optimization of spatial resolution for peripheral magnetic resonance angiography

Optimization of Spatial Resolution for PeripheralMagnetic Resonance Angiography1

An Tang, Guy Cloutier, Eric Therasse, Gilles Beaudoin, Salah D. Qanadli, Marie-France Giroux, Nicolas BoussionJacques A. de Guise, Vincent L. Oliva, Gilles Soulez

Rationale and Objectives. To determine optimum spatial resolution when imaging peripheral arteries with magnetic reso-nance angiography (MRA).

Materials and Methods. Eight vessel diameters ranging from 1.0 to 8.0 mm were simulated in a vascular phantom. Atotal of 40 three-dimensional flash MRA sequences were acquired with incremental variations of fields of view, matrixsize, and slice thickness. The accurately known eight diameters were combined pairwise to generate 22 “exact” degrees ofstenosis ranging from 42% to 87%. Then, the diameters were measured in the MRA images by three independent observ-ers and with quantitative angiography (QA) software and used to compute the degrees of stenosis corresponding to the 22“exact” ones. The accuracy and reproducibility of vessel diameter measurements and stenosis calculations were assessedfor vessel size ranging from 6 to 8 mm (iliac artery), 4 to 5 mm (femoro-popliteal arteries), and 1 to 3 mm (infrapoplitealarteries). Maximum pixel dimension and slice thickness to obtain a mean error in stenosis evaluation of less than 10%were determined by linear regression analysis.

Results. Mean errors on stenosis quantification were 8.8% � 6.3% for 6- to 8-mm vessels, 15.5% � 8.2% for 4- to 5-mmvessels, and 18.9% � 7.5% for 1- to 3-mm vessels. Mean errors on stenosis calculation were 12.3% � 8.2% for observ-ers and 11.4% � 15.1% for QA software (P � .0342). To evaluate stenosis with a mean error of less than 10%, maxi-mum pixel surface, the pixel size in the phase direction, and the slice thickness should be less than 1.56 mm2, 1.34 mm,1.70 mm, respectively (voxel size 2.65 mm3) for 6- to 8-mm vessels; 1.31 mm2, 1.10 mm, 1.34 mm (voxel size 1.76mm3), for 4- to 5-mm vessels; and 1.17 mm2, 0.90 mm, 0.9 mm (voxel size 1.05 mm3) for 1- to 3-mm vessels.

Conclusion. Higher spatial resolution than currently used should be selected for imaging peripheral vessels.

Key Words. Magnetic resonance angiography; peripheral arteries; experimental studies.©
AUR, 2007
Acad Radiol 2007; 14:54–61

1 From the Department of Radiology (A.T., E.T., G.B., M.-F.G., V.L.O., G.S.),the Laboratory of Biorheology and Medical Ultrasonics-Research Center(G.C., N.B.), and Laboratory of Orthopedic Imaging-Research Center(J.D.G.), Centre Hospitalier de l’Université de Montréal (CHUM)-HôpitalNotre-Dame, Montreal, Quebec, Canada; and the Department of Radiology,Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland (S.D.Q.).Received March 6, 2006; accepted October 13, 2006. Supported by grantsfrom the Canadian Institutes of Health Research (G.C., G.S.; #MOP-53244)and Valorisation-Recherche Québec (group grant #2200-094), and by a Na-tional scientist award (G.C.) and clinical research scholarship awards fromFonds de la recherche en santé du Québec (G.C., G.S.). Address corre-spondence to: G.S. e-mail: [email protected]

©
AUR, 2007doi:10.1016/j.acra.2006.10.007
54

Magnetic resonance angiography (MRA) has gained wide-spread clinical acceptance for imaging the aorta and itsmajor branches (1–4). Technical advances in contrast-enhanced bolus chase three-dimensional (3D)-MRA withfast-gradient echo-recalled sequences, moving table, andtime-resolved imaging have improved diagnostic accuracy(5–11).

