Received: 24 April 2001Revised: 28 August 2001Accepted: 31 August 2001Published online: 15 December 2001© Springer-Verlag 2001

Abstract The aim of this study wasto verify the feasibility of a respira-tory motion compensation technique(motion-adapted gating, MAG) forvisualization of coronary arteries(CA) by correlation with selectivecoronary angiography (SCA). Fif-teen subjects (11 patients, mean age61.3 years, age range 41–73 years;and 4 healthy volunteers, mean age32.3 years, age range 31–35 years)were investigated. A Philips Gyro-scan ACS-NT was used, operating at1.5 T, was combined with the Power-Trak 6000 gradient system. AnECG-triggered, respiratory motion-gated 3D turbo field echo sequencewas used. The real-time algorithmutilized the concept of k-spaceweighting in combination with auto-matic analysis of respiratory motion.The main CA were investigated.Qualitative analysis was performedby three blinded investigators. Visi-bility was graded on a five-pointscale (0=not visualized, 1=insuffi-cient, 2=sufficient, 3=good, 4=excel-

lent). Segments graded 2–4 were de-fined as adequately visualized. Six-ty-two of 88 assessable CA segmentsin patient, and 22 of 32 in volunteergroup were adequately visualized.Visibility of CA was classified as ex-cellent for proximal RCA (avg.3.6±0.5), good for LM, proximalLAD, proximal LCX, middle RCAand sufficient for middle LAD. Sen-sitivity, specificity, positive and neg-ative predictive values for coronaryMRA in detection of CA stenoseswith luminal narrowing ≥50% were88, 94, 83, and 96%, respectively.Magnetic resonance imaging in com-bination with MAG has proven to bea promising technique for noninva-sive imaging of CA due to good im-age quality and a patient convenientfree-breathing technique.

Keywords Heart · Magnetic resonance · Coronary angiography ·Coronary vessels · Stenosis or obstruction · Motion correction

Eur Radiol (2002) 12:718–726DOI 10.1007/s00330-001-1152-x C A R D I A C

Christoph WeberPaul SteinerRalph SinkusThorsten DillPeter BörnertGerhard Adam

Correlation of 3D MR coronary angiographywith selective coronary angiography: feasibilityof the motion-adapted gating technique

Introduction

Cardiovascular disease remains the leading cause ofdeath in most developed western industrial nations.Presently, selective X-ray based coronary angiography(SCA) is the gold standard for the evaluation of coro-nary artery anatomy, luminal stenoses, or occlusions.Despite its indisputable standing as a reference for de-tection of coronary artery disease, cardiac catheteriza-tion procedures entail high costs, require an injection of

iodinated contrast agent, and constitute an exposure toradiation.

Reliable noninvasive assessment of coronary artery stenosis and occlusions would offer an advantage in thecare of patients with known or suspected coronary arterydisease [1]. Coronary MR angiography (MRA) is a possible approach. Several impediments, however, limit its clinical utility. These obstacles include compensationfor cardiac and respiratory motion, need for sub-millime-ter spatial resolution, visualization of the complex three-

C. Weber (✉ ) · P. Steiner · G. AdamDepartment of Diagnostic and Interventional Radiology, University Hospital Hamburg-Eppendorf,Martinistrasse 52, 20246 Hamburg, Germanye-mail: C.Weber@uke.uni-hamburg.deTel.: +49-40-428034029Fax: +49-40-428033802

T. DillDepartment of Cardiology, University Hospital Hamburg-Eppendorf,Martinistrasse 52, 20246 Hamburg, Germany

R. Sinkus · P. BörnertPhilips Research Laboratories, Division Technical Systems, Röntgenstrasse 24–26, 22335 Hamburg, Germany

dimensional anatomy of coronary arteries, and suppressionof signals from adjacent epicardial fat and myocardium.

Thus far, reports with varying approaches to coronaryMRA have been published [2]. Initial approaches usedtwo-dimensional (2D) gradient-echo strategies to take ad-vantage of unsaturated blood inflow and breath holding tominimize respiratory motion [3, 4, 5]. So-called free-breathing coronary MRA is coupled with MR navigatorechoes that assess diaphragmatic or cardiac position [6, 7,8, 9, 10, 11, 12, 13, 14, 15], thus eliminating time con-straints of breath holding and enabling high-resolutionMRA acquisition. Three-dimensional MRA techniques[16, 17] can be performed with navigator techniques [6,11, 12] resulting in even higher spatial resolution and per-mitting post-processing multiplanar reformatting of 3Ddata. Gating based on navigator echoes is one of the mostpromising ways to reduce respiratory motion artifacts incardiac imaging. Displacements induced by respiratorymotion can be measured with a high spatial accuracy us-ing navigators based on 2D radio-frequency (RF) pulses.In real-time gating methods this information can be usedto drive the MR data acquisition by deciding which MRdata should be acquired next. Furthermore, motion statis-tics can be obtained during scanning that can serve tomonitor changes in the respiratory pattern of the patient.Particularly in long cardiac MR scans, the respiratorymotion statistics may change because of diaphragm drifteffects or macroscopic patient motion [18]; thus, the gat-ing algorithm can automatically change certain gating pa-rameters during scanning to optimize scan efficiency. Theinformation about the current motion state derived fromthe navigator can further be used to improve image quali-ty by applying appropriate corrections. Image artifactscaused by the through-plane and the in-plane componentsof the respiratory motion can be reduced. All of thesemeasures increase the quality and the reproducibility ofcardiac images. Furthermore, the scan efficiency can beincreased while the need for operator interaction and/orsignificant patient cooperation can be reduced significant-ly. The more elaborate gating algorithm such, as the mo-tion-adapted gating (MAG), shows promising potentialfor handling these problems. The lower coronary blood-to-myocardium contrast inherent to 3D MRA techniquescan be improved by adopting a T2 preparation pulse [19].

