中華放射醫誌 Chin J Radiol 2009; 34: 67-78 67

To compare the image quality and interpret-ability of two delayed phase protocols of cardiac CT under the same low radiation dose: prospec-tive ECG-triggering sequential scan (modif ied from the calcium score protocol) versus retrospec-tive ECG-gating spiral scan with ECG-pulsing.

From March to Apr i l of 20 07, i n a l l the patients referred for cardiac CT, prospective ECG-triggering sequential scan was used for the delayed phase. In May and June, retrospec-tive 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 com-pared.

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

included 31 patients for comparison. The prospec-tive 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 uninter-pretable 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 prospec-tive ECG-triggering sequential scan (modif ied 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 signif icant portion of patients.

There have been many studies on the delayed hyperenhancement of cardiac magnetic resonance image (MRI) [1-5]. MRI delayed phase is consid-ered to be a relatively mature imaging technique [3]. With technical advance of multi-detector row CT (MDCT), the effect ive temporal resolut ion has improved due to shorter rotation time and the mult i-segment reconst ruction (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 tomog-raphy (SPECT) [7]. Mahnken et al. showed that

The Image Quality and Feasibility of DelayedPhase in Cardiac CT: Prospective ECG-Triggering Sequential Scan VersusRetrospective ECG-Gating Spiral Scan Usingthe Same Radiation DoseWEI-LIn TSaI1,2,3 I-ChEn TSaI1,2 MIn-ChI ChEn1 Pao-Chun LIn1 SI-Wa Chan1 huI-ChIng ho4 Wan-Chun LIao1 CLayTon ChI-Chang ChEn1

Department of Radiology1, Biostatistics Task Force, Department of Education & Research4, Taichung Veterans General Hospital Department of Medicine and Institute of Clinical Medicine2, National Yang Ming University Department of Radiology3, Chiayi Veterans Hospital

Reprint requests to: Dr. I-Chen TsaiDepartment of Radiology, Taichung Veterans general hospital.no. 160, Sec. 3, Taichung harbor Road, Taichung 407, Taiwan, R.o.C.

Delayed phase cardiac CT: Prospective VS retrospective68

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 pro-vide another role for CT in cardiovascular medicine.

There are two major methods for synchronizing hear t beat and scanning on MDCT. The f irst is prospective electrocardiography (ECg)-triggering sequential scan. The second is retrospective ECg-gating 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 radia-tion dose [16-19].

There a re t wo major d i f ferences bet ween delayed phase and calcium score: the interpreting plane and the interpreted target. First, the measure-ment 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 mil-limeters 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 interpre-table delayed-phase image.

In order to avoid motion artifact and freeze the myocardium, synchronization with ECg is neces-sary. Therefore, we only compared prospective ECg-triggering sequential scan with retrospective ECg-gating spiral scan with ECg-pulsing. The cardiac axes used for left ventricle (LV) myocardial assess-ment are different from the direction of tube rota-tion. Even though the slab artifact will not impair the axial image interpretation, it will impair the quality of cardiac axes images [21]. Concerning the detec-tion 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 struc-

tures [21, 22], the artifact should not exceed this range (20 hu). Because the delayed phase is usu-ally performed after CT coronary angiography, low radiation dose for delayed phase is an important pre-requisite.

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 pro-spective 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 selectionT he s t udy proposa l was approved by t he

Institutional Review Board of our hospital. our car-diac CT routine for patients with suspected coronary artery disease includes CT coronary angiography and a 6-minute delayed phase. The study popula-tion included all patients referred for cardiac CT from March to June 2007. 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 coro-nary angiography (14 and 17 patients), impaired LV motion (5 and 7 patients), focal thinning or hyper-trophy 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 preparationFrom March to april, all patients referred for

cardiac CT were asked if they would consent to have their image data used for fur ther scientif ic analysis and publication. In May and June, explana-tion about protocol changes was also given. We also emphasized that the CT coronary angiography would

Delayed phase cardiac CT: Prospective VS retrospective 69

always be performed under the same standard pro-tocol 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 contraindica-tion, 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 f low rate of 4 mL/s, and then followed by 30 mL saline chaser under the same f low 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 groupsThe prospective ECg-triggering sequential scan

was a protocol modif ied 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 cur rent 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 0.017 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 131.7 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 cal-culations, the average DLP was 118.9 mgy*cm, con-verting 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 220.0 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) 120 120

Tube current (ma) 280 67.2

Effective tube current per slice (mas/slice) 140 168

Rotation time (s) 0.5 0.5

Reconstruction Thinnest: 1.25 mm Thinnest: 1.4 mm, increment: 0.7 mm

acquired phase during RR-interval 70% 70%

others Pitch = 0.2ECg-pulsing

Delayed phase cardiac CT: Prospective VS retrospective70

was 220.0 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 radia-tion dose.

