After excluding patients with known coronary artery disease or myocardial lesions, each group

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1 中華放射醫誌 Chin J Radiol 2009; 34: The Image Quality and Feasibility of Delayed Phase in Cardiac CT: Prospective ECG-Triggering Sequential Scan Versus Retrospective ECG-Gating Spiral Scan Using the Same Radiation Dose Wei-Lin Tsai 1,2,3 I-Chen Tsai 1,2 Min-Chi Chen 1 Pao-Chun Lin 1 Si-Wa Chan 1 Hui-Ching Ho 4 Wan-Chun Liao 1 Clayton Chi-Chang Chen 1 Department of Radiology 1, Biostatistics Task Force, Department of Education & Research 4, Taichung Veterans General Hospital Department of Medicine and Institute of Clinical Medicine 2, National Yang Ming University Department of Radiology 3, Chiayi Veterans Hospital To compare the image quality and interpretability of two delayed phase protocols of cardiac CT under the same low radiation dose: prospective ECG-triggering sequential scan (modified from the calcium score protocol) versus retrospective ECG-gating spiral scan with ECG-pulsing. From March to Apr i l of 20 07, i n al l the patients referred for cardiac CT, prospective ECG-triggering sequential scan was used for the delayed phase. In May and June, retrospective ECG-gating spiral scan was used in all such patients. The radiation dose was pre-calculated to match the two protocols. The marginal sharpness, slab artifact and beam hardening artifact were graded 1 (worst) to 5 (best) semi-quantitatively for comparison. Grades 1 and 2 were considered uninterpretable. In addition, the demographic data of the patients, image noise, scan time, and radiation dose by dose-length-product were compared. After excluding patients with known coronary artery disease or myocardial lesions, each group Reprint requests to: Dr. I-Chen Tsai Department of Radiology, Taichung Veterans General Hospital. No. 160, Sec. 3, Taichung Harbor Road, Taichung 407, Taiwan, R.O.C. included 31 patients for comparison. The prospective ECG-triggering sequential scan rated good in less noise and better marginal sharpness, and the retrospective ECG-gating spiral scan rated good in less slab and beam hardening artifact. Considering the interpretability, in the sequential group, 41.9% and 85.5% of scans were uninterpretable due to severe slab and beam hardening artifact, respectively, whereas the spiral group reached 100% interpretability. Conclusion: Under the similar low radiation dose, retrospective ECG-gating spiral scan with ECG-pulsing should be used instead of prospective ECG-triggering sequential scan (modified from the calcium score protocol) for delayed phase of cardiac CT. The sequential protocol is not clinically feasible due to severe slab and beam hardening artifact in a significant portion of patients. There have been many studies on the delayed hyperenhancement of cardiac magnetic resonance image (MRI) [1-5]. MRI delayed phase is considered to be a relatively mature imaging technique [3]. With technical advance of multi-detector row CT (MDCT), the effective temporal resolution has improved due to shorter rotation time and the multi-segment reconstruction (MSR) technique [6]. Paul et al. demonstrated that the late defect on MDCT is closely related to the residual perfusion defect on single photon emission computed tomography (SPECT) [7]. Mahnken et al. showed that

2 68 Delayed phase cardiac CT: Prospective VS retrospective late enhancement on MDCT is as reliable as MRI in assessing infarct size and myocardial viability in acute myocardial infarction [8]. Because a close relationship between the prognosis and either SPECT or MRI has been established, the lesions seen in the delayed phase of CT could probably also correlate with the patient s prognosis [5, 9-13]. This may provide another role for CT in cardiovascular medicine. There are two major methods for synchronizing heart beat and scanning on MDCT. The first is prospective electrocardiography (ECG)-triggering sequential scan. The second is retrospective ECGgating spiral scan. The difference between two methods has been discussed on calcium score [14, 15]. The reproducibility of retrospective ECG-gating spiral scan is better, but its limitation is high radiation dose [16-19]. There are t wo major differences bet ween delayed phase and calcium score: the interpreting plane and the interpreted target. First, the measurement of calcium score is done in the axial plane [20]. But since the myocardium is the major concern in the delayed phase [7, 8], cardiac axes including short, horizontal and vertical axes are important [8]. It is obvious that spiral scan provides better continuity of the