The aim of this study was to evaluate the feasibility ofthe navigator-based, free-breathing, high-resolution 3DMRA technique MAG [20, 21] in correlation with the goldstandard SCA for visualization of the coronary arteries.

Materials and methods

Subjects

Fifteen subjects (1 woman and 14 men), representing 11 patientswith known or suspected coronary artery stenoses, and 4 healthyvolunteers without a history of cardiovascular disease, were in-

cluded in this study. Mean age in the volunteer group was32.3 years (age range 31–35 years). Mean age in the patient groupwas 61.3 years (age range 41–73 years). All patients had anginapectoris and were referred to our hospital for SCA. Patients re-vealed stable sinus rhythm and clinical conditions, equivalent toNYHA grades I–II. Written informed consent was obtained fromall participants, and the protocol was approved by the local ethicscommittee. Patients underwent both coronary MRA and SCAwithin 4 weeks. In the case of the volunteers, only coronary MRAwas performed.

Coronary MR angiography

Coronary MRA in combination with the MAG algorithm was per-formed on a 1.5-T whole-body scanner (Gyroscan, ACS-NT 15,Philips, Best, The Netherlands) equipped with a self-shielded gra-dient system (PowerTrak 6000) providing 23 mT/m, maximumslew rate 100 mT/m ms–1. Two surface coils, one on the chest andthe other on the back, were used to receive the signal for the MRAimages. The navigator signal was received via the body coil.

The MRA study protocol consisted of one scout and two high-resolution scans for the visualization of the right and the left coro-nary artery tree. The scout utilized an echo-planar imaging (EPI)sequence, which was ECG triggered and navigator gated. Leftmain coronary artery (LM), proximal, middle, and distal part ofleft anterior descending coronary artery (LAD) and proximal partof left circumflex coronary artery (LCX), as shown on the scout,were used as landmark for positioning of the high-resolution slabin accordance with their anatomic route. In correspondence, proxi-mal, middle, and distal part of the right coronary artery (RCA), asshown on the scout, were used as landmark to position the high-resolution slab; therefore, the orientation for the two high-resolu-tion scans was chosen such that LM, LAD, proximal LCX, andRCA were fully contained within the data sets. The LCX was notscanned in its whole length separately. Images from the LCX wereobtained only for its proximal third by the high-resolution slab ofthe left coronary artery tree. Data was acquired at end expirationand in mid-diastole. The high-resolution scans consisted of a 3Dturbo field echo (TFE) sequence which was ECG triggered andnavigator gated in combination with a T2-preparation pulse(TR=8 ms, TE=4 ms, flip angle 30°) and a fat-suppression pulse.The 3D TFE sequence was a steady state. The T2-preparationpulse sequence (approximately 50-ms duration) was used to en-hance the contrast between the myocardial muscle tissue and thearterial blood in the coronary arteries. The second pulse was a fat-saturation pulse (approximately 15-ms duration) to suppress epi-cardial fat signal in which the coronary arteries are embedded. Tocompensate for respiratory motion artifacts, a navigator echo tech-nique was used to sample the respiratory motion. The subsequent-ly applied navigator was based on a 2D RF pulse (6-ms duration)that excites a pencil-beam shaped volume (25-mm diameter) fol-lowed by a gradient-echo readout. This navigator was placed onthe diaphragm’s dome to monitor the respiratory motion. The dis-placement information, which was derived from a cross correla-tion in real-time, was used for real-time gating with an acceptancewindow of 5 mm. In addition, prospective adaptive navigator-based correction of the slab position was performed using a fixed,dimensionless correction of 0.6. The entire duration of the naviga-tor sequence, including the correlation analysis, was 20 ms. Thenavigator was applied very close to the actual MR data acquisitionto achieve a high accuracy of the motion information and the real-time corrections derived from it. For high temporal resolution,three navigator echoes were obtained during each RR interval, twobefore and one after the image acquisition (Fig. 1); however, onlythe navigator just before the TFE block was used to steer theMAG algorithm, whereas the other two were used for motion pat-tern analysis. The image data acquisition was placed between twoadjacent RR peaks. The data acquisition window for coronary

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MRA within the cardiac cycle was 96 ms to minimize motion arti-fact. During one RR interval 12 echoes were acquired within theTFE imaging block (Fig. 1). Imaging parameters were: field ofview 340×340 mm to include the full length of the coronary arte-ries; 512×512×12 pixel matrix resulting in an in-plane resolutionof 0.7×0.7 mm2; and a slice thickness of 2 mm.