However, suboptimal correlations with Digital subtrac-tion angiography (DSA) have often been reported becauseof limited spatial resolution and frequent venous contami-nation observed with the sequence used for bolus-chasingMRA (12,13). For infrapopliteal vessels, better resultshave been obtained when the examination was performed
in two steps: a first injection combined with high-resolu-

Academic Radiology, Vol 14, No 1, January 2007 OPTIMIZATION OF SPATIAL RESOLUTION FOR PERIPHERAL MRA

tion acquisition on the distal leg and foot followed by asecond injection to image proximal vessels (14). How-ever, this approach requires two consecutive injectionsand acquisitions to image the entire runoff.

To the best of our knowledge, no study has assessedthe minimal spatial resolution when imaging iliac, femo-ropopliteal, and infrapopliteal arteries by gadolinium-en-hanced MRA. Also, the best tradeoff between spatial resolu-tion and acquisition time for bolus-chasing MRA needs tobe defined to avoid motion artifacts, minimize venous en-hancement (15), and exploit a single contrast-bolus injectioncombined with the moving-bed technique (16).

The aim of this study was to establish the optimumvoxel dimension for the three acquisitions undertaken inperipheral runoff studies: aorto-iliac, femoropopliteal, andinfrapopliteal vessels.

MATERIALS AND METHODS

Vascular PhantomsTwo multimodality vascular phantoms were built to

simulate vessels of size comparable to those of lowerlimb arteries (17). We simulated the process of stenosismeasurement by combining different lumen diameters,rather than physically simulate stenoses per se.

With a lost-material casting technique (18,19), we ap-

Figure 1. Phantom construction using lost-material casting tech-nique. (a) Two bronze rods with segment diameters ranging from1.0 to 8.0 mm. (b) Filling of vascular phantom with tissue-mimick-ing agar-based gel. (c) Latex vessels after removal of rod-latexunit. (d) Multimodality vascular phantom. Outer view.

plied a 1-mm thin latex layer simulating a vessel wall

around a cylindrical bronze rod made of four adjacent20-mm long pieces with different diameters (Fig 1). Twophantoms were built, one with segment diameters of 7.91,6.29, 4.39, 2.39, and 0.99 mm, and the other with seg-ment diameters of 5.10, 3.21, and 1.87 mm. The accu-rately known eight diameters were combined pairwise togenerate 22 “exact” degrees of stenosis ranging from 42%to 87% (Table 1).

The rod-latex unit was then molded in a tissue-mim-icking, agar-based gel inside a polyethylene phantomframe. After the agar solidified, the rod was removed,letting us a vascular conduit of known geometry that wasconnected to tubings at its extremities.

The lumen of the latex vessel was filled with gadolin-ium solution (gadopentate dimeglumine, Magnevist; BerlexCanada, Lachine, Quebec, Canada) of 1.8 mmol/L dilutedwith 0.9% NaCl solution with concentration comparable tothat of a bolus (20,21). The vessel lumen was maintained instatic conditions with a constant 100-mm Hg lumen pressureto avoid vessel collapse (17). The accuracy and precision ofphantom lumen diameters in these conditions were estimated

Table 1Pairs of Vessels and Corresponding Exact Degrees ofStenosis for the Stations “Infrapopliteal,” “Femoropopliteal,”and “Iliac”

LevelPairs of Vessels

(mm/mm)Stenosis

Percentage (%)

Infrapopliteal 1.87/3.21 41.74(1- to 3-mm vessels) 0.99/1.87 47.06

0.99/2.39 58.580.99/3.21 69.16

Femoropopliteal 3.21/5.10 37.06(4- to 5-mm vessels) 2.39/4.39 45.56

2.39/5.10 53.141.87/4.39 57.401.87/5.10 63.330.99/4.39 77.450.99/5.10 80.59

Iliac 4.39/6.29 30.21(6- to 8-mm vessels) 5.10/7.91 35.52

4.39/7.91 44.503.21/6.29 48.973.21/7.91 59.422.39/6.29 62.002.39/7.91 69.791.87/6.29 70.271.87/7.91 76.360.99/6.29 84.260.99/7.91 87.48

previously at �0.004 � 0.18 mm (17).