images [14, 15]. The second difference is the size of the interpreted target. Calcium score is used to measure tiny calcifications which are only a few millimeters in size [20]. On the other hand, the delayed phase is for myocardium, which may be several millimeters to centimeters in thickness. Thus, the requirements of noise control and spatial resolution might not be so critical in the delayed phase. With these concerns, we believe that even if we lower the tube current in the spiral scan to achieve the same low radiation dose as with sequential scan, the image quality might still meet our criteria for an interpretable delayed-phase image. In order to avoid motion artifact and freeze the myocardium, synchronization with ECG is necessary. Therefore, we only compared prospective ECGtriggering sequential scan with retrospective ECGgating spiral scan with ECG-pulsing. The cardiac axes used for left ventricle (LV) myocardial assessment are different from the direction of tube rotation. Even though the slab artifact will not impair the axial image interpretation, it will impair the quality of cardiac axes images [21]. Concerning the detection of attenuation change, the threshold is defined as 20 HU in Mahnken s study on correlation with MRI, so even if the normal myocardium is affected by the beam hardening artifact caused by bony structures [21, 22], the artifact should not exceed this range (20 HU). Because the delayed phase is usually performed after CT coronary angiography, low radiation dose for delayed phase is an important prerequisite. In this study, we tried to define the necessary conditions of a good delayed phase protocol, and then used this scoring system to evaluate the prospective ECG-triggering sequential scan (modified from the calcium score protocol) and retrospective ECG-gating spiral scan. In addition, we wanted to answer the question: under the same low radiation dose, which one is the better protocol for delayed phase of cardiac CT? MATERIALS AND METHODS Patient selection T he st udy proposal was approved by the Institutional Review Board of our hospital. Our cardiac CT routine for patients with suspected coronary artery disease includes CT coronary angiography and a 6-minute delayed phase. The study population included all patients referred for cardiac CT from March to June There were 69 patients in March and April, and delayed phases were scanned with the prospective ECG-triggering sequential scan. The 78 patients in May and June were scanned with the retrospective ECG-gating spiral scan. Because this was a study of image quality, only patients with normal myocardium were included. We excluded patients with history of myocardial infarction (15 and 21 patients in the sequential group and spiral group, respectively), significant stenosis in CT coronary angiography (14 and 17 patients), impaired LV motion (5 and 7 patients), focal thinning or hypertrophy of the LV wall (9 and 8 patients), symptoms of typical angina (10 and 14 patients), diabetes mellitus (4 and 3 patients), and autoimmune disease (1 and 2 patients). Some of the patients had more than one exclusion criteria. After exclusion of patients for the abovementioned criteria, 31 patients in March and April and 31 patients in May and June were included. Patient preparation From March to April, all patients referred for cardiac CT were asked if they would consent to have their image data used for further scientific analysis and publication. In May and June, explanation about protocol changes was also given. We also emphasized that the CT coronary angiography would

3 Delayed phase cardiac CT: Prospective VS retrospective 69 always be performed under the same standard protocol regardless of which delayed phase protocol was used. Patients were asked if they had any history of renal insufficiency or asthma. The serum creatinine data on the chart was checked. If no contraindication, the possible adverse effects of beta-blocker and iodinated contrast medium were explained. Then the heart rate was measured. For those with heart rate higher than 71 bpm, 40 mg propranolol was administered orally 60 minutes before scanning. If heart rate was lower than 70 bpm, the patient was scanned immediately. The scan was performed with a 40-detector row CT (Brilliance 40, Philips, Best, the Netherlands). In CT coronary angiography, 100 ml of Iohexol (Omnipaque 350, Amersham, Cork, Ireland) was injected at a flow rate of 4 ml/s, and then followed by 30 ml saline chaser under the same flow rate. Bolus tracking technique was used for synchronization between scanning and contrast injection. The scan started 5 