Motion-adapted gating

The MAG algorithm enables an instant and automatic analysis ofthe motion statistics of the diaphragm in real-time. If necessary,the algorithm changes gating parameters without user interactionbeing required [20].

MAG is based on the concept of k-space weighting [21]. Cen-tral regions of the k-space are more sensitive to motion-inducederrors than outer regions; thus, the central portion should be mea-sured carefully, whereas the outer regions in k-space could bemeasured with less accuracy; therefore, a k-space-dependent gat-ing acceptance window can be introduced that minimizes the re-sidual motion-induced errors. The choice of a particular weightedgating function represents a tradeoff between the final image qual-ity and the scan time and could be tailored to the individual pa-tient. According to the current motion state, the MAG algorithmdecides in real-time which lines of k-space could be measurednext, always considering the imposed weighted gating function.By analyzing motion statistics, the most probable motion state ofthe diaphragm is determined and used as a reference. Dependingon the difference between the current motion state and the refer-ence position of the diaphragm, the k-space lines are acquired ifthe difference does not exceed a predefined threshold. The centralk-lines are recorded for small deviations. For large deviations be-tween the current motion state and the reference position the moreperipheral k-lines are acquired. Three echoes were obtained to ob-

viate the optimal position of the diaphragm for acquisition of cen-tral k-space data within the predefined reference level by theMAG algorithm. But only the navigator just before the TFE blockwas used when it was consistent with the reference level for dataacquisition for the central lines of k-space; otherwise, it was usedfor the peripheral ones, which were less important for high tempo-ral resolution. No zero padding was used. It is assumed that themotion statistics change significantly. If the gating algorithm werenot capable of changing the gating parameters, the scan efficiencywould drop to zero; however, the MAG algorithm decides in real-time if the reference position (seed) has to be changed according-ly. Each imaging block records 12 lines with ky fixed and is cho-sen by the MAG algorithm. The z-encoding gradient varies ac-cording to the number of slices; thus, the concept of k-spaceweighting is only applied to the ky phase-encoding direction.

Postprocessing

For the visualization of the coronary arteries a vessel-based maxi-mum intensity projection was used. The slice depicting most ofthe vessel was chosen as the central slice. Those parts of the vesselwhich were inevitably contained in adjacent slices relative to theselected central one were projected similar to a maximum intensi-ty projection onto the selected slice. Average time required forpostprocessing of the coronary arteries was 12 min (±4 min). Ad-ditional clinical evaluation was based on source and reformattedimages.

Coronary angiography

The selective coronary angiographies (SCA) were performed bytransfemoral Judkins technique. All angiograms were documentedon cine angiographic films. Two experienced cardiologists, un-aware of the coronary MRA results, reviewed and analyzed thefilms by consensus.

Data analysis

Segmentation of the coronary arteries was performed according tothe American Heart Association (AHA) segmentation scheme[22]. The LAD and RCA were subdivided into three segments(proximal, middle, and distal segments). Additionally, LM and theproximal part of the LCX were evaluated. The middle and distalpart of the LCX were not contained within the 3D slab and there-fore excluded from analyses. Eighty-eight (patient group) and 32coronary artery segments (volunteers) formed the basis of theevaluation.

For both MRA and SCA the visibility of each coronary arterysegment was graded on a five-point scale ranging from 0 to 4(0=not visualized, 1=insufficient, 2=sufficient, 3=good, 4=excel-lent). Segments graded 2–4 were defined as “adequately visual-ized.” Length of continuously visualized coronary arteries wasdocumented. Additionally, all segments were evaluated for thepresence of stenoses. Stenoses were defined as signal loss, as de-scribed in other reports [4]. Stenoses were divided into those withluminal narrowing of more or less than 50%.

The results of the patient group were compared with SCA. An-alyses of all coronary MRA data were performed by three investi-gators in a blinded fashion (two radiologists, one cardiologist).The results were compared and differences in judgment discusseduntil consensus was achieved. Mean values and standard deviationwere calculated. Statistical analysis was performed by the pairedStudent’s t-test, p<0.05 interpreted as statistically significant. In-terobserver variability was tested by means of Kendall’s W coeffi-cient of concordance.

Fig. 1 The motion-adapted gating (MAG) sequence. NAV naviga-tor; T2 pre-pulse; F fat saturation; TFE imaging block. Schematicrepresentation of the applied pulse sequence for magnetic reso-nance angiography. Three navigators are distributed over one RRinterval. The data of the three navigator echoes enables a previewof the most likely diaphragm position in real-time by establishinga breathing curve. They optimize data acquisition and enable asampling almost evenly over time. All navigators contribute to theanalysis of the motion pattern; however, only the one precedingthe turbo field echo (TFE) block is used to steer the MAG algo-rithm. A T2 preparation pulse and a fat-suppression pulse serve toimprove the contrast of the arteries within the myocardium and thesurrounding fatty tissue. The gating algorithm selects for eachTFE block the ky values according to the concept of k-spaceweighting. The kz-encoding direction is not weighted; thus, allslices within the 3D slab are treated equally in terms of their kyweighting

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Results

Selective coronary angiography

Results from the SCA are displayed in Table 1. Therewere 22 coronary segments with stenotic alterations. In16 segments stenoses exceeding 50% were detected.Two segments (LAD-P, RCA-M) revealed complete oc-clusions. Anatomic norm-variance was found in 2 pa-tients; both revealed an irregular branching of the inter-mediate branch at the LAD/LCX bifurcation.

