Coronary CT angiography for acute chest pain triage: Techniques for radiation exposure reduction;128 vs. 64 multidetector CT

Abstract

Background

Coronary CT angiography (CCTA) is used daily in acute chest pain triage, although exposing patients to significant radiation dosage. CCTA using prospective ECG gating (PG CCTA) enables significant radiation reduction

Purpose

To determine whether the routine use of 128 vs. 64 multidetector CT (MDCT) can increase the proportion of patients scanned using PG CCTA technique, lowering radiation exposure, without decreasing image quality.

Material and Methods

The study comprised 232 patients, 116 consecutive patients scanned using 128 MDCT (mean age 49 years, 79 men, BMI 28) and 116 consecutive patients (mean age 50 years, 75 men, BMI 28) which were scanned using 64 MDCT. PG CCTA was performed whenever technically permissible by each type of scanner: 64 MDCT = stable heart rate (HR) <60/min and weight <110 kg; 128 MDCT = stable HR < 70/min and weight <140 kg. All coronary segments were evaluated for image quality using a visual scale of 1–5. An estimated radiation dose was recorded.

Results

PC CCTA was performed in 84% and 49% of the 128 and 64 MDCT groups, respectively (P < 0.0001). Average image quality score were 4.6 ± 0.3 and 4.7 ± 0.1 for the 128 and 64 MDCT, respectively (P = 0.08). The mean radiation dose exposure was 6.2 ± 4.8 mSv and 10.4 ± 7.5 mSv for the 128 and 64 MDCT, respectively (P = 0.008).

Conclusion

The 128 MDCT scanner enables utilization of PG CCTA technique in a greater proportion of patients, thereby decreasing the related radiation significantly, without hampering image quality.

Keywords

Cardiac CT angiography heart adults

Coronary CT angiography (CCTA) has become an established non-invasive modality for coronary artery disease assessment (1, 2). CCTA is presently performed for patient triage in the evaluation of acute chest pain (ACP) in dedicated chest pain centers (CPC) (3–5). The utilization of CCTA for ACP triage in moderate risk patients has been suggested by the AHA/ACC guidelines (1) The recently revised appropriateness criteria for the use of CT and MRI in cardiovascular diseases proposed that CCTA could be suitable for the evaluation of both low and medium risk patients with ACP (6) Thus, MDCT CCTA is currently part of the non-invasive cardiology arsenal. Along with the technological advances and the increased utilization of CCTA in CPC triage, the awareness to the CCTA radiation ‘price tag’ is rising. Extensive efforts are taken in order to reduce the radiation exposure related to CCTA. ‘Tailored CCTA’ employs alteration of technical parameters, according to individual patient characteristics. This includes decrease of tube amperage (mAs), voltage (KvP), and ECG gating modifications such as dose modulation and prospective ECG gating (PG CCTA; also known as ‘step and shoot’) (7–10). Combining all these factors may lead to significant radiation dose reduction.

This study aimed to assess and quantify whether the introduction of 128 MDCT has decreased the per-patient and per-cohort radiation exposure in comparison to 64 MDCT, in a dedicated CPC triage. For this purpose, CCTA using 128 MDCT was evaluated for coronary image quality and radiation exposure quantification as compared with 64 MDCT.

Material and Methods

Patients

The study included 232 patients who underwent CCTA for ACP triage in a dedicated CPC. The patients were divided in two groups. The first group included 116 consecutive patients scanned using 128 MDCT (ICT SP, Philips Medical Systems, Cleveland, OH, USA). The control group comprised a cohort of 116 consecutive patients who underwent CCTA using 64 MDCT (Brilliance 64, Philips Medical Systems, Cleveland, OH, USA), during the period immediately preceding the installation of the 128 MDCT scanner. All patients were admitted to CPC due to acute chest pain lasting more then 5 min, suspected for ischemic origin. No electrocardiographic or cardiac enzyme (Troponin) abnormalities were detected during a 12-h monitored period.

