive visualization of the coronary arteries and modern MSCT-scanners permit the detection of coronary lesions with an overall good sensitivity (sens) and specificity (spec). 3 7 Apart from the heart rate and respiration, obesity is supposed to be one of the factors to influence image quality. However, at present, there are no data whether the accuracy of MSCT depends on the body mass index (BMI). Thus, we compared retrospectively the catheter-controlled MSCT

2 570 results from normal weight and obese pts in a cohort of 117 consecutive pts with regard to sens, spec, positive predictive value (PPV), negative predictive value (NPV) and image quality. Methods Patients and study protocol A retrospective analysis of data from 117 pts enrolled in a previously published study 7 was performed. All pts were scheduled for invasive coronary angiography due to suspicion of CAD or progression of previously diagnosed CAD. The study protocol had been approved by the local ethics committee and all participants had given informed consent. An inclusion criterion was the indication for invasive coronary angiography because of known or suspected CAD. MSCT and invasive coronary angiography was performed in all pts. Clinical exclusion criteria were renal failure (creatinine 41.5 mg/dl), unstable angina pectoris, acute myocardial infarction, known allergic reaction to contrast media (CM), increased exposure to radiation in the last 12 months (415 msv (millisievert)), hyperthyroidism (basal thyroidstimulation hormone o0.03 mu/liter in combination with elevated thyroid hormone levels in the peripheral blood), known epilepsy, liver dysfunction (glutamic oxaloacetic and glutamic pyruvic transaminase values 43 reference value), and advanced heart failure (New York Heart Association III and IV). To reduce motion artefacts caused by elevated heart rate 8,9 additional b-blockade with mg metoprolol per os (p.o.) was performed within 30 min prior to the CT scan in pts with heart rates 465/min. MSCT MSCT was performed by using a Sensation 16 Speed 4 Dt (Siemens, Forchheim, Germany) scanner. This technique allows the application of dedicated spiral algorithms that provide up to 188 ms of temporal resolution. ECG-gated heart phase selective imaging reconstruction was used in all pts. After a low dose precontrast spiral scan (collimation mm, 3.8 mm table feed/rotation, 120 kv, 133 mas (milli ampere seconds), rotation time 375 ms) with simultaneously recorded ECG signal, a test bolus of 20 ml of contrast medium and a chaser bolus of 20 ml of physiological saline solution were injected through an 18-gauge catheter into an antecubital vein to determine the circulation time. The following scan protocol was used: 0.75 mm collimation, caudocranial scan direction, 80 cc CM (400 mg iodine/ cc) with a biphasic injection protocol (50 ml at 4 ml/s and 30 ml at 2.5 ml/s), gantry rotation time 375 ms, temporal resolution 188 ms, effective slice thickness 1.0 mm, 120 kv, maximal 650 mas. All scans could be performed within one single breath hold (15 20 s). Algorithms optimized for retrospective ECG-gated multislice spiral CT were used for reconstructing the raw data. According to prior studies, image reconstruction was performed in the diastolic phase with a relative retrospective gating of 60% for all coronary arteries a first step. In case of impaired image quality additional image reconstruction was performed at different RR-intervals after a test series. The reconstructed data of the MSCT angiography (MSCTA) were transferred to a computer workstation for further processing (Leonardot, Siemens, Forchheim, Germany or Vitrea 2t, Vital Images, Minnetonlea, USA). The analyses were performed on conventional contrast enhanced axial slices, as well as on 3D volume rendering images (maximal intensity protection (MIP); multiplanar reformation (MIP)). Definitions The coronary tree was divided into 13 segments (sgts) according to a modified American Heart Association scheme. 10 The image quality of each sgt was determined as: (1) excellent free of motion artefacts, (2) good mild motion artefacts, (3) relevant artefacts but still diagnostic value, (4) severe calcification and (5) insufficient image quality missing sgt. Sgts with minor calcifications with or without additional motion artefacts were classified as sgts with relevant artefacts in case of a still diagnostic image quality. Otherwise they were classified as sgts with insufficient image quality. As previously reported each narrowing of more than 50% of the vessel diameter was determined to be a significant stenosis. 11 The degree of stenosis severity was classified by visual judgement. Vessel occlusion was defined as a total interruption of the contrast-enhanced lumen, whereas a missing segment showed no contrast enhancement at all. BMI was calculated as body weight in kilograms divided by the square of height in meters. According to the BMI the pts were categorized in three different groups: Group I BMIo25 kg/m 2, group II BMI kg/m 2, group III of 36 obese pts BMI 430 kg/m 2. Statistics Continuous variables are described as means and standard deviations. Categorical data are presented with absolute frequencies and percentages. Unpaired t-tests were used to compare continuous variables. w 2 tests were performed to evaluate differences between patient groups. Values of Po0.05 were considered to reveal statistically significant differences. All statistic analyses were performed using Prism 3.0t (GraphPad Software Inc., San Diego, CA, USA). Results Study population The study group consisted of 117 consecutive pts (79 male, 38 female) scheduled for conventional coronary angiography.