Coronary MRA

In all subjects the coronary MRA studies were per-formed successfully and without complications. The av-erage time needed for the investigation was 32 min persubject (LAD/LCX-P and RCA), i.e., the scout and twohigh-resolution scans. Length of continuously visualizedLM was 12.5±0.5 mm (mean±standard deviation), LAD116.9±20.7 mm, proximal LCX 47.2±4.5 mm, and RCA128.9±18.8 mm. Interobserver variability showed a Ken-dall’s W coefficient of concordance equal to 0.89(p<0.0001) concerning the visualization of coronary ar-

tery segments and 0.96 (p<0.0001) concerning the detec-tion of coronary artery stenoses.

Volunteer group

Twenty-two of 32 assessable coronary artery segmentsobtained a visibility graded 2–4 as displayed in Table 2.Two stenotic segments were visualized in the group ofvolunteers, one in the proximal LCX and one in theproximal RCA. According to the assumption that volun-teers are healthy, these observations were rated as falsepositive. On source image data the false-positive stenosisexceeding 50% in the proximal LCX segment in the vol-unteer group could not be verified by source data. Themean values for the qualitative visualization of the ves-sel segments are shown in Fig. 2a.

Patient group

Sixty-two of 88 (70.4%) coronary artery segments re-vealed a visibility graded 2–4 (Table 3). Mean valuesand standard deviation of qualitative analyses of the pa-tient group are presented in Fig. 2b. Qualitative visual-ization revealed an improved (p<0.001) detectability of

Table 1 Results of selective coronary angiography (n=11). SCAselective coronary angiography; LM left main coronary artery;LAD-P proximal left anterior descending coronary artery; LAD-Mmiddle left anterior descending coronary artery; LAD-D distal left

anterior descending coronary artery; LCX-P proximal left circum-flex coronary artery; RCA-P proximal right coronary artery;RCA-M middle right coronary artery; RCA-D distal right coronaryartery; AHA American Heart Association

Segments No. of segments according Segments visualized Stenoses ≥50% Stenoses <50to AHA by SCA

LM 11 11 of 11 0 0LAD-P 12 11 of 11 3 0LAD-M 13 11 of 11 5 4LAD-D 14 11 of 11 0 0LCX-P 18 11 of 11 1 0RCA-P 1 11 of 11 0 0RCA-M 2 11 of 11 4 2RCA-D 3 11 of 11 3 0Total 88 of 88 16 6

Table 2 Results of coronarymagnetic resonance angiography(MRA) in volunteer group(n=4). LM left main coronary ar-tery; LAD-P proximal left anteri-or descending coronary artery;LAD-M middle left anterior de-scending coronary artery; LAD-D distal left anterior descendingcoronary artery; LCX-P proxi-mal left circumflex coronary ar-tery; RCA-P proximal right cor-onary artery; RCA-M middleright coronary artery; RCA-Ddistal right coronary artery

Segments Segments visualized Stenoses ≥50% Stenoses <50% by MRA (grades 2–4) in MRA in MRA

LM 3 of 4 0 0LAD-P 4 of 4 0 0LAD-M 1 of 4 0 0LAD-D 0 of 4 0 0LCX-P 4 of 4 1 0RCA-P 4 of 4 1 0RCA-M 4 of 4 0 0RCA-D 2 of 4 0 0Total 22 of 32 2 0

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the proximal segments of LAD and RCA (Fig. 3) incomparison with the distal parts. This observation wasalso found in the volunteer group. The difference be-tween proximal and middle segments was less pro-nounced, and except for the RCA (p<0.05), no signifi-cant difference was found.

The LM could be evaluated in 10 of 11 patients(90%), proximal LAD in 11 of 11 patients (100%), mid-dle LAD in 9 of 11 patients (81%), distal LAD in only 4of 11 patients (36%), proximal LCX in 8 of 11 patients(73%), proximal RCA in 11 of 11 patients (100%), mid-dle RCA in 6 of 11 patients (55%), and distal RCA inonly 3 of 11 patients (27%).

The diagnostic accuracy of the free-breathing ECG-triggered coronary MRA adopting the MAG algorithm incomparison with SCA is documented in Table 3.

