J A C C : C A R D I O V A S C U L A R I M A G I N G VO L . 8 , N O . 8 , 2 0 1 5

ª 2 0 1 5 B Y T H E AM E R I C A N C O L L E G E O F C A R D I O L O G Y F O U N DA T I O N I S S N 1 9 3 6 - 8 7 8 X / $ 3 6 . 0 0

P U B L I S H E D B Y E L S E V I E R I N C . h t t p : / / d x . d o i . o r g / 1 0 . 1 0 1 6 / j . j c m g . 2 0 1 5 . 0 2 . 0 2 1

EDITORIAL COMMENT

Radiation Dose Reduction inCoronary CT AngiographyTime to Buckle Down*

Andrew J. Einstein, MD, PHD

S ince the inception of coronary computedtomography angiography (CTA), minimizingionizing radiation exposure has posed a clinical

challenge. In a 1984 paper chronicling their pioneer-ing work with the dynamic spatial reconstructor(DSR), the first scanner used for coronary CTA, Block,Bove, and Ritman (1) wrote that “an adult patient canexpect to receive approximately 0.9 rads/s of DSRscan time.” Translating the details (2) into current ter-minology, a study performed on this scanner could beassociated with an effective dose (ED) of up to 20 mSv,equivalent to 7 years of background radiation.

The ED from coronary CTA decreased dramatically,to 1.5 to 2.0 mSv (3), with the advent of the electronbeam scanner; however, electron beam computedtomography (CT) had limited spatial resolution andgave way to multidetector-row (MDCT) scanners. Asperformed with early 4-slice MDCT scanners, coro-nary CTA was again associated with higher EDs,typically 6 to 13 mSv (3). An important factor thatcontributed to the increased dose was longer x-rayexposure time from these scanners, whose x-raytubes and detector arrays operated in a helical modewith considerable overlap. The potential radiationdoses imparted by coronary CTA continued to in-crease as MDCT technology advanced from 4 to 64slices. Subsequent scanners used greater numbers ofrows of thinner detectors, to improve z-axis coverage

*Editorials published in JACC: Cardiovascular Imaging reflect the views of

the authors and do not necessarily represent the views of JACC:

Cardiovascular Imaging or the American College of Cardiology.

From the Department of Medicine, Cardiology Division, and Department

of Radiology, Columbia University Medical Center and New York-

Presbyterian Hospital, New York, New York. Dr. Einstein was supported

by National Institutes of Health grant R01 HL109711, by a Herbert Irving

Associate Professorship, and as a Victoria and Esther Aboodi Cardiology

Researcher. Dr. Einstein has received support for other research from

GE Healthcare, Philips Healthcare, and Toshiba America Medical

Systems.

and spatial resolution, but required lower pitch forcoronary CTA (i.e., more overlap), which increaseddose. Their more powerful x-ray tubes enabled op-erators to maintain or even decrease noise leveldespite the improved spatial resolution, therebyimproving image quality (IQ), but at the cost ofincreased dose. Thus, at the beginning of the 64-sliceera, just as coronary CTA became widely used, the EDreached high levels not found since the days of theDSR. It was in this context that initial estimates pro-jected surprisingly high cancer risks from a singlecoronary CTA scan, particularly in younger femalepatients (4). Although these estimates were derivedfrom radioepidemiological models based on cancersfrom other radiation exposure scenarios, not epide-miological studies of actual cancers in patientsundergoing coronary CTA, they nevertheless appro-priately raised awareness of the need for radiationdose reduction.

The first widespread characterization of radiationdoses from cardiac CT in the 64-slice era occurred inthe PROTECTION I (Prospective Multicenter Studyon Radiation Dose Estimates of Cardiac CT Angio-graphy in Daily Practice I) study, a cross-sectional,international, observational study by Hausleiteret al. (5) describing doses in 2007 to nearly 2,000patients at 50 centers, selected on the basis of pre-vious publications on coronary CTA and personalcontacts (5). Its primary outcome measure, dose-length product (DLP), is a dosimetric quantity re-ported by CT scanners for each scan. Some earlydose-reduction techniques were used in PROTEC-TION I. Nevertheless, mean DLP was 885 mGy$cm;with a standard, albeit controversial (6), conver-sion factor (0.014 mSv$mGy�1 $ cm�1), which mayunderestimate ED, this would translate to an ED of12 mSv. Of note, median DLPs ranged 7-fold betweensites. PROTECTION I established a baseline ofearly 64-slice coronary CTA practice in high-qualitycenters.