To avoid biased selection all patients were evaluated in the CPC implementing the same triage protocol, as previously published (3).The study was approved by the institutional review board.

CCTA protocol

Scans were performed using 128- and 64-slice MDCT scanners (ICT SP and Brilliance 64, Philips Medical Systems, Cleveland, OH, USA

ECG gating options included: retrospective EGC gating (RG CCTA), retrospective EGC gating with dose modulation (RG DM CCTA) and prospective EGC gating (PG CCTA). For the 128 MDCT scanner a new option of PG CCTA with 5% tolerance (also known as ‘padding’) was added. Using this option the ECG triggers the X-ray tube during specific targeted mid-diastolic phases (70, 75 and 80%) during the R-R interval. Thus, using PG CCTA with 5% tolerance 3 diastolic phases, namely 70%, 75%, and 80% of the R-R interval, are available for reconstruction. The decision which gating protocol to use was based on the following parameters: patient weight, a stable heart rate (HR) with HR variability <10 beats per minute (bpm) and the relevant inclusion criteria for each scanner. Inclusion criteria for 128 MDCT PG CCTA: stable HR < 70/min and weight < 140 kg. Inclusion criteria for 64 MDCT PG CCTA: stable HR < 60/min and weight < 110 kg.

Oral and or intravenous β blockers (P.O Propranolol 10–40 mg; I.V. Metoprolol 5–15 mg) were administered to all patients with HR >65 bpm. Non-ionic contrast (Iopamiro 370, Iopamidol, BRACCO s.p.a, Milan, Italy) was administered into an antecubital vein via a dual head injector (GE Nemoto dual head injector, GE Healthcare, Milwaukee, WI, USA). Technical parameters were set according to each scanner's characteristics (Table 1).

Table 1
Scanning technical parameters for the 128 MDCT and 64 MDCT groups

128 MDCT 64 MDCT

Mean contrast volume (mL) 84 87

Mean injection rate (mL/s) 5 5

Tube voltage 100 (KvP) 88 (76%) NA^†

Tube voltage 120 (KvP) 28* (24%) 116 (100%)

Tube amperage (mAs) mean (range) 362 (150–1200) 533 (150–1235)

Gantry rotation (s) 0.27 0.42

KvP of 120 was used in the 128 MDCT in patients > 90 kg

^†NA = Non-applicable; voltage of 100 Kvp was not available on the 64-slice MDCT scanner

Dose-length-product (DLP) (measured in milligray/cm) was recorded per patient. Effective dose (mSv) was calculated by multiplying DLP by k, a constant equal to 0.017 (mSv X mGy⁻¹ X cm⁻¹) as previously described (11). According to the manufacturers specifications a typical ±15% deviation of measurements is acceptable with a maximal ±30% for both scanner types.

Image analysis

All 15 coronary segments were evaluated according to the American Heart Association classification (12). Image quality was assessed using a previously described 5-point visual scale as follows: 5 = no artifacts (continuous vessels course, no step artifacts); 4 = minor artifacts (discrete blurring of vessel margin, minor step artifacts, not affecting vessel evaluation); 3 = moderate artifacts (moderate blurring, or moderate step artifacts); 2 = severe artifacts (doubling or severe step artifacts causing coronary segments discontinuity); 1 = un-assessable (vessel structures not differentiable). A score of 3 or higher was considered of diagnostic quality (13).

Curved multiplanar reformats were reconstructed for each coronary artery (Extended Workspace, version 4.5 Philips Medical Systems, Cleveland, Ohio, USA). Each study was interpreted as either normal (no evidence of coronary artery disease); non-significant coronary artery disease (stenosis ≤ 50%); significant coronary artery disease (stenosis > 50%).

Statistical analysis

Comparison of continuous variables (calculated effective radiation dose and image quality scores) was done using a two-tailed t-test. Contingency tables (used to evaluate differences in utilization frequencies of specific ECG gating techniques) were analyzed using the Fisher exact test (GraphPad InStat version 3.00 for Windows, GraphPad Software, San Diego, CA, USA).