mm below the carina to heart base. The delayed phase was begun 6 minutes after the start of contrast injection. Protocols for the sequential and spiral groups The prospective ECG-triggering sequential scan was a protocol modified from the calcium score, using 120 kv tube voltage, 280 ma tube current, 0.5 s rotation time, effective tube current of 140 mas and 32 x 1.25 mm collimation to acquire the image at a 70% RR-interval. The spiral protocol included 120 kv tube voltage, 67.2 ma tube current, pitch of 0.2, rotation time of 0.5 s, effective tube current of 168 mas/slice, and 32 x 1.25 mm collimation with ECG pulsing (Table 1). The tube current in the spiral protocol was determined by the calculation based on the dose of sequential protocol. We used CT dose index (CTDI) and dose length product (DLP) to estimate and calculate the radiation dose. A factor of msv/(mgy-cm) was used to convert DLP into an effective dose [23]. In the delayed phase acquired by sequential scan, because the coverage was 40 mm, the scan length had to be 4, 8, 12 or 16 cm. If the scan length is 12 cm, then the DLP will be 98.8 mgy*cm. If the scan length is 16 cm, then the DLP will be mgy*cm. Twelve centimeters length (3 x 40 mm) was used in 38.9% (49/126) of patients. But 61.1% (77/126) of patients needed 16 cm (4x40 mm) to cover the whole heart. According to our calculations, the average DLP was mgy*cm, converting to an effective dose of 2.02 msv. According to a phantom study of ECG-pulsing, this new ECG modulated tube current control technique could save 46% radiation dose [16, 19]. However, because one of the prerequisites of CTDI and DLP is a constant tube current during scanning [24], the scanner can not display the real DLP after dose modulation. The displayed DLP will be the dose without ECG-pulsing [18]. Accordingly, we divided 2.02 msv by 54% and the result was 3.74 msv, which is equal to a DLP of mgy*cm. Using the CT console to calculate the radiation dose, the tube current would be 168 mas/slice in spiral protocol. Thus, when we used 168 mas/slice in spiral protocol, the displayed DLP Table 1. Protocol parameters for sequential and spiral scans Scan mode Sequential Spiral ECg synchronization Prospective ECg-triggering Retrospective ECg-gating Collimation 32 x 1.25 mm 32 x 1.25 mm Tube voltage (kv) Tube current (ma) Effective tube current per slice (mas/slice) Rotation time (s) Reconstruction Thinnest: 1.25 mm Thinnest: 1.4 mm, increment: 0.7 mm acquired phase during RR-interval 70% 70% others Pitch = 0.2 ECg-pulsing

4 70 Delayed phase cardiac CT: Prospective VS retrospective was mgy*cm, but actually we only exposed 54% of the dose to the patient, which was equal to the radiation dose in the sequential scan. Thus, we believe that the two protocols utilize the same radiation dose. Regarding the reconstruction, the thinnest reconstruction thickness was used for both protocols. Due to the continuous nature of the spiral mode [25], an increment of 0.7 mm with 50% overlap was used in reconstruction of the spiral protocol. Though the radiation doses in both protocols were calculated, the DLP displayed on the console was still recorded. The DLP in the sequential group was directly recorded, whereas the DLP in the spiral group was timed by 54% before recording. Then the radiation dose was compared. Evaluation of image quality Independent image evaluations were conducted by two cardiac radiologists at different times. One had 4 years experience with cardiac MR and had interpreted more than 100 cases of cardiac CT. The other had interpreted more than 300 cases of cardiac CT. Images of delayed phase were evaluated on the workstation (Extended Brilliance Workspace, Philips, Best, the Netherlands). The window level and width were set at 100/250, respectively, and the thickness was set at 5 mm with average slab. The interpreting radiologist could freely evaluate the six axes including axial, coronal, sagittal, short, horizontal long and vertical long axes (Figures 1, 2). Images with good quality should have good anatomic delineation (which may be impaired by poor marginal sharpness, slab artifact, and noise) and homogenous myocardial attenuation (may be impaired by slab artifact, beam hardening artifact, and noise). A five-grade scoring system is used for evaluating marginal sharpness, slab artifact and beam hardening artifact. Grade 1 is the worst, grade 5 is the best. Grades 1 and 2 are considered uninterpretable. Grades 3, 4 and 5 are considered interpretable. The detailed definitions of each evaluation are as follows: Margin sharpness: Can the outer and inner borders of the LV wall be