comparison of coronary ct angiography image quality with and without breast shields

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AJR:200, March 2013

all patients [57]. Therefore, by the as low

reasonably achievable [4] principle of usi

the lowest radiation possible to achieve a

agnosis, continued methods of radiation

duction should be sought to minimize pati

radiation exposure without significantly d

grading diagnostic imaging quality.

One potential practical strategy that ph

sicians could employ is the use of bismu

shields [8, 9] to reduce radiation exposu

to breast tissue among younger women d

ing coronary CTA (Fig. 1). Although dosi

etry data are available with radiation reduct

at the breast tissue level ranging from 30%

almost 60% [10], this technique has not gnered widespread use and is not currently r

ommended by guidelines because of conce

that unshielded techniques, such as tube c

rent modulation, may be superior to shie

ing [10], concerns that breast shields co

increase radiation dose through scatter [1

and a lack of data concerning their effect

image quality [1113]. The impact of bre

shields on image quality of nongated thora

and pulmonary embolus CT studies has be

Comparison of Coronary CTAngiography Image Quality Withand Without Breast Shields

Edward Hulten1,2

Patrick Devine1

Timothy Welch1

Irwin Feuerstein3

Allen Taylor4

Sara Petrillo5

Minnetta Luncheon1

Binh Nguyen1

Todd C. Villines1

Hulten E, Devine P, Welch T, et al.

1Cardiology Service, Walter Reed National Military

Medical Center, Bethesda, MD. Address correspondence

to E. Hulten (ehulten@partners .org).

2Present address: Noninvasive Cardiovascular Imaging,

Departments of Medicine and Radiology, Brigham and

Womens Hospital, Harvard Medical School, Boston,

MA 02115.

3Food and Drug Administration, Silver Spring, MD.

4Washington Hospital Center, Washington, DC.

5Mid-Atlantic Kaiser Permanente Group, Rockville, MD.

Cardiopulmonary Imaging Original Research

AJR2013; 200:5295 36

0361803X/13/2003529

American Roentgen Ray Society

Coronary CT angiography (CTA) is

a highly accurate method for the

noninvasive evaluation of coro-

nary artery and heart disease [1].

Increased clinical utilization of coronary CTA

has resulted in concerns over potential long-

term risks of cumulative ionizing radiation re-

lated to coronary CT and other medical imaging

modalities [2]. Recent estimates of the theoretic

imposed risk from coronary CTA and other

medical ionizing radiation imaging techniques

have varied widely, and although the risk is

generally thought to be low, it may be higher

among younger women [24]. Women young-

er than 50 years who are at significant lifetimerisk for breast cancer (the commonest malig-

nancy of women) may undergo a significant

chest radiation dose during coronary CTA. Al-

though current dose-reduction strategies, such

as prospective ECG-triggered scanning, lower

tube potential (kilovoltage), decreased scan

length, and novel scan techniques, have been

shown to markedly reduce the estimated radia-

tion dose in coronary CTA, these techniques are

not consistently applied and are not feasible in

Keywords:breast shield, cardiac CT, coronary CT

angiography, radiation

DOI:10.2214/AJR.11.8302

Received November 23, 2011; accepted after revision

February 29, 2012.

T. C. Villines reports moderate speaker honoraria from

Boehringer-Ingelheim Pharmaceuticals unrelated to the

topic of this manuscript. All other authors have no

financial disclosures or conflicts of interest to declare.All authors had full access to all of the data in the study

and take responsibility for the integrity of the data and

the accuracy of the manuscript.

The opinions and assertions contained herein are the

authors alone and do not represent the views of the

Walter Reed National Military Medical Center, the U.S.

Army, or the Department of Defense.

JO URNAL CLUB FOCUSON:

OBJECTIVE.The purpose of this study is to compare the image quality of coronary

angiography performed with and without breast shields.

MATERIALS AND METHODS.This study involved a retrospective cohort of 72 wom

with possible angina who underwent 64-MDCT retrospective ECG-gated coronary CT angio

raphy at a single academic tertiary medical center. Images of 36 women scanned while weari

bismuth-coated latex breast shields and 36 control subjects scanned without shields, match

by heart rate and body mass index, were graded on a standardized Likert scale for image quity, stenosis, and plaque by two independent board-certified readers blinded to breast shields

RESULTS.Seventy-two patients (mean [ SD] age, 53 9 years) were included. The presc

heart rate, body mass index, and Agatston score did not differ between groups. The median

timated radiation dose was 13.4 versus 16.1 mSv for those with and without breast shields (p

0.003). For shielded versus unshielded scans, 86% versus 83% of coronary segments were r

ed excellent or above average (p= 0.4), median image quality was 2.0 for both groups, me

signal was 474 75 and 452 91 HU (p= 0.27), mean noise was 33.9 8.5 and 29.8

HU (p= 0.04), and median signal-to-noise ratio was 14.4 and 14.7 (p= 0.56), respectively

CONCLUSION.Breast shields for women undergoing coronary CT angiography slig

ly increased noise but did not negatively affect signal, signal-to-noise ratio, quality, or int

pretability. Breast shield use warrants further study.