TABLE 1 The PROTE

Study C

PROTECTION II

PROTECTION III

PROTECTION IV

PROTECTION V

Values are n, mean � SD,

DLP ¼ dose-length prodRadiation Dose Estimates o

Einstein J A C C : C A R D I O V A S C U L A R I M A G I N G , V O L . 8 , N O . 8 , 2 0 1 5

Editorial Comment A U G U S T 2 0 1 5 : 8 9 7 – 9

898

Numerous subsequent studies have evaluated theeffect on IQ and radiation dose of newer dose-reducing technological advances. Each advance aimsto reduce the time during which patients are exposedto x-rays, the tube potential (which determines theenergies of x-rays), or the tube current (which de-termines the rate at which x-rays are released). Thisliterature mostly consists of single-center observa-tional studies. Uniquely, the PROTECTION programhas sought to validate these novel dose-reductionmethods in a series of multicenter, multivendor, in-ternational randomized trials. Following on the suc-cess of PROTECTION I, PROTECTION studies IIthrough V (Table 1) have, respectively, comparedearlier coronary CTA methodology with reduced-tube-potential scanning (7), prospectively triggeredaxial imaging (8), high-pitch helical scanning fordual-source CT (9), and now in this issue of iJACC,reduced tube current with iterative image re-construction (IR) (10), a computationally more

SEE PAGE 888

demanding but improved approach to the recon-struction of images from raw data. It has beenobserved in numerous single-center and single-vendor studies that with IR, x-ray tube current canbe decreased while IQ is maintained. PROTECTION Vthus compared standard reconstruction with filteredback-projection to IR with a 30% reduction in tubecurrent in 400 patients at 8 centers, imaged withuse of scanners and reconstruction algorithms fromthe 4 largest manufacturers of CT equipment. Siteinvestigators were encouraged to use other dose-reducing techniques, such as reduced tube potential(validated in PROTECTION II) and axial imaging(validated in PROTECTION III), as clinically appro-priate. The authors found IQ to be noninferior in theIR group, whereas mean DLP was 29% lower, whichwas not surprising because radiation dose is linearlyrelated to tube current. The median DLP of 157mGy$cm in the IR group would translate to an ED of 2to 4 mSv, depending on the conversion factor used.

CTION Randomized Controlled Trials

enters n DR Strategy Standard Strategy

8 400 100 kVp (nonobese) 120 kVp (nonobese)

9 400 Axial mode Helical mode

3 303 High-pitch helical first Conventional first

8 400 IR, 30% lower TC FBP, standard TC

or median (interquartile range). Image quality was graded on a 4-point scale ranging from

uct (in mGy$cm); DR ¼ dose reduction; FBP ¼ filtered back-projection; IQ ¼ image quality; If Cardiac CT Angiography in Daily Practice; S ¼ standard; TC ¼ tube current.

In fact, the investigators’ use of a 30% reduction intube current was admittedly conservative, and itmay well be possible with IR to reduce tube currentmore while preserving IQ and diagnostic accuracy.More advanced “model-based” IR algorithms thatmodel optical system geometry and image noise,although not yet available for coronary CTA, are onthe horizon and offer potential to further reduce tubecurrent. Thus, using a combination of technologicaldevelopments such as reduced tube potential innonobese patients, reduced tube current with IR, andaxial, volume, or high-pitch helical scanning, it ispossible to perform coronary CTA with very low ra-diation doses. Indeed, multiple groups have pub-lished experience performing coronary CTA, inselected patients, with an ED of <1 mSv. Is radiationdose from coronary CTA, as some have suggested, nolonger a significant issue we need to concern our-selves with?

I would contend that the answer to this question isa resounding “no.” The potential to use multipledose-reduction methods, and their successful imple-mentation in the context of clinical research con-ducted at expert centers, need not imply that this isthe standard of care received by patients undergoingcoronary CTA worldwide. PROTECTION I demon-strated great between-center and within-center vari-ation in DLP. The introduction into practice, as wellas validation in PROTECTION II through V, of newerdose-reduction strategies, does not automaticallytranslate into their subsequent adoption whereverappropriate. Numerous centers still do not havetechnology available to perform axial (or prospec-tively triggered helical) imaging and iterative recon-struction; I have visited 2 such sites in the past year orso. These are costly upgrades to the first generation of64-slice scanners, not offset by increased reimburse-ment. Other centers opt for routine helical scan pro-tocols with high tube potential and current tominimize noise and optimize IQ; I recently visited 1such site with a typical DLP around 2,000 mGy$cm.We need more current data as to real-world coronary

IQ:DR IQ:S DLP:DR DLP:S

3.30 � 0.67 3.28 � 0.68 599 � 255 868 � 317

3.36 � 0.59 3.37 � 0.59 252 � 147 802 � 419

3.81 � 0.35 3.83 � 0.37 140 � 169 333 � 344

3.5 (3.0–4.0) 3.4 (2.8–4.0) 157 (114–239) 222 (141–319)

1 ¼ nondiagnostic to 4 ¼ excellent.