clearly identified? Grade 5, very clear; grade 4, distinguishable but smoother than grade 5; grade 3, faintly visible; grade 2, only visible if window width and level are adjusted; grade 1, the border can not be identified. Slab artifact: Slab artifact is defined as a sharp and obvious stepladder inconsistency between slices while viewed in coronal and sagittal axes [21]: grade 5, no slab artifact; grade 4, minimal slab mismatching without attenuation change; grade 3, minor attenuation change less than 20 HU with or without slab mismatching; grade 2, obvious slab mismatching more than 5 mm with or without attenuation change; grade 1, obvious attenuation difference of more than 20.1 HU with or without slab mismatching. Beam hardening artifact: Beam hardening artifact is defined as a fan or radiating shape of attenuation difference related to the dense bony structures such as spine, scapula or ribs. The axial plane is the only plane used for evaluation. The attenuation difference can be higher (software correction of the artifact) or lower (beam hardening artifact itself) than the surrounding tissue [21, 22]. Then the attenuation difference between the myocardium affected by the artifact and normal myocardium is measured: grade 5, no beam hardening artifact; grade 4, mild beam hardening artifact with attenuation difference less than 10 HU; grade 3, moderate beam hardening artifact with attenuation difference between 10.1 HU and 20 HU; grade 2, marked beam hardening artifact with the attenuation difference between 20.1 to 50 HU; grade 1, severe beam hardening artifact with attenuation difference more than 50.1 HU. Then the noise of the image is measured by the operating technologist. The first image in the axial plane is used. A circular region of interest with area of 1 cm2 is placed in the center of the ascending aorta to measure the mean CT number and standard deviation. The standard deviation is recorded as noise measurement [18] (Figures 1, 2). Statistical analysis SPSS version 11.5 (Chicago, Illinois, USA) was used for statistical analysis. Values were given as mean ± standard deviation. The demographic data, average and standard deviation of heart rate, estimated effective dose, and real dose length product of both groups were compared by independent t test to determine any significant difference between the two groups. The marginal sharpness, slab artifact, beam hardening artifact, noise and scan time of the two groups were compared by using the Mann- Whitney U test due to its non-parametric nature. Then the inter-observer agreement was calculated by kappa statistics. The kappa statistics were weighted as follows: 0 to 0.2 low, 0.21 to 0.4 moderate, 0.41 to 0.6 good, 0.61 to 0.8 substantial, and >0.81 perfect agreement. Significant difference was considered when p<0.05.

5 Delayed phase cardiac CT: Prospective VS retrospective 71 1b 1a 1d 1c Figure 1. Representative case in the sequential group: a 64 year-old male. a. The axial image shows very clear margin of the left ventricle, which was graded 5. (arrow) b. The coronal image shows marked slab artifact. The upper slab artifact (arrow) shows only spatial mismatching. But the lower slab artifact (arrowheads) shows not only spatial mismatching but also attenuation change. In this case, the slab artifact was graded 1. c. At the level of densely calcified vertebral body, marked beam hardening artifact can be identified (arrow). The fan-shape artifact produced attenuation difference of more than 20 HU between the affected and normal myocardium, which interfered with the interpretation. Thus, the beam hardening artifact was graded 2. d. On the first axial image, a circular region of interest was placed at the center of the ascending aorta to measure the noise. In this case, the noise measurement was 12.5 HU. There was generally less noise in the sequential group than in the spiral group.

6 72 Delayed phase cardiac CT: Prospective VS retrospective 2a 2b 2d 2c Figure 2. Representative case in the spiral group: a 62-year-old male. a. The axial image shows distinguishable but smoother than very clear left ventricular myocardium (arrow). The marginal sharpness was graded 4. b. No obvious slab artifact can be identified in the coronal image. The grade of slab artifact for this image was 5. c. At the level of vertebral body, no definite beam hardening artifact can be identified. Scrolling through the whole study (not shown), the grade of beam hardening in this case was 5. d. On the first axial image, the noise measurement was 15.9 HU. The noise was generally higher in the spiral group than in the sequential group.