Hulten et al.Coronary CT Angiography With and Wi thout Breast Shields

Cardiopulmonary ImagingOriginal Research

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530 AJR:200, March 2013

Hulten et al.

reported, but data for use in coronary CTA are

not available [11, 1416]. This validation re-

mains important because coronary anatomy

by CT is more technically demanding and in-

cumbent on intricately detailed image qualitywhen compared with nongated CT of thoracic

anatomy. Because of the potential for breast

shields to offer simple but clinically impor-

tant radiation protection, the relative lack of

breast shield use for coronary CTA vis--vis

alternative radiation protection methods, and

the lack of available image quality studies, we

undertook a comparative image quality study

of coronary CTA with and without in-plane

bismuth breast shields. Thus, the purpose of

this study is to evaluate coronary CTA im-

age quality with and without in-plane bismuth

breast shield use.

Materials and Methods

Patients

We retrospectively identified a cohort of wom-

en older than 18 years without known coronary ar-

tery disease (CAD) referred for symptoms of pos-

sible angina who underwent coronary CTA scans

with bismuth-coated in-plane latex breast shields

(AttenuRad, Cone Instruments). Such shields con-

sist of a 1-mm-thick bismuth sheet impregnated in

rubber that mounts in a foam offset. The shields

were placed on the anterior chest exterior to the

patient garments, allowed repeat use, and did not

require any specia l hygiene considerations beyond

that of any other medical equipment exposed topatient contact. The cost for a medium-size breast

shield used in our study was $98.25 [17]. The

breast-shielded patients served as a convenience

sample, the use of shields having been mandated

by our institutional review boards radiation safe-

ty officer before approval of a research protocol

evaluating chest pain [18]. However, because the

risk-to-benefit ratio of breast shields remains un-

proven, beyond the requirements of this research

protocol, bismuth shields are not otherwise re-

quired in our hospital for coronary CTA. Thus,

these patients were compared with a control pop-

ulation of 36 clinically scanned patients random-

ly selected among all women who had undergone

coronary CTA using a comparable CT acquisition

protocol without breast shields during the study

period. Patients were matched by heart rate (HR)

and body mass index (BMI), because these covar-

iates have been consistently shown to be among

the strongest patient characteristics that influ-

ence image quality on coronary CTA. All coro-

nary CTA studies were performed at Walter Reed

Army Medical Center, a single-center university-

affiliated urban tertiary medical center, from Jan-

uary 2006 through February 2010. The research

protocol was approved by our hospital institu-

tional review board. Clinical information was ob-

tained from electronic health databases (inpatient,

outpatient, laboratory, and radiologic) of the De-

partment of Defense Military Healthcare System.

Coronary CTA

Per usual protocol for coronary CTA at our in-

stitution [19], all patients were prescribed variable

doses (typically 50100 mg) of oral metoprolol to

be taken 1 hour before the scheduled scan [20]. Ad-

ditional metoprolol was administered IV, if need-

ed, immediately before coronary CTA to obtain a

goal prescan HR of less than 60 beats/min. Nitro-

glycerin 0.40.8 mg sublingual was given 1 minute

before contrast-enhanced image acquisition.

All scans were performed using the same 64-

MDCT scanner (LightSpeed VCT, GE Health-

care). An initial unenhanced prospectively ECG-

triggered scan was acquired without breast shields

for calcium scoring and contrast-enhanced scan

planning. After a timing bolus series, a contrast-

enhanced scan was obtained with contrast agent

(Isovue, Bracco Diagnostics) injected IV at flow

rates of 4.56.0 mL/s through an antecubital vein,

followed by a 40-mL normal saline flush. Sixty-

four overlapping 0.625-mm slices were acquired

per rotation, with a rotation time of 350 ms. All

scans were performed using a tube potential of

120 kV. Additional acquisition variables were ad-

justed to individually optimize scans (pitch range,

0.160.25; range of tube current, 400750 mA).