In comparison with SCA, 14 of 16 stenoses exceeding50% were correctly diagnosed (Figs. 4, 5). With 26 CAsegments graded 0–1 and excluded from the analysis, 3 of 3 stenoses exceeding 50% of proximal LAD, 4 of 5of middle LAD, 1 of 1 of proximal LCX, and 3 of 4 of

Table 3 Results of coronary MRA in patient group (n=11)

Segments Segments visualized True-positive stenoses True-positive stenoses False-positive stenoses False-positive stenosesby MRA (grades 2–4) <50% in MRA ≥50% in MRA <50% in MRA ≥50% in MRA

LM 10 of 11 0 of 0 0 of 0 0 0LAD-P 11 of 11 0 of 0 3 of 3 0 0LAD-M 9 of 11 4 of 4 4 of 5 0 1LAD-D 4 of 11 0 of 0 0 of 0 0 0LCX-P 8 of 11 0 of 0 1 of 1 2 2RCA-P 11 of 11 0 of 0 0 of 0 0 0RCA-M 6 of 11 2/2 3 of 4 0 0RCA-D 3 of 11 0 of 0 3 of 3 0 0Total 62 of 88 6 of 6 14 of 16 2 3

Fig. 2 a Visibility of coronary arteries (CA) in volunteer group.LM left main coronary artery; LAD left anterior descending coro-nary artery; LCX left circumflex coronary artery; RCA right coro-nary artery. The mean values for qualitative visualization were2.5±1 for LM, 3.0±1 for proximal LAD, and 1.0±1 for the middleLAD. The distal LAD could not be visualized. For the proximalLCX the visibility was rated 3.3±0.4, for proximal RCA 3.8±0.5,for the middle RCA 3.0±1, and 2.0±2 for distal RCA. b Visibilityof CA in patient group. LM left main coronary artery; LAD left an-terior descending coronary artery; LCX left circumflex coronaryartery; RCA right coronary artery. The values were 3.3±1.3 forLM, 3.1±1.2 for proximal LAD, and 2.5±1.4 for the middle LAD.The distal LAD could not be visualized (0.9±0). For the proximalLCX the visibility was rated 2.2±0.5, for proximal RCA 3.5±0.5,and for the middle and distal RCA 2.0±1.2 and 0.9±2.3, respec-tively

Fig. 3 Non-enhanced, ECG-triggered, respiratory-motion-gated3D TFE sequence of the RCA obtained in a healthy volunteer using MAG. Acquisition time=13 min. Excellent visualization ofthe proximal and middle segments, whereas the distal segment isvisualized insufficiently in the postprocessed image

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middle RCA, as well as 3 of 3 stenoses of distal RCA,were correctly detected by coronary MRA. In one middle segment each of LAD and RCA, stenoses ex-ceeding 50% were missed by MRA. Three false-positiveresults were obtained in two proximal segments of LCXand one middle segment of LAD (Fig. 6). On source im-age data one false-positive stenosis exceeding 50% in theproximal LCX segment in the patient group could not beverified by source data. The SCA revealed no luminalnarrowing even less than 50% in proximal LCX, where-

Fig. 4a, b Correlation of a postprocessed image of LM, LAD, andproximal LCX with selective coronary angiography. a Non-enhanced, ECG-triggered, respiratory-motion-gated 3D TFE se-quence obtained in a patient using MAG; postprocessed image.Proximal, middle LAD, and proximal LCX are well visible.

Middle LAD shows a 40% stenosis (arrow), LCX appears shortand occluded (double arrow). b Selective coronary angiographyreveals the stenosis of middle LAD (arrow) and the subtotal oc-clusion of proximal LCX (double arrow), slight retrograde fillingof middle, and distal LCX by collaterals

Fig. 5a, b Non-enhanced, ECG-triggered, respiratory-motion-gated3D TFE sequence of the RCA obtained in a patient using MAG.a Postprocessed image with arteriosclerotic plaque in the proximalRCA (arrow), significant stenosis (90%) in middle RCA (doublearrow), and diffuse narrowing in the distal segment. b Selectivecoronary angiography reveals the arteriosclerotic plaque in theproximal RCA (arrow), the significant stenosis (90%) in middleRCA (double arrow), and narrowing in the distal segment

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as luminal narrowing estimated less than 50% by SCAcould be found in middle RCA. No stenoses were foundby SCA in the segments that were not visualized by cor-onary MRA. The sensitivity, specificity, and positive andnegative predictive values for coronary MRA in the de-tection of clinically significant stenoses accounted for88, 94, 83, and 96%, respectively.

Six of six stenoses with luminal narrowing of lessthan 50% were correctly detected by coronary MRA;however, two false-positive results were obtained inproximal segments of LCX. The corresponding sensitivi-ty, specificity, and positive and negative predictive val-ues for detection of stenoses less than 50% all accountedfor 100, 96.5, 75, and 100%, respectively.

Discussion

High-resolution, ECG-triggered free-breathing 3D MRAwith MAG has the potential to be a feasible techniquefor visualization of coronary arteries. Qualitatively ade-quate visualization of proximal and middle segments ofcoronary vessels, both in healthy volunteers and patients,was achieved despite cardiac and respiratory motion.The combination with the T2-prep pulse for signal sup-pression from cardiac muscle further increases the de-tectability of the small vascular structures [19]. Thisstudy shows that sensitivity and specificity (88 and 94%)

are adequate for detection of significant coronary arterystenoses in a small group of patients with coronary arterydiseases referred for SCA. These results are comparableor even favorable to recent studies adopting intravenouscontrast-enhanced 3D MRA techniques. Additionally,the stenoses of less than 50% lumenal narrowing werecorrectly identified and were not reported to be morethan 50%.