7 Delayed phase cardiac CT: Prospective VS retrospective 73 RESULTS Demographic data After exclusion of patients with known coronary artery disease or myocardial lesions, there were 31 patients in each group. No significant difference between the two groups was identified in gender, age, height, body weight, body mass index, mean heart rate, heart rate standard deviation and estimated effective dose (Table 2). Scan time The scan time was significantly longer in the spiral group (Table 3). The scan time of the spiral group was shorter than that of CT coronary angiography because the table feed was faster. In CT coronary angiography, the parameters were 40x0.625 mm, pitch 0.2, rotation time 0.42 s and the actual table feed was 12 mm/s. The parameters in the delayed-phase spiral protocol were 32x1.25 mm, pitch 0.2, rotation time 0.5 s and the actual table feed was 16 mm/s. Image quality (Table 3) Both observers reported significantly better marginal sharpness and less noise in the sequential group. However, the scores of marginal sharpness in the two groups were either equal or more than three, which means the image quality in both groups for marginal sharpness was interpretable. Both observers reported significantly more slab and beam hardening artifact for the sequential group. Furthermore, a significant portion of patients images were not considered interpretable by either observer. For observer 1, in the sequential group, 45.2% of patients images were not interpretable due to slab artifact and 83.9% of patients images were not interpretable due to beam hardening artifact. For observer 2, also in the sequential group, the slab and beam hardening artifact were so severe that 38.7% and 87.1%, respectively, of patients images were not interpretable. Averaging the results of both observers, the uninterpretable ratio was 41.9% for slab artifact and 85.5% for beam hardening artifact. All the cases in the spiral group were considered interpretable. Inter-observer agreement Regarding the inter-observer agreement, combining all 62 cases, the kappa statistics for both observers in marginal sharpness, slab artifact and beam hardening artifact were 0.792, 0.842, and 0.872, respectively. This means a substantial to perfect agreement in image quality even with different observers. Table 2. Demographic data between groups group a (Sequential) (no.= 31) group B (Spiral) (no.= 31) P-value a gender F 8 (25.8%) 11 (35.5%) M 23 (74.2%) 20 (64.5%) age ± ± height (cm) ± ± Body mass index (m/kg 2 ) ± ± Body weight (kg) ± ± Mean heart rate (bpm) ± ± heart rate standard deviation (bpm) 7.71 ± ± Estimated dose length product (mgy*cm) ± ± Estimated effective dose (msv) 1.79 ± ± a Independent t test.

8 74 Delayed phase cardiac CT: Prospective VS retrospective Table 3. Image quality and scan time comparison observer 1 group a (Sequential) (no.= 31) group B (Spiral) (no.= 31) p value a Margin sharpness ** 1 b 0 (0.0 %) 0 (0.0 %) 2 b 0 (0.0 %) 0 (0.0 %) 3 7 (22.6 %) 14 (45.2 %) 4 17 (54.8 %) 17 (54.8 %) 5 7 (22.6 %) 0 (0.0 %) Slab artifact < ** 1 b 0 (0.0 %) 0 (0.0 %) 2 b 14 (45.2 %) 0 (0.0 %) 3 12 (38.7 %) 0 (0.0 %) 4 5 (16.1 %) 5 (16.1 %) 5 0 (0.0 %) 26 (83.9 %) Beam hardening artifact < ** 1 b 12 (38.7 %) 0 (0.0 %) 2 b 15 (48.4 %) 0 (0.0 %) 3 4 (12.9 %) 4 (12.9 %) 4 0 (0.0 %) 5 (16.1 %) 5 0 (0.0 %) 22 (71.0 %) observer 2 Margin sharpness * 1 b 0 (0.0 %) 0 (0.0 %) 2 b 0 (0.0 %) 0 (0.0 %) 3 6 (19.4 %) 13 (41.9 %) 4 22 (71.0 %) 17 (54.8 %) 5 3 (9.7 %) 1 (3.2 %) Slab artifact < ** 1 b 0 (0.0 %) 0 (0.0 %) 2 b 12 (38.7 %) 0 (0.0 %) 3 14 (45.2 %) 0 (0.0 %) 4 5 (16.1 %) 6 (19.4 %) 5 0 (0.0 %) 25 (80.6 %) Beam hardening artifact < ** 1 b 13 (41.9 %) 0 (0.0 %) 2 b 14 (45.2 %) 0 (0.0 %) 3 4 (12.9 %) 3 (9.7 %) 4 0 (0.0 %) 6 (19.4 %) 5 0 (0.0 %) 22 (71.0 %) noise (hu) ± ± 5.74 < ** Scan time (sec) 5.56 ± ± 1.00 < ** a Mann-Whitney u test. *: P < 0.05; **: P < 0.01 b Score of 1 or 2 was considered uninterpretable.