When appropriate, ECG-dose modulation of tubecurrent was used, with maximal current output

generally occurring between 40% and 80% of the

R-R interval. Because prospectively ECG-trig-

gered scanning was not in widespread use at the

beginning of the study period, all coronary CTA

studies included in this study were retrospective-

ly ECG-gated. Scans for all patients were recon-

structed at 0.625-mm slice thickness using filtered

back projection and the GE Healthcare standard

soft-tissue kernel.

Existing scans were previously interpreted

jointly by a card iologist and a radiologist tra ined

in the performance and interpretation of coronary

CTA. For the assessment of image quality for our

study, noncardiac findings were not reassessed.

For the assessment of coronary findings for our

retrospective study, the scans were independent-

ly reread by two independent cardiac CT board-

certified readers using the Society of Cardiovas-

cular CT 18-segment coronary model [21] while

blinded to use of breast shields, clinical variables,

symptoms, and the original scan clinical interpre-

tation. To achieve blinding, one researcher loaded

Fig. 1Appearance of bismuth in-plane breast shields.A,Shield is shown in position on patients chest.B,Shield is shown with 10 -cm ruler.C andD,Breast shield can be seen at top of axial slice from64-MDCT cardiac CT angiography image (C) and in 3Dreconstruction of same image (D).

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AJR:200, March 2013

Coronary CT Angiography With and Without Breast Shields

all image series and panned the breast shield and

anterior chest tissue from the FOV on the work-

station before coronary evaluation by the two

blinded readers. After independent interpretation,

each segment was then read and scored by consen-

sus. Complete datasets from 36 women scanned

with bismuth-coated latex breast shields and 36

HR- and BMI-matched female control subjects

scanned without breast shields were randomly

graded on a standardized Likert scale for quality,

stenosis, and plaque presence and type (i.e., non-

calcified, partially calcified, or entirely calcified)

[21, 22]. Specifically, segments were graded us-

ing all available acquired phases for image qual-

ity and for the worst stenosis within each segment,

in accordance with Society of Cardiovascular CT

guidelines [21]. Quality ranged from 1 (excellent),

2 (good), 3 (below average), to 4 (uninterpretable).

Stenosis severity was graded as 0 (no stenosis), 1

(< 25% diameter worst stenosis), 2 (2549% ste-

nosis), 3 (5069% stenosis), 4 ( 70% stenosis),

and 5 (100% stenosis). Plaque within each seg-

ment was rated as noncalcified (no appreciable

calcium within the plaque or segment), partially

calcified, or entirely calcified by each rater. Seg-

ments smaller than 1.5 mm were not assessed, and

anatomically absent segments (e.g., ramus) were

coded as missing. Coronary calcium was quanti-

fied according to the Agatston method [23]. Im-

age signal and image noise were measured as the

mean ( SD) of Hounsfield units within a 1-cm di-

ameter region of interest in the aortic root and at

the level of the left main coronary artery origin,

respectively [21]. Signal-to-noise ratio was calcu-

lated as the signal divided by the noise. Estimated

radiation dose in millisieverts was calculated as

the dose-length product multiplied by the conv

sion factor for chest CT of 0.014 [6].

Statistical Analysis

Continuous variables with normal distributi

were expressed as mean ( SD) and were co

pared using Student ttest for independent gro

and one-way analysis of variance for betw

group comparisons. Categorical variables w

expressed as frequencies (percentages) and w

compared by the Pearson chi-square test. V

ables with skewed distributions were expressed

median (interquartile range [IQR]) and were co

pared by Mann-Whitney Utest. Interrater relia

ity was measured with the kappa statist ic. Beca

of a skewed distribution, calcium scores were n

ural logtransformed before analysis for tren

one was added to zero calcium scores. A tw

tailedpvalue less than or equal to 0.05 was c

sidered significant. All analyses were perform

using Stata (version 11.0, StataCorp).

Results

Seventy-two patients with a mean age

53 9 years were included. The prescan H

was 56 10 beats/min, BMI was 30 5 kg/m

median Agatston score was 0 (IQR, 014; 3

of scores were > 0), and the estimated radiat

dose was 14 5 mSv (Tables 1 and 2).