Regarding the reconst ruct ion, 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 qualityIndependent 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 car-diac 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, hori-zontal 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 ar tifact and beam hardening artifact. grade 1 is the worst, grade 5 is the best. grades 1 and 2 are considered uninter-pretable. grades 3, 4 and 5 are considered interpre-table. The detailed definitions of each evaluation are as follows:

Margin sharpness: Can the outer and inner bor-ders 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 sagit tal 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 mis-matching more than 5 mm with or without attenua-tion change; grade 1, obvious attenuation difference of more than 20.1 hu with or without slab mis-matching.

Beam hardening artifact: Beam hardening arti-fact is defined as a fan or radiating shape of attenu-ation 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 dif-ference can be higher (software correction of the artifact) or lower (beam hardening artifact itself ) than the surrounding tissue [21, 22]. Then the attenu-ation 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 analysisSPSS 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, esti-mated 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 ar tifact, 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.

Delayed phase cardiac CT: Prospective VS retrospective 71

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 arti-fact (arrow) shows only spatial mismatching. But the lower slab artifact (arrowheads) shows not only spatial mismatch-ing 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.

1a

1b

1d

1c

Delayed phase cardiac CT: Prospective VS retrospective72

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 gen-erally higher in the spiral group than in the sequential group.

2a2b

2d

2c

Delayed phase cardiac CT: Prospective VS retrospective 73

RESULTS

Demographic dataafter exclusion of patients with known coro-

nary 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 esti-mated effective dose (Table 2).

Scan timeThe 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 angi-ography 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 signif icantly 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 repor ted signif icantly 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 ar tifact 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 agreementRegarding the inter-observer agreement, com-

bining all 62 cases, the kappa statistics for both observers in marginal sharpness, slab artifact and beam hardening ar t ifact were 0.792, 0.842, and 0.872, respectively. This means a substantial to per-fect 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-valuea

gender 　 0.582

F 8 (25.8%) 11 (35.5%) 　

M 23 (74.2%) 20 (64.5%) 　

age 61.87 ± 12.35 63.84 ± 10.61 0.504

height (cm) 163.65 ± 6.59 162.39 ± 9.21 0.539

Body mass index (m/kg2) 25.36 ± 3.21 24.18 ± 3.66 0.183

Body weight (kg) 68.02 ± 9.98 63.98 ± 11.90 0.154

Mean heart rate (bpm) 62.16 ± 5.99 61.10 ± 4.53 0.433

heart rate standard deviation (bpm) 7.71 ± 4.11 7.08 ± 3.90 0.541

Estimated dose length product (mgy*cm) 105.17 ± 13.21 108.04 ± 8.34 0.310

Estimated effective dose (mSv) 1.79 ± 0.22 1.84 ± 0.14 0.310aIndependent t test.

Delayed phase cardiac CT: Prospective VS retrospective74

Table 3. Image quality and scan time comparison

group a (Sequential) (no.= 31) group B (Spiral) (no.= 31) p value a

observer 1

Margin sharpness 0.008 **

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 < 0.0001 **

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 < 0.0001 **

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 0.042 *

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 < 0.0001 **

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 < 0.0001 **

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) 11.59 ± 1.83 18.43 ± 5.74 < 0.0001 **

Scan time (sec) 5.56 ± 1.46 12.92 ± 1.00 < 0.0001 **a Mann-Whitney u test. *: P < 0.05; **: P < 0.01b Score of 1 or 2 was considered uninterpretable.

Delayed phase cardiac CT: Prospective VS retrospective 75

DISCUSSION

Delayed phase of cardiac CTThis study tried to answer the question: under

the same low radiation dose, which protocol is the better protocol for delayed phase of cardiac CT: prospect ive ECg-t r igger ing sequent ial 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 uninter-pretable 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 clini-cally 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 reconstruc-tion 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 pos-sible 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 hard-ening artifact on the myocardium. The spiral scan can avoid this by over-sampling, MSR and inter-polation. 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 implicationWith 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 reduc-tion. our study suggests that, if the scanner sup-ports 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 agreementEven 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 f ive-grade system. Therefore, we obtained great inter-observer agreement.

Radiation estimateRegarding 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

Delayed phase cardiac CT: Prospective VS retrospective76

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 angi-ography. Therefore, we applied the result from the study with the lower heart rate [16].

LimitationIn 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 resolu-tion. 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 ref-erence 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 sequen-tial scan (modified from the calcium score protocol) for delayed phase of cardiac CT. The sequential pro-tocol is not clinically feasible in a significant portion of patients due to slab artifact and beam hardening artifact.

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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。