Presently, most clinical experience in coronary MRAhas been accumulated with ultrafast 2D segmented turbofast low-angle shot (FLASH) gradient-echo techniqueswith breath-holding in order to avoid excessive blurringfrom respiratory motion [3, 4, 5]. The periods of breathholding and the long imaging times are a limitation forthese techniques due to lack of patient cooperation. Thesignal-to-noise ratio (SNR) and the required length ofthe breath-hold limit the effective spatial resolution of2D coronary MRA. Furthermore, inconsistent breath-holding position, heart motion, and beat-to-beat varia-tions may blur the vessel detail and can lead to slice mis-registration [23]. Adopting 2D segmented turbo FLASHMRA, sensitivity (56–90%) and specificity (82–97%)varied for detection of significant (≥50%) stenoses in theproximal parts of the coronary arteries [3, 4].

Significant progress is being made with 3D MRA.The 3D imaging allows acquisition of thin continuoussections as well as image postprocessing, resulting in anincreased SNR, higher spatial resolution, and easy imageplane setup [24]. Multiple techniques have been evaluat-ed since then, such as magnetization-prepared 3D seg-mented EPI with a single breath-hold volumetric imag-ing, providing a rapid anatomical and functional exami-nation of the heart [25]; however, all of these techniqueshave the disadvantages of breath-hold periods and/or in-jection of contrast agent in common.

For the time being, state-of-the-art coronary MRA iscomprised of 3D MRA techniques with the use of navi-gator echo [6, 8, 11, 26, 27]. Ideally, prospective or ret-rospective navigator gating during free breathing resultsin image quality similar to that achievable with breathholding; however, craniocaudal registration errors aresignificantly reduced (75% reduction compared withbreath holding) [26]. The decision to accept or reject da-ta can be made in real-time [28], like in our study, or ret-rospectively by resorting the data after it has been com-piled [9, 27]. A previous study adopting retrospectiverespiratory-gated MRA with two navigator spin echoesachieved a sensitivity of 48%, specificity of 92%, andpositive and negative predictive value of 67 and 85%, re-spectively [28]; however, there is a considerable debateabout the reproducibility of common navigator tech-niques and hence the associated image qualities.

In comparison with the aforementioned navigator tech-niques, MAG offers a high level of patient comfort by en-abling a free breathing mode. One problem of respiratorygating is the change in the respiratory pattern due to anxi-

Fig. 6 Non-enhanced, ECG-triggered, respiratory-motion-gated3D TFE sequence of LM, LAD, and proximal LCX obtained in apatient using MAG. The postprocessed image reveals an irregularbranching of the intermediate branch at the LAD/LCX bifurcation(arrow). False-positive stenosis ≥50% was reported by all investi-gators in middle LAD (double arrow). First diagonal branch ofLAD is slightly visualized (triple arrow)

ety, sedation, or sudden macroscopic shift or motion of thepatient. To obviate that problem and furthermore to mini-mize imaging time while enabling a free breathing pattern,the MAG algorithm has been developed. In situations ofchanges in the respiratory pattern it is necessary to decidewhether to restart the acquisition, to proceed with the cur-rent sub-optimal gating parameters, or to terminate thescan. In the event of a significant drop in scan efficiencywith a large amount of data remaining to be acquired, theMAG algorithm independently considers changing thereference position in real-time (seed). The MAG providesan intelligent and robust algorithm which, when these de-viations exceed a predefined threshold, automaticallychanges the parameters in real-time during scanning. Thesystem decides to change gating parameters, based on aparameter that registers changes of the motion pattern anda second indicator that expresses the quality of the data setalready obtained [20]; therefore, scan efficiency can be in-creased and scan time reduced. The investigations can becarried out even for patients with limited cooperative abil-ities. The MAG technique is suitable in all those instanceswhen acquisition time is long [20]. In this context it is ob-vious that the MAG technique facilitates a free breathingpattern for high-resolution coronary MRA.

If the distal segments of the LAD and RCA had beentaken into account, there would have been 70 and 69% ofthe CA segments in the patient and volunteer group, re-spectively, available for further analyses (p<0.05). If onlyproximal and mid-portions of CA had been taken intoconsideration, the detectability would have increased to 83and 87%, comparable to previous studies primarily focus-ing on these vascular regions [8, 9, 11]. The results of thisstudy suggest that image quality seems not to be affectedby the clinical state of the subject. Of coronary artery ste-noses, 85% were correctly assessed. The sensitivity of91% and specificity of 90% reached for detection of ste-noses are acceptable results. Most clinically significantcoronary artery stenoses are located in the proximal seg-ments of the coronary tree [29]; thus, a method that canimage these segments in a reliable, noninvasive, rapid,and cost-effective fashion would have a major effect onthe management of patients suffering from coronary arterydisease and could evolve into a screening method [8].