Of 1296 theoretically possible coron

segments for 72 patients using an 18-segm

coronary model, 17% were anatomically

sent and 14% were rated as too small (<

mm). Arteries that were depicted on the t

oretic 18-segment model but were absenttoo small were most commonly ramus, l

posterolateral, left posterior descending, a

second obtuse marginal segments. Amo

891 segments rated, 99% of graded coron

segments were evaluable (10 segments w

noninterpretable because of poor image qu

ity secondary to coronary motion). The m

dian quality rating for both shielded and u

shielded patients was good (2 [IQR, 220

There was no significant difference betwe

groups in the prevalence of normal, nono

structive (< 50% worst stenosis), and o

structive (50%) CAD (Table 3). There w

slightly higher rates of no coronary plaq(9% prevalence difference; p < 0.001) a

slightly lower rates of noncalcified plaq

(2% difference;p=0.012) and partially c

cified plaque (6% difference; p < 0.001)

the shield group, although rates of calcifi

and noncalcified plaque were similar (Table

Evaluation of segments located closer to

shield did not identify an effect of proximity

breast shield placement on image quality sco

TABLE 1: Demographics of Patients Who Underwent Coronary CT

Angiography With and Without Breast Shields

CharacteristicWithout Shield

(n= 36)With Shield

(n= 36) p

Age (y), mean SD 54.1 1.7 51.3 1.5 0.27a

Body mass index, mean SD 29.9 0.9 29.2 0.8 0.84a

Hypertension, no. (%) of patients 22 (61) 19 (53) 0.48a

Hyperlipidemia, no. (%) of patients 21 (58) 15 (42) 0.16a

Smoking, no. (%) of patients 2 (6) 4 (11) 0.39a

Family history, no. (%) of patients 15 (42) 5 (14) 0.01a

Diabetes mellitus, no. (%) of patients 4 (11) 2 (6) 0.39a

Heart rate (beats/min), median (IQR) 54 (4762) 55 (5061) 0.66b

Left ventricular ejection fraction (%), median (IQR) 72 (6277) 66 (5873) 0.29b

NoteIQR = interquartile range.aStudent ttest or chi-square test.bWilcoxon rank-sum test.

TABLE 2: Results of Coronary CT Angiography With and WithoutBreast Shields

Result Without Shield (n= 36) With Shield (n= 36) p

Agatston score

Median (IQR) 0 (029) 0 (02) 0.44a

Minimum 0 0 Not applicable

Maximum 563 784 Not applicable

Tube current (mA), median (IQR) 593 (573642) 606 (575643) 0.47a

Dose-length product, median (IQR) 1151 (8891405) 956 (8301060) 0.003a

Radiation dose (mSv), median (IQR) 16.1 (12.420.0) 13.4 (11.614.8) 0.003a

Signal (HU), mean SD 452 91 474 75 0.27b

Noise (HU), mean SD 29.8 8.3 33.9 8.5 0.04b

Signal-to-noise ratio, median (IQR) 14.7 (11.521.7) 14.4 (11.717.9) 0.56a

Image quality score, median (IQR) 2 (22) 2 (22) 0.50a

NoteIQR = interquartile range.aWilcoxon rank-sum test.bStudent ttest or chi-square.

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532 AJR:200, March 2013

Hulten et al.

(for shielded vs unshielded patients, distal left

anterior descending quality, 2 vs 2 [p= 0.4];

mid-left anterior descending quality, 2 vs 2

[p= 0.8]; and proximal right coronary artery,

2 vs 2 [p= 0.4]).

Table 4 shows predictors of image quality

by univariate and multivariate linear regres-

sion. BMI (= 0.018;p< 0.0001) was signif-

icant after multivariate analysis. As depicted

in Figure 2, comparing scans with and with-

out breast shields, 86% versus 83% of coro-

nary segments were rated as excellent or good

(p= 0.4). There was no statistically significant

difference in the percentage of below-average

(poor) quality or nonevaluable segments for

shielded patients (14%) compared to unshield-

ed patients (17%;p= 0.39). Patients without

shields had a small, but significantly higher,

number of uninterpretable segments (n = 9)

compared with shielded patients (n= 1; p=

0.01). There was a small but statistically sig-

nificant increase in image noise of 4 HU (14%

relative increase) between scans performed

with (33.9 1.4 HU) and without (29.8 1.4

HU) breast shields (p= 0.04). There was no

statistically significant difference in mean sig-

nal (474 75 vs 452 91 HU;p= 0.27) and

median signal-to-noise ratio (14.4 vs 14.7;p=

0.56) for scans performed with and without

shields. At the segment level, none of the 18

coronary segments was significantly differ-

ent between shielded and unshielded patients

for quality, stenosis severity, or plaque score.

Interrater agreement was excellent for quality

(= 0.82;p< 0.001), stenosis (= 0.82;p

comparison of coronary ct angiography image quality with and without breast shields

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