55

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TANG ET AL Academic Radiology, Vol 14, No 1, January 2007

Description of the System and AcquisitionProtocols

A 1.5-Tesla magnet (Magnetom Vision; Siemens, Er-langen, Germany) and a body array coil were used for thephantom study. The phantom with simulated vessel seg-ments was positioned parallel to B0. Forty 3D, fast, lowangle shot (3D Flash) MRA acquisitions were performed

Table 2Pixel Dimensions for All Magnetic Resonance Angiography Acq

Acquisition # Field of View (mm) Matrix Size Pix

1 390 � 390 512 � 4102 390 � 390 512 � 4103 390 � 390 512 � 4104 390 � 390 512 � 4105 390 � 390 512 � 4106 390 � 390 512 � 3207 390 � 390 512 � 3208 390 � 390 512 � 3209 390 � 390 512 � 320

10 390 � 390 512 � 32011 390 � 390 512 � 26012 390 � 390 512 � 26013 390 � 390 512 � 26014 390 � 390 512 � 26015 390 � 390 512 � 26016 390 � 390 256 � 19017 390 � 390 256 � 19018 390 � 390 256 � 19019 390 � 390 256 � 19020 390 � 390 256 � 19021 490 � 490 512 � 41022 490 � 490 512 � 41023 490 � 490 512 � 41024 490 � 490 512 � 41025 490 � 490 512 � 41026 490 � 490 512 � 32027 490 � 490 512 � 32028 490 � 490 512 � 32029 490 � 490 512 � 32030 490 � 490 512 � 32031 490 � 490 512 � 26032 490 � 490 512 � 26033 490 � 490 512 � 26034 490 � 490 512 � 26035 490 � 490 512 � 26036 490 � 490 256 � 19037 490 � 490 256 � 19038 490 � 490 256 � 19039 490 � 490 256 � 19040 490 � 490 256 � 190

Note: All other acquisition parameters remained constant: time o

in the coronal plane with two different fields of views

56

(390 � 390 mm and 490 � 490 mm), four different ma-trix sizes (512 � 410, 512 � 320, 512 � 260, 256 �190), and five different slice thicknesses (1.31, 1.50, 2.00,2.50, 3.00 mm). The pixel and voxel sizes used in theacquisitions are detailed in Table 2. No pixel interpolationwas used. All other parameters remained constant: time ofrepetition/time of echo 4.6/1.8; flip angle 30°; number of

ion Parameters

e (mm � mm) Slice Thickness (mm) Voxel Size (mm3)

6 � 0.95 1.31 0.956 � 0.95 1.50 1.086 � 0.95 2.00 1.446 � 0.95 2.50 1.816 � 0.95 3.00 2.176 � 1.22 1.31 1.216 � 1.22 1.50 1.396 � 1.22 2.00 1.856 � 1.22 2.50 2.316 � 1.22 3.00 2.786 � 1.50 1.31 1.496 � 1.50 1.50 1.716 � 1.50 2.00 2.286 � 1.50 2.50 2.856 � 1.50 3.00 3.422 � 2.05 1.31 4.082 � 2.05 1.50 4.672 � 2.05 2.00 6.232 � 2.05 2.50 7.792 � 2.05 3.00 9.356 � 1.20 1.31 1.516 � 1.20 1.50 1.736 � 1.20 2.00 2.306 � 1.20 2.50 2.886 � 1.20 3.00 3.466 � 1.53 1.31 1.926 � 1.53 1.50 2.206 � 1.53 2.00 2.936 � 1.53 2.50 3.676 � 1.53 3.00 4.416 � 1.88 1.31 2.366 � 1.88 1.50 2.716 � 1.88 2.00 3.616 � 1.88 2.50 4.516 � 1.88 3.00 5.411 � 2.58 1.31 6.461 � 2.58 1.50 7.391 � 2.58 2.00 9.861 � 2.58 2.50 12.321 � 2.58 3.00 14.78

etition 4.6/time of echo 1.8/flip angle 30°/number of excitation 1.

uisit

el Siz

0.70.70.70.70.70.70.70.70.70.70.70.70.70.70.71.51.51.51.51.50.90.90.90.90.90.90.90.90.90.90.90.90.90.90.91.91.91.91.91.9

excitation 1; slab thickness 70 mm.

lf the


Description of the Measurement ProtocolsFor each acquisition, one two-dimensional coronal

source image and one coronal and one sagittal maximumintensity projection images were generated on a 3D view-ing workstation (3D Virtuoso; Siemens). Each generatedimage included eight segments of different diameters ac-quired with the same acquisition parameters. This resultedin 960 segments (40 acquisitions � 3 images/acquisition� 8), which were independently measured by three expe-rienced vascular radiologists and by a quantitative angiog-raphy (QA) system and used to compute the degrees ofstenosis corresponding to the 22 “exact” ones.