The false-positive findings of stenoses, 5 in the patientgroup and 2 in the volunteer group, still remain a technicalproblem to be solved. The evaluation of source images re-vealed an overestimation of two stenoses on reformattedimages. No further discordance was found in the evalua-tion of coronary artery stenosis on source and reconstruct-ed image data. Overestimation of the stenoses on refor-matted image data might have been due to reconstructionartifacts in terms of vessel wall partial-volume effects.Furthermore, the concept of assigning little importance tothe peripheral k-space data by the MAG technique mighthave had an effect on sharp edge definition and high spa-tial resolution. It seems possible that limited sharpness of

MRA might have contributed to the false stenoses report-ed. In 1 patient it is assumed that LM was not adequatelypositioned within the high-resolution slab and only mar-ginally obtained due to an atypical high branching off. Itseems likely that coronary MRA might be of clinical val-ue in the demonstration of anomalous coronary arteryanatomy [11], as anatomic norm-variances in the branch-ing of the main CA were depicted in this study.

Several limitations of our study must be addressed.The number of subjects studied, both volunteers and pa-tients, was small. The local ethics commission dictatesthat only such a small number be examined in the proto-type MR at the research lab; however, as this work wasdesigned as a feasibility study, we assume that the resultsof this study still hold true. Only patients with minorsymptoms of coronary heart disease (NYHA I-II) couldbe included. Although this poses a source of bias for im-age quality, we are convinced that the free breathing ap-proach would render good image quality even in patientswith more severe cardiopulmonary impairment; however,this remains to be proven. The MAG concept has a sub-stantial limitation. It assigns more importance to contrastthan to edge definition. Edge definition and sharpness areprerequisites for stenosis detection and quantification. Inorder to increase spatial resolution in all three dimen-sions, the thickness of the volume stack was limited;therefore, the distal segments of the LCX could not be in-cluded in that volume and were not eligible for analysis.The same applies to the marginal and diagonal branches.Consensus evaluation was chosen to simplify illustrationof results, but interobserver variability still must be con-sidered. Improvements and refinements of the softwarethat controls the respiratory motion pattern would be de-sirable to also accept unusual breathing patterns. Investi-gations with another navigator echo placed on the heartapex/base to register and overcome anterior–posteriormotion artifacts is in trial. Depiction improvements of theCA themselves are still necessary, i.e., with attempts witha higher pixel matrix (1024×1024). Further improve-ments in SNR, spatial resolution, and volume coveragewith broader stack volumes are essential to visualize dis-tal segments of the coronary arteries. Intravasal blood-pool contrast media offer one promising approach to in-crease SNR and the visibility of the coronary arteries[30]. The MAG in combination with the application of anbloodpool contrast agent would increase spatial resolu-tion of the coronary arteries and still increase image qual-ity. This study is in progress. Another possible approachfocuses on the use of a specially designed surface coil.

The results obtained in this study offer hope that thepresented 3D, free-breathing, ECG-triggered, navigator-gated-based coronary MRA with MAG algorithm couldbe helpful as a screening test in certain patient groups toexclude clinically relevant stenosis; however, clinical tri-als involving large patient groups are needed to definethe clinical usefulness and restrictions.

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References

1. Achenbach S, Moshage W, Ropers D,Nossen J, Daniel WG (1998) Value ofelectron-beam computed tomographyfor the noninvasive detection of high-grade coronary artery stenoses and oc-clusions. N Engl J Med 339:1964–1971

2. Wielopolski PA, Geuns RJM van, Feyter PJ de, Oudkerk M (1998) Coro-nary arteries. Eur Radiol 8:873–885

3. Manning W, Li W, Edelmann RR(1993) A preliminary report comparingmagnetic resonance coronary angiogra-phy with conventional angiography. N Engl J Med 328:828–832

4. Pennell DJ, Gogren HG, Keegan J, Firmin DN, Underwood SR (1996) Assessment of coronary artery stenosisby magnetic resonance imaging. Heart75:127–135

5. Edelmann RR, Manning WJ, BurnsteinD, Paulin S (1991) Coronary arteries:breath-hold MR angiography. Radiology 181:641–643

6. Wang Y, Rossman PJ, Grimm RC,Riederer SJ, Ehman RL (1996) Naviga-tor-echo-based real-time respiratorygating and triggering for reduction ofrespiration effects in three-dimensionalcoronary MR angiography. Radiology198:55–60

7. Hofman MBM, Paschal CB, Li D, Haacke EM, Rossum AC van, Sprenger M (1995) MRI of coronary arteries: 2D breath-hold versus 3D respiratory-gated acquisition. J Comput AssistTomogr 19:56–62

8. Müller MF, Fleisch M, Kroeker R,Chatterjee T, Meier B, Vock P (1997)Proximal coronary stenosis: three-dimensional MRI with fat saturationand navigator echo. J Magn Reson Imaging 7:644–651

9. Woodard PK, Li D, Haacke EM et al.(1998) Detection of coronary stenosison source and projection images usingthree-dimensional MR angiographywith retrospective respiratory gating:preliminary experience. Am J Roent-genol 170:883–888

10. Li D, Kaushikkar S, Haacke EM et al.(1996) Three-dimensional imaging ofcoronary arteries with retrospectiverespiratory gating. Radiology 201:857–863