9 Delayed phase cardiac CT: Prospective VS retrospective 75 DISCUSSION Delayed phase of cardiac CT This study tried to answer the question: Under the same low radiation dose, which protocol is the better protocol for delayed phase of cardiac CT: prospective ECG-triggering sequential scan or retrospective ECG-gating spiral scan? Our study revealed that the spiral protocol can produce studies with 100% interpretability, whereas the sequential protocol produces a significant portion of uninterpretable images due to severe slab artifact (41.9%) and beam hardening artifact (85.5%). Even though the sequential scan has advantages of low noise, better marginal sharpness and short scan time, it is not a clinically feasible protocol for delayed phase of cardiac CT. Thus, spiral protocol is the protocol of choice. The delayed phase of cardiac CT mainly focuses on the attenuation change. Some studies have used 20 HU as criteria [8], and some have used subjective observation [7]. In our study, the beam hardening artifact easily exceeded 20 HU attenuation change in the sequential group. Thus, it was judged as clinically uninterpretable. According to Mahnken s study [8], the average difference between the delayed phase and lesion was 33.5 HU. But in our study, the beam hardening artifact created attenuation change of more than 50 HU in 38.7% and 41.9% of patients as evaluated by the two observers, respectively. If the lesions are located right on the beam hardening artifact, they will be easily missed. We propose several reasons for the marked slab and beam hardening artifact reduction in spiral scan compared with that of the sequential scan. First, there is the MSR technique. MSR can improve the effective temporal resolution [14, 15]. Therefore, the myocardium can be frozen better. Second, the nature of over-sampling and interpolation of spiral scan makes the image smoother. Third, spiral scan uses retrospective ECG-gating, and the reconstruction can be adjusted by the ECG signal acquired retrospectively. But in the sequential scan, the scan is triggered prospectively according to the previous heart beat. If the heart rate changes suddenly, the scan might be triggered at the wrong time, and then slab artifact will appear. This phenomenon has been reported in previous studies on calcium score: when the heart rate changes, the image quality degrades more in the sequential scan than in the spiral scan [14, 15]. And finally, why was the beam hardening artifact in the sequential scan so severe? One possible reason may be that the sequential scan acquires images in a continuous half scan. If during the 180o rotation, the X-ray traverses the scapula or vertebral body, the bone mass will cause severe beam hardening artifact on the myocardium. The spiral scan can avoid this by over-sampling, MSR and interpolation. Under the same radiation dose, even the tube current at any time during the spiral protocol is lower than during the sequential scan (67.2 ma vs. 280 ma). However, the interpretability is better, probably due to the above reasons. There is even more noise with the spiral group, but for myocardial interpretation, the attenuation change is a focal area, not a single pixel. Noise is always scattered, and does not interfere with the interpretation. Clinical implication With more and more MDCT being installed, radiologists are continuing to investigate the field of the heart. Following the calcium score [20] and CT coronary angiography [26], delayed