Diameter measurements by these radiologists weretaken using standardized reading parameters (windowwidth: 912; window level: 473; magnification: 4.0�) on aPACS workstation (IMPAX 3.5; Agfa Medical Imaging,Greenville, SC). The radiologists positioned electroniccalipers at vessel borders perpendicularly to the long axisof the vessel. Objective image analysis was performedwith a public domain software (NIH Image J, version1.27z; National Institute of Health, Bethesda, MD) thatallows QA measurements using an intensity profile as thewidth at middle height of the peak of intensity (full widthat half maximum technique) (Fig 2). This technique al-lows window-setting independent measurements (22,23).

Statistical Analyses of Phantom StudyIndividual measurements of lumen diameters taken

at the mid-portion of each vessel segment were com-pared with true phantom diameters. Segment vesselsdeemed too blurred to be measured were excluded forvessel diameter analysis, and the error of stenosisquantification was arbitrarily set at 100% for these seg-

Figure 2. Quantitative automated measurement technique. (a) Imline perpendicular to the vessel axis indicating the measurement lequantitative diameter measurement corresponds to full width of ha

ments.

Signed mean errors, which are the mean of signed ab-solute differences (in percent) between measured valuesand the known diameters or known degree of stenoses,were calculated with their standard deviation and confi-dence intervals. Each radiologist’s dataset was analyzedindividually; the individual results were then averaged.Intraclass correlation coefficients between the radiologistsand QA measurements were computed for diameter mea-surement and stenosis calculation.

Paired Student t-test was applied to underline differ-ences between radiologists and QA measurements.

In addition, linear regression analysis with calculationsof Pearson’s correlation coefficients was performed toassess the correlations between mean errors of stenosisestimation and pixel width, surface, and slice thickness.Mean measurements of coronal source and coronal maxi-mum intensity projection images served to assess the cor-relations of these errors with the pixel surface or spatialresolution in the phase direction, whereas sagittal maxi-mum intensity projection measurements were used to as-sess the correlation for slice thicknesses.

Based on the results of the linear regression analyses,we calculated hypothetical thresholds for maximal pixelsize in the phase direction, pixel surface, and slice thick-ness when a 90% accuracy in stenosis measurement isrequired (mean error inferior to 10%). Although arbitrary,these accuracy thresholds were deemed to be clinicallyacceptable to provide the physician information for thera-peutic planning. These evaluations of stenosis measure-ments were performed separately for the diameters rang-ing from 6 to 8, 4 to 5, and 1 to 3 mm because wewanted to take into account the three usual steps of pe-

hows a coronal view of the vascular phantom with a transversea) Signal intensity along the transverse line in the left panel. Themaximum intensity (full width at half maximum technique).

age svel. (

ripheral MRA acquisition (aorto-iliac, femoropopliteal,

57

class


and infrapopliteal). The obtained thresholds yield the opti-mal pixel dimensions.

Mean errors of diameter and stenosis measurements ofthe three observers and QA were recalculated, with mea-surements taken only on acquisitions having a spatial res-olution equivalent to or higher than the previously definedthresholds.

In all cases, statistical analysis was performed withcommercially available software (SAS version 8.2, SASInstitute Inc, Cary, NC).

RESULTS

Diameter MeasurementsEach observer performed 960 diameter measure-

ments. Of a total of 3,840 diameters, 109 images weredeemed too blurred to be measured by the radiologistsor returned aberrant values with QA software. Meanerrors on diameter measurement were smaller with QAsoftware (�0.08 � 0.83 mm) than for the