3 Table 1 Patient characteristics Group I Group II Group III 571 Number of pts Male 12 (57%) 43 (72%) 24 (67%) Female 9 (43%) 17 (28%) 12 (33%) Age (years) Number of risk factors *,# Body mass index (range) ( ) ( ) ( ) Additional beta-blockade prior to CT scan 15/21 (71%) 33/60 (55%) 20/36 (56%) Mean heart rate prior to CT scan (bpm) Agatston score *Po0.05 vs group II, # Po0.05 vs group III. Group I consisted of 21 normal weight pts (BMIo25 kg/m 2, years, number of risk factors , 12 male), group II of 60 pts with mild overweight (BMI kg/m 2, years, number of risk factors , 43 male) and group III of 36 obese pts (BMI 430 kg/m 2, years, number of risk factors , 24 male). Three pts had to be excluded from the analysis (group I 1 patient with incomplete breath hold, group II 1 patient with extravasation and one patient due to technical problems with the ECG-electrode). The number of risk factors was significantly lower in group I than in the other groups. The mean calcium burden expressed as Agatston score was comparable in all groups and the mean heartbeat prior to the scan was not significantly different. The patient characteristics are summarized in Table 1. Image quality Group I: 109/260 (42%) scanned sgts showed excellent image quality. 51/260 (20%) sgts could be visualized with good image quality. Relevant artefacts which allowed only to distinguish between the presence or absence of vessel occlusion were seen in 32/260 (12%). 15/260 (6%) sgts presented severe calcifications or stented lesions that did not allow to assess vessel narrowing accurately. 18/260 sgts (7%) could not be visualized by MSCT. Group II: 185/754 (25%) scanned sgts showed excellent, 263/754 (35%) sgts good image quality. Relevant artefacts were seen in 140/754 (19%). 109/754 (14%) sgts presented severe calcifications or stented lesions. 47/754 sgts (6%) could not be visualized. Group III: 106/468 (23%) scanned sgts showed excellent, 185/468 (39%) sgts good image quality. Relevant artefacts were seen in 88/468 (19%). 50/468 (11%) sgts presented severe calcifications or stented lesions that did not allow to assess vessel narrowing accurately. 39/468 sgts (8%) could not be visualized. The number of sgt with very good image quality was significantly higher in group I than in the other groups (Po0.0001). The total number of sgt with diagnostic (class I III) or nondiagnostic image quality (IV V) did not differ between the groups. Lesion detection Group I: 59 of 67 (88%) significant stenosis (450%) of the coronary vessels were correctly detected by MSCT. 187of 193 (97%) sgts without significant stenosis were correctly classified by MSCT. Sens, spec, PPV and NPV were calculated as follows: 0.88/0.97/0.91/0.96. Group II: 138 of 167 (83%) significant stenosis were correctly detected by MSCT. 569 of 587 (97%) sgts without significant stenosis were correctly classified by MSCT. Sens, spec, PPV and NPV: 0.83/0.97/0.88/0.95. Group III: 92 of 106 (87%) significant stenosis were correctly detected by MSCT. 358 of 362 (99%) sgts without significant stenosis were correctly classified by MSCT. Sens, spec, PPV and NPV: 0.87/0.99/0.96/0.96. Sens, spec, PPV and NPP were statistically not different in all three groups (w 2 test) (Figure 1). The data for sens, spec, PPV and NPV for each group and the total cohort are given in Table 2. Figure 2 gives an example of a mild stenosis of the proximal left anterior descending artery. Figure 1 Image example for excellent (1a þ 1d), good (1b þ 1e) and impaired but still diagnostic image quality (1cþ 1f). First row: Axial slices showing the left anterior descending artery (arrow). Second row: Volume rendering images of the corresponding vessel (fat arrow). 1a þ 1d: patient with a body mass index (BMI) o25 kg/m 2, 1bþ 1e: patient with a BMI between 25 and 30 kg/m 2,1cþ1f: obese patient (BMI 430 kg/m 2 ).