phase might be another new field of cardiac CT [7, 8]. We have to utilize the radiation dose effectively to provide interpretable images. The protocol design is one of the most important parts of radiation dose reduction. Our study suggests that, if the scanner supports the function of ECG-pulsing, using the same radiation dose, retrospective ECG-gating spiral scan can obtain images with better interpretability than prospective ECG-triggering sequential scan. Thus, utilizing retrospective ECG-gating spiral scan in delayed phase of cardiac CT should be encouraged. Inter-observer agreement Even though both radiologists had different training backgrounds, one in cardiac MR, the other in cardiac CT, their evaluations of image quality showed substantial to perfect agreement. The high kappa value might be due to the semi-quantitative five-grade system. Therefore, we obtained great inter-observer agreement. Radiation estimate Regarding how much radiation dose could be saved by applying ECG-pulsing, different results have been obtained in different studies [16-19]. But the different results might have been caused by the different heart rate in different studies. In those studies with lower heart rate, such as 60 bpm, 46% dose could be reduced [16, 17]. With faster heart

10 76 Delayed phase cardiac CT: Prospective VS retrospective rate, the reduction would be less. Gerber s study only had a 28% dose reduction in 70 bpm [18]. In our patient group, the heart rate was reduced to near 60 bpm for better image quality in CT coronary angiography. Therefore, we applied the result from the study with the lower heart rate [16]. Limitation In our study, we used 120 kv rather than 80 kv [7, 8] for delayed phase of cardiac CT. To reduce the radiation dose, some studies used 80 kv and raised the ma slightly to achieve a lower radiation dose than a standard 120 kv protocol in pediatric application [27, 28]. However, there is still some debate about the image quality and contrast resolution. Some phantom studies showed severe beam hardening artifact even in pediatric phantom [29]. To date, there has not been any systemic comparison of 80 kv and 120 kv in the delayed phase of adult cardiac CT. Furthermore, because the image contrast changes when the energy is reduced from 120 kv to 80 kv, the attenuation measured in the delay phase can not be compared with the attenuation measured on the 120 kv CT coronary angiography. Another limitation is if we want to compare these two protocols, the best way is to have the same patients scanned with both protocols, reducing the patient bias. However, we think it is unethical to expose a patient twice if another study design can be used. Therefore, we recruited two patient groups with comparable demographic data in this study. The third limitation is that only the patients with normal myocardium were included in this study, because this was a study of image quality. Therefore, which protocol to be better at identifying infarcted myocardium needs further studies with reference standard. CONCLUSION In conclusion, under the similar low radiation dose, tube voltage, and rotation time, retrospective ECG-gating spiral scan with ECG-pulsing should be used instead of prospective ECG-triggering sequential scan (modified from the calcium score protocol) for delayed phase of cardiac CT. The sequential protocol is not clinically feasible in a significant portion of patients due to slab artifact and beam hardening artifact. REFERENCES 