Results

Patients' baseline characteristics for the 128 and 64 MDCT groups are presented in Table 2. The different ECG gating techniques and calculated effective dosages are presented in Table 3.

Table 2
Baseline patient characteristics for the 128 MDCT and 64 MDCT groups

128 MDCT 64 MDCT

Patients (n) 116 116

Age ± SD (years) 49 ± 9 50 ± 9

Gender (% men) 79 75

Weight (kg); BMI (m²/kg) 83 ± 18; 28 ± 6 82 ± 14; 28 ± 4

Heart rate (bpm) 57 ± 8 59 ± 7

Smoking (%) 36 30

Hypertension (%) 25 23

Diabetes (%) 5 6

Hyperlipidemia (%) 45 42

Family history (ischemic heart disease) (%) 32 32

Table 3
Average effective dose (mSv) and ECG gating techniques for the different scanning techniques in both 128 MDCT and 64 MDCT groups

Technique Average mSv 128 MDCT (n = 116) Average mSv 64 MDCT (n = 116)

PG CCTA 3.2 (n = 21) 4.0 (n = 56)

PG CCTA ± 5%^† 5.1 (n = 79) NA (n = 0)

RG^‡ 15.1 (n = 1) 23.1 (n = 7)

RG DM (40–80%)^§ 14.35 (n = 8) 14.4 (n = 32)

RG DM (70–80%)** NA (n = 0) 10.6 (n = 9)

PG CCTA & DM (40–80%)^†† 14.8 (n = 6) 17.6 (n = 8)

DM (40–80%) twice^‡‡ NA 27 (n = 3)

PG CCTA twice^§§ 5.7 (n = 1) 6.9 (n = 1)

*Prospective gating CCTA (single diastolic phase – 70%)

^†Prospective gating CCTA (three diastolic phases – 70%, 75%, 80%)

^‡Retrospective gating (full exposure during the R-R whole interval period)

^§Retrospective gating CCTA with dose modulation (full exposure during 40–80% of the R-R interval)

**Retrospective gating CCTA with dose modulation (full exposure during 70–80% of the R-R interval)

^††PG CCTA was acquired first and was non-diagnostic, followed by DM (40–80%)

^‡‡Dose modulation was acquired first and was non-diagnostic, followed by the same repeated technique*

^§§Prospective gating CCTA was acquired first and was non-diagnostic, followed by the same repeated technique

PG CCTA was performed in 84% of the 128 MDCT group as compared with 49% in the 64 MDCT group (P < 0.0001). A tube current of 100 kVp was used in 76% of patients in the 128 MDCT group. All the patients in the control group (64 MDCT) were scanned using a tube voltage of 120 kVp (Table 2). The mean radiation dose exposure using 128 MDCT was 6.2 ± 4.8 mSv compared with 10.4 ± 7.5 mSv using 64 MDCT; P value < 0.00001. The radiation exposure dosages for each scanning technique are presented in Table 3 and Fig. 1.

Fig. 1
Average effective radiation for the different scanning techniques in both 128 MDCT and 64 MDCT groups. DLP = dose length product mGy/Cm2; S&S single phase = Step and shoot only 75% of R-R; S&S 5% tolerance = Step and shoot 70%,75%,80% of R-R; RG = retrospective gating; DM (40,70,75,80%) = Dose modulation for 40%, 70%, 75%, 80% of R-R; DM (70,75,80%) = Dose modulation for 70%, 75%, 80% of R-R; S&S twice = Step and shoot twice due to study failure; S&S & DM = Step and shoot followed by dose modulation due to study failure; DM twice = dose modulation twice due to study failure

Image quality was good for both groups. Average quality scores were 4.6 ± 0.3 and 4.7 ± 0.1 for the 128 and 64 MDCT, respectively (P = 0.08) (Figs. 2–4). There was a trend toward more non-diagnostic studies (image quality uniformly non-assessable) in the 64 vs. the 128 MDCT groups (10.3% vs. 6%, respectively; P = 0.13). This was due to HR acceleration and variability during the studies. These studies were repeated in order to achieve a diagnostic study.