11. Post JC, Rossum AC van, HofmanMBM, Valk J, Visser CA (1996) Three-dimensional resipratory-gated MR an-giography of coronary arteries: com-parison with conventional coronary angiography. Am J Roentgenol166:1399–1404

12. Kessler W, Achenbach S, Moshage Wet al. (1997) Usefulness of respiratorygated magnetic resonance coronary an-giography in assessing narrowings≥50% in diameter in native coronaryarteries and in aortocoronary bypassconduits. Am J Cardiol 80:989–993

13. Sandstede J, Pabst T, Beer M, Geis N,Kenn W, Neubauer S, Hahn D (1999)Three-dimensional MR coronary angio-graphy using the navigator techniquecompared with conventional coronaryangiography. Am J Roentgenol172:135–139

14. Hofman MBM, Paschal CB, Li D,Haacke EM, Rossum AC van, Sprenger M (1995) MRI of coronaryarteries, 2-D breath-hold versus 3-Drespiratory-gated acquisition. J ComputAssist Tomogr 19:56–62

15. Stuber M, Botnar RM, Danias PG, Sodickson DK, Kissinger KV, VanCauteren M, DeBecker J, Manning WJ(1999) Double-oblique free-breathinghigh resolution three-dimensional coro-nary magnetic resonance angiography.J Am Coll Cardiol 34:524–531

16. Wielopolski PA, Feyter PJ de, Bruin Hde, Bongaerts A, Oudkerk M (1997)Coronary screening with breath-hold 3-D MR coronary angiography. Eur Radiol 7:224

17. Goldfarb JW, Edelman RR (1998) Coronary arteries: breath-hold, gado-linium-enhanced, three-dimensionalMR angiography. Radiology 206:830–834

18. Taylor AM, Jhooti P, Wiesmann F, Keegan J, Firmin DN, Pennel DJ(1997) MR navigator-echo monitoringof temporal changes in diaphragm po-sition: implications for MR coronaryangiography. J Magn Reson Imaging7:629–636

19. Botnar R, Stuber M, Danias PG, Kissinger KV, Manning WJ (1999) Improved coronary artery definitionwith T2-weighted, free-breathing,three-dimensional coronary MRA. Circulation 99:3139–3148

20. Sinkus R, Börnert P (1999) Motionpattern adapted real-time respiratorygating. Magn Reson Med 41:148–155

21. Weiger M, Börnert P, Proska R, Schäffter T, Haase A (1997) Motionadapted gating based on k-spaceweighting for reduction of respiratorymotion artifacts. Mag Reson Med38:328–333

22. Scanlon PJ, Faxon DP, Audet AM, Carabello B, Dehmer GJ, Eagle KA,Legako RD, Leon DF, Murray JA, Nissen SE, Pepine CJ, Watson RM,Ritchie JL, Gibbons RJ, Cheitlin MD,Gardner TJ, Garson A, Russell RO,Ryan TJ, Smith SC (1999) ACC/AHAguidelines for coronary angiography. A report of the American College ofCardiology/American Heart Associa-tion Task Force on Practice Guidelines(Committee on Coronary Angiogra-phy). J Am Coll Cardiol 6:1756–1824

23. Duerinckx AJ, Atkinson DP, Mintorovith J, Simonetti OP, VrmanMK (1996) Two-dimensional coronaryMRA: limitations and artifacts. Eur Radiol 6:312–325

24. Börnert P, Jensen D (1995) Coronaryartery imaging at 0.5 T using segmen-ted 3D echo planar imaging. Magn Reson Med 34:779–785

25. Wielopolski PA, Manning WJ, Edel-man RR (1996) Breathhold volumetricimaging of the heart using magnetiza-tion prepared 3-D segmented echo pla-nar imaging. J Magn Reson Imaging4:403–409

26. McConnell MV, Khasgiwala VC, Savord BJ, Chen MH, Chuang ML,Edelmann RR, Manning WJ (1997)Comparison of respiratory supressionmethods and navigator locations forMR coronary angiography. Am J Roentgenol 168:1369–1375

27. Lin D, Kaushikkar S, Haacke EM et al.(1996) Three-dimensional imaging ofcoronary arteries with retrospectiverespiratory gating. Radiology 201:857–863

28. Sachs TS, Meyer CH, Hu BS, Kohli J,Nishimara DG, Macovski A (1994)Real-time motion detection in spiralMRI using navigators. Magn ResonMed 32:639–645

29. Detrano R, Hsiai T, Wang S, PuentesG, Fallavollita J, Shields P, Stanford W,Wolfkiel C, Georgiou D, Budoff M,Reed J (1996) Prognostic value of cor-onary calcification and angiographicstenoses in patients undergoing coro-nary angiography. J Am Coll Cardiol27:285–290

30. Taylor AM, Panting JR, Keegan J,Gatehouse PD, Amin D, Jhooti P, YangGZ, McGill S, Buman ED, Francis JM,Firmin DN, Pennell DJ (1999) Safetyand preliminary findings with the intra-vascular contrast agent NC 1001150 in-jection for MR coronary angiography. J Magn Reson Imaging 9:220–227