R ¼ iterative image reconstruction; PROTECTION ¼ Prospective Multicenter Study on

J A C C : C A R D I O V A S C U L A R I M A G I N G , V O L . 8 , N O . 8 , 2 0 1 5 EinsteinA U G U S T 2 0 1 5 : 8 9 7 – 9 Editorial Comment

899

CTA dosimetry. I suspect this would show improve-ment since PROTECTION I; however, undoubtedlythere remain many patients who do not receive thelow doses observed in PROTECTION V.

A useful analogy can be made to seatbelts, whichreduce the risk of fatality from a motor vehicle acci-dent by 45%. Although Congress mandated theirinstallation in automobiles in 1966, and thus they arein virtually all cars on the road today, 2010 data fromthe Centers for Disease Control and Prevention foundthe prevalence of always wearing a seatbelt rangedfrom 62% to 94%, depending on the state, with asteady increase between 2002 and 2010 (11). TheCenters for Disease Control and Prevention in-vestigators concluded, on the basis of these data, thatenactment of statewide primary seatbelt enforcementregulations and enhanced enforcement of seatbeltlaws were 2 effective strategies to increase seatbeltuse and reduce traffic fatalities. The existence ofsafety technology, even if ubiquitous, does not

ensure its use. Implementation can remain a chal-lenge and can vary depending on modifiable systemsfactors. A burgeoning new field of implementationscience is beginning to address this important un-dertaking (12).

Hausleiter et al. (10) have done a great service byvalidating our box of dose-reduction tools, particu-larly now in providing evidence that validates IR,which can be used for virtually all patients. Ourcommunity’s next challenge is to develop addi-tional methods and systems to ensure each patientreceives an optimal patient-centered, indication-specific protocol using the right tools to ensurediagnostic-quality information while minimizing ra-diation exposure.

REPRINT REQUESTS AND CORRESPONDENCE: Dr.Andrew J. Einstein, Columbia University Medical Center,622 West 168th Street, PH 10-203A, New York, NewYork 10032. E-mail: andrew.einstein@columbia.edu.

RE F E RENCE S

1. Block M, Bove AA, Ritman EL. Coronary angio-graphic examination with the dynamic spatialreconstructor. Circulation 1984;70:209–16.

2. Kinsey JH, Orvis AL. High repetition rate volu-metric x-ray CT scanning. IEEE Trans Nucl Sci 1981;NS-28:1732–5.

3. Hunold P, Vogt FM, Schmermund A, et al. Ra-diation exposure during cardiac CT: effective dosesat multi-detector row CT and electron-beam CT.Radiology 2003;226:145–52.

4. Einstein AJ, Henzlova MJ, Rajagopalan S.Estimating risk of cancer associated with radia-tion exposure from 64-slice computed tomogra-phy coronary angiography. JAMA 2007;298:317–23.

5. Hausleiter J, Meyer T, Hermann F, et al. Esti-mated radiation dose associated with cardiac CTangiography. JAMA 2009;301:500–7.

6. Gosling OE, Roobottom CA. Radiation exposurefrom cardiac computed tomography. J Am CollCardiol Img 2010;3:1201–2.

7. Hausleiter J, Martinoff S, Hadamitzky M, et al.Image quality and radiation exposure with a lowtube voltage protocol for coronary CT angiog-raphy: results of the PROTECTION II trial. J AmColl Cardiol Img 2010;3:1113–23.

8. Hausleiter J,Meyer TS,Martuscelli E, et al. Imagequality and radiation exposure with prospectivelyECG-triggered axial scanning for coronary CTangiography: the multicenter, multivendor, ran-domized PROTECTION-III study. J Am Coll CardiolImg 2012;5:484–93.

9. Deseive S, Pugliese F, Meave A, et al. Imagequality and radiation dose of a prospectivelyelectrocardiography-triggered high-pitch dataacquisition strategy for coronary CT angiography:the multicenter, randomized PROTECTION IV

study. J Cardiovasc Comput Tomogr 2015;9:278–85.

10. Deseive S, Chen MY, Korosoglou G, et al. Pro-spective randomized trial on radiation dose estimatesof CT angiography applying iterative image recon-struction: the PROTECTIONV study. J AmColl CardiolImg 2015;8:888–96.

11. Shults RA, Beck LF. Self-reported seatbelt use,United States, 2002-2010: does prevalence varyby state and type of seatbelt law? J Safety Res2012;43:417–20.

12. Mensah GA. Embracing dissemination andimplementation research in cardiac critical care.Global Heart 2014;9:363–6.

KEY WORDS computed tomographyangiography, coronary, iterative imagereconstruction, radiation dose, reconstruction