Fig. 2
CCTA using prospective ECG gating (64 slice MDCT). A 43-year-old man with chest pain. (a) LAD: curved multiplanar reformat. Complex plaque (calcified and non-calcified components) in the mid LAD causing stensosis of 50%. Subtle ‘step’ artifacts (black arrows) (image quality score = 4). (b) LAD at conventional angiography: 50% stenosis in the MID LAD

Fig. 3
CCTA using prospective ECG gating (128 slice MDCT). A 57-year-old man with chest pain. (a) RCA: curved multiplanar reformat. Non-calcified plaques in proximal and mid RCA causing significant stensosis (white arrows). Subtle ‘step’ artifacts (black arrowheads) (image quality score = 4). (b) RCA at conventional angiography: significant stenosis in the proximal and mid RCA

Fig. 4
CCTA using prospective and retrospective ECG gating in the same patient (128-slice MDCT). A 43-year-old woman with chest pain. (a) M1 curved multiplanar reformat: CCTA using prospective ECG gating; Severe ‘step’ artifacts causing coronary segments discontinuity (white arrow) (image quality score = 2). (b) M1 curved multiplanar reformat: A repeat CCTA (in the same patient) using retrospective ECG gating; No ‘step’ artifacts (image quality score = 5). (c) LCX curved multiplanar re-format: CCTA using prospective ECG gating; Severe ‘step’ artifacts causing coronary segments discontinuity (white arrow) (image quality score = 2). (d) LCX curved multiplanar re-format: A repeat CCTA (in the same patient) using retrospective ECG gating; No ‘step’ artifacts (image quality score = 5)

Discussion

The radiation exposure entailed in CT scans has been criticized and addressed as a significant source of medical exposure (14–17). Radiation burden related to CCTA has been studied in depth compared with other CT scans (15–17) . The estimated average effective dose using 64 MDCT scanners for CCTA was previously reported to range between 7–23 mSv, depending on the manufacturer, the scanning technique and patient-related factors (18). CCTA was suggested as the modality of choice for ACP triage in patients with low to intermediate pretest probability for coronary artery disease (1, 6). This patient subgroup was previously shown to be of relatively young age (average age of 50 years), similar to the average age of our cohort (3). A recent study calculated the life attributable risk (LAR) for cancer for women at the age of 40 undergoing CCTA to be 1:270. For men of similar age the LAR was calculated as 1:595 (14). Thus, any efforts to lower radiation dose exposure associated with CCTA are of utmost importance, especially in the subset of patients encountered in CPC.

The present study demonstrates that the use of a new generation 128 MDCT scanner allows a significant reduction of radiation exposure for patients with ACP, when compared with the previously used 64 MDCT scanner. This study shows that in ‘real life’, even when including the repeated studies obtained due to non-diagnostic scans, the use of 128 MDCT leads to a radiation exposure reduction greater than 40%. The mean radiation exposure using 128 MDCT was significantly reduced to 6.2 mSv when compared with 10.4 mSv using the 64 MDCT.

The main factor responsible for this change is the higher percentage of studies applicable for PG CCTA using the 128 MDCT – 84% VS 49% when using the 64 MDCT. Several technical developments available on the 128 MDCT are responsible for this. These include new PG CCTA options and faster gantry rotation.

The 128 MDCT scanner permits a modified PG CCTA option. This PG CCTA alternative, also known as PG CCTA with 5% tolerance or PG CCTA ‘padding’, utilizes a slightly wider diastolic window in which the tube current is enabled, allowing reconstruction of three diastolic phases, namely 70%, 75%, and 80% of the R-R interval. The availability of three different diastolic phases for reconstruction often solves motion artifacts present in one of the phases but not in others. This adjustment renders this PG CCTA mode less rigid and applicable in slightly higher HRs (up to 70 bpm) leading to its utilization in a larger portion of our patients.