1. Ingkanisorn WP, Rhoads KL, Aletras AH, Kellman P, Arai AE. Gadolinium delayed enhancement cardiovascular magnetic resonance correlates with clinical measures of myocardial infarction. J Am Coll Cardiol 2004; 43: Allard M, Doucet D, Kien P, Bonnemain B, Caille JM. Experimental study of DOTA-gadolinium. Pharmacokinetics and pharmacologic properties. Invest Radiol 1988; 23 Suppl 1: S Thomson LE, Kim RJ, Judd RM. Magnetic resonance imaging for the assessment of myocardial viability. J Magn Reson Imaging 2004; 19: Hillenbrand HB, Kim RJ, Parker MA, Fieno DS, Judd RM. Early assessment of myocardial salvage by contrast-enhanced magnetic resonance imaging. Circulation 2000; 102: Gerber BL, Garot J, Bluemke DA, Wu KC, Lima JA. Accuracy of contrast-enhanced magnetic resonance imaging in predicting improvement of regional myocardial function in patients after acute myocardial infarction. Circulation 2002; 106: Flohr T, Ohnesorge B. Heart-rate adaptive optimization of spatial and temporal resolution for ECG-gated multislice spiral CT of the heart. J Comp Assist T 2001; 25: Paul JF, Wartski M, Caussin C, et al. Late defect on delayed contrast-enhanced multi-detector row CT scans in the prediction of SPECT infarct size after reperfused acute myocardial infarction: initial experience. Radiology 2005; 236: Mahnken AH, Koos R, Katoh M, et al. Assessment of myocardial viability in reperfused acute myocardial infarction using 16-slice computed tomography in comparison to magnetic resonance imaging. J Am Coll Cardiol 2005; 45: Miller TD, Christian TF, Hopfenspirger MR, Hodge DO, Gersh BJ, Gibbons RJ. Infarct size after acute myocardial infarction measured by quantitative tomographic 99mTc sestamibi imaging predicts subsequent mortality. Circulation 1995; 92: Burns RJ, Gibbons RJ, Yi Q, et al. The relationships of left ventricular ejection fraction, end-systolic volume index and infarct size to 6-month mortality after hospital discharge following myocardial infarction treated by thrombolysis. J Am Coll Cardiol 2002; 39: Gibbons RJ, Valeti US, Araoz PA, Jaffe AS. The quantification of infarct size. J Am Coll Cardiol 2004; 44: Volpi A, De Vita C, Franzosi MG, et al. Determinants of 6-month mortality in survivors of myocardial infarction after thrombolysis: results of the GISSI-2 data base. The ad hoc working group of the Gruppo Italiano per lo Studio della Sopravvivenza nell Infarto Miocardico (GISSI)-2 data base. Circulation 1993; 88: Miller TD, Christian TF, Hopfenspirger MR, Hodge DO, Gersh BJ, Gibbons RJ. Technetium-99m sestamibi infarct size predicts mortality. Am J Cardiol 1998; 81:

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12 78 Delayed phase cardiac CT: Prospective VS retrospective 心臟電腦斷層延遲相的影像品質和可行性分析 : 相同輻射劑量下前瞻性心電圖觸發順序掃描與回溯性心電圖門控螺旋掃描之比較 蔡瑋琳 1,2,3 蔡依橙 1,2 陳明至 1 林保鐘 1 陳詩華 1 何惠卿 4 廖婉君 1 陳啟昌 1 台中榮民總醫院放射線部 1 教學研究部生物統計小組 4 國立陽明大學醫學系與臨床醫學研究所 2 嘉義榮民醫院放射線部 3 比較在相同輻射劑量下, 以前瞻性心電圖觸發順序掃描 ( 修改自鈣化分數掃描技術 ) 與回溯性心電圖門控螺旋掃描, 施作延遲相心臟電腦斷層之影像品質與可行性自 2007 年三月到四月, 以前瞻性心電圖觸發順序掃描技術施作所有延遲相之心臟電腦斷層 2007 年五月到六月的患者則以回溯性心電圖門控螺旋掃描施作 輻射劑量皆經事先計算和設定以調整至相同等級 影像邊緣銳利度 層狀假影 射束硬化假影以 grade 1( 最差 ) 到 grade 5( 最佳 ) 作半定量分析, 其中 grade 1 和 grade 2 定義為無法判讀 並以統計方法比較病患基本資料 影像雜訊 假影差異 掃描時間 以及輻射劑量 排除已知有冠狀動脈或心肌疾病的病患後, 兩種掃描方式各自有 31 名患者列入研究 前瞻性心電圖觸發順序掃描在影像雜訊和邊緣銳利度上表現較佳 ; 回溯性心電圖門控螺旋掃描在層狀假影和射束硬化假影上表現較佳 若考慮是否可以判讀, 則前瞻性心電圖觸發順序掃描組因嚴重層狀假影和射束硬化假影分別造成 41.9% 和 85.5% 的不可判讀比例 ; 而螺旋掃描組則 100% 皆可判讀 施作心臟電腦斷層延遲相時, 若使用相同輻射劑量, 則應使用回溯性心電圖門控螺旋掃描而非前瞻性心電圖觸發順序掃描 由於前瞻性心電圖觸發順序掃描會造成嚴重的層狀假影和射束硬化假影, 在臨床使用上是不可行的