4 572 Table 2 Diagnostic accuracy of MDCT in detecting/excluding significant lesions Group I Group II Group III Total Sensitivity (%) Specificity (%) PPV (%) NPV (%) TP FP TN FN Invisible Segments 18/260 (7%) 47/754 (6%) 39/468 (8%) 104/1482 (7%) PPV ¼ positive predictive value, NPV ¼ negative predictive value, TP ¼ true positive, FP ¼ false positive, TN ¼ true negative, FN ¼ false negative. Figure 2 Image example showing a mild narrowing of the proximal left anterior descending artery by a soft plaque. (a) MSCT angiography (arrow indicates soft plaque) (b) corresponding invasive angiogram demonstrating the mild narrowing (arrow). Discussion The most important finding of the present study is that obesity actually had an influence on image quality, which did, however, not affect the diagnostic accuracy of noninvasive MSCT imaging. Although the number of sgts with very good image quality was significantly higher in normal weight pts, the total number of sgts with still diagnostic image quality was comparable in obese pts. In addition, the previously reported data on noninvasive lesion detection using 16-slice scanners 3,11 could be reproduced in our cohort. Just recently data from 64-slice scanners were published showing an even more accurate assessment of the coronary arteries and plaque area. However, in contrast to some 64-slice data pts with known CAD were not excluded in our cohort. As at the beginning of the MSCT era, coronary imaging was limited to the proximal vessel sgts, 15 continuous modifications of hardware and scan protocol, for example, lowering of the heart rate by application of betablockers, 8,9 has led to a significant stabilization and improvement of image quality. 16 Important determinants are temporal and spatial resolution. A breakthrough could be achieved by the 16-row MSCT scanner generation with a temporal resolution of 188 ms and an effective slice thickness of 1.0. The total scan time could be reduced to s and a complete heart scan could, thus for, be performed within one single breath hold, reducing breathing artefacts. Despite all technical improvements there are still limitations for noninvasive coronary angiography using MSCT. Patients with typical clinical symptoms or pathological stress exams should be transferred to invasive angiography as MSCT remains a sole diagnostic tool without the option to perform angioplasty. Although coronary artery bypass grafts can be easily visualized, the assessment of the native vessels in these pts remains a major limitation due to the severe calcifications in advanced CAD. Moreover, reliable stent assessment seems not to be possible with 16-slice computed tomography. Obesity is considered to be another limiting factor of image quality in MSCT imaging, because of an impaired image quality. Our data show, that, as suspected, the total number of sgts with very good image quality was higher in normal weight pts. However, the scan protocol used and the total amount of CM administered was the same in all pts and not adjusted to BMI. An adaptation of the scan protocol with an increase of tube voltage and amount of CM could presumably have led to an improvement of image quality also in obese pts. This concept would have an influence on possible side effects, especially on renal function, and would lead to an additional increase of radiation exposure. To reduce radiation dose ECG-pulsed tube current modulation was used in all pts. However, as shown in prior studies 20,21 a radiation exposure of at least 6 msv has to be calculated for calcium scoring and MSCT angiography. This remains one main limitation of noninvasive MSCT coronary angiography. It was found of special interest, that obesity did not affect the diagnostic accuracy at least in our study population with a mean BMI of Sens and spec for the detection or exclusion of severe lesions were not significantly different in all groups. This might be caused by the fact that the number of analyzable sgt was equal in normal weight and obese pts. MSCT as a first-line imaging technique is already used in clinical practice. 22 Based on our findings noninvasive coronary angiography using MSCT might be helpful even in obese pts, especially to exclude CAD. However, despite

5 continuous technical improvements pts without stable heart rhythm, severe renal insufficiency or contraindications for the administration of iodinated contrast agents are not suitable for MSCT. Study limitations The data of this study are based on retrospective findings. Prospective studies with larger patient number are required to re-evaluate our results. In addition, our most obese pt had a BMI of 38.1 kg/m 2. Thus, our conclusions cannot be transferred to even more obese pts. Conclusions The 16-slice MSCT technology with a fast gantry rotation time does allow for the detection of significant coronary lesions with high sens and spec. Overweight and obesity have an impact on MSCT image quality but did not hamper the diagnostic accuracy. Thus, MSCT is a noninvasive method to detect or rule out CAD also in pts with higher BMI. These retrospective data have to be confirmed in larger prospective trials. 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