Gantry rotation of 0.27 s is an additional feature of the 128 MDCT compared with a gantry rotation of 0.42 s in the 64 MDCT scanner. This feature results in less motion artifacts and may explain the lower percentage of non-diagnostic studies in our study when using 128 vs 64 MDCT (6% of 128 MDCT vs 10.3% of 64 MDCT).

Another important factor is the option of lowering tube voltage to 100 kVp, implemented in 75% of patients in the 128 MDCT group. Tube voltage affects the peak photon energy and improves image contrast in studies using iodinated contrast agents (17, 18). Tube voltage is correlated to the radiation dosage varying approximately with the square of the KvP. In the 128 MDCT, reduction of tube voltage to 100 KvP was made available, enabling a significant dose reduction in our study. Our findings are in accordance with a recent multicenter study which demonstrated a 53% reduction in radiation exposure for CCTA when reducing tube voltage from 120 to 100 kV (19). Tube voltage reduction, is associated with increased image noise, making the use of 100 kV recommended only for non-obese patients with body weight of less than 85 kg or BMI of less than 30 kg/m2) (19). Adequate selective use of low voltage in our series resulted in uncompromised diagnostic image quality when compared with 64 MDCT images. Image quality remained close to optimal with average quality scores were 4.6 ± 0.3 and 4.7 ± 0.1 for the 128 and 64 MDCT, respectively (P = 0.08); this is in concordance with previous series as well (17–19).

Two additional technical alterations in the 128 scanner offer further dose reduction. Eighty mm scan coverage per rotation allows the performance of PG CCTA with less ‘steps’ causing less ‘step’ artifacts as well as a dedicated cardiac filter (‘bow tie’ filter) which lowers the skin dose.

There are some limitations of this study. The current study was not randomized. The control group was composed of a cohort of 116 consecutive patients who underwent CCTA using 64 MDCT, during the period immediately preceding the installation of the 128 MDCT scanner. However, the baseline characteristics were similar in both groups and not statistically different, rendering this comparison reasonable.

In conclusion, significant radiation dose reduction along with uncompromised image quality was possible utilizing a new 128 MDCT scanner. A steady significant increase in CT utilization over the past decade in the setup of urgent patient triage emphasizes the importance of the results of this study.

Conflict of interest: None.

	128 MDCT	64 MDCT
Mean contrast volume (mL)	84	87
Mean injection rate (mL/s)	5	5
Tube voltage 100 (KvP)	88 (76%)	NA^†
Tube voltage 120 (KvP)	28* (24%)	116 (100%)
Tube amperage (mAs) mean (range)	362 (150–1200)	533 (150–1235)
Gantry rotation (s)	0.27	0.42

	128 MDCT	64 MDCT
Patients (n)	116	116
Age ± SD (years)	49 ± 9	50 ± 9
Gender (% men)	79	75
Weight (kg); BMI (m²/kg)	83 ± 18; 28 ± 6	82 ± 14; 28 ± 4
Heart rate (bpm)	57 ± 8	59 ± 7
Smoking (%)	36	30
Hypertension (%)	25	23
Diabetes (%)	5	6
Hyperlipidemia (%)	45	42
Family history (ischemic heart disease) (%)	32	32

Technique	Average mSv 128 MDCT (n = 116)	Average mSv 64 MDCT (n = 116)
PG CCTA*	3.2 (n = 21)	4.0 (n = 56)
PG CCTA ± 5%^†	5.1 (n = 79)	NA (n = 0)
RG^‡	15.1 (n = 1)	23.1 (n = 7)
RG DM (40–80%)^§	14.35 (n = 8)	14.4 (n = 32)
RG DM (70–80%)**	NA (n = 0)	10.6 (n = 9)
PG CCTA & DM (40–80%)^††	14.8 (n = 6)	17.6 (n = 8)
DM (40–80%) twice^‡‡	NA	27 (n = 3)
PG CCTA twice^§§	5.7 (n = 1)	6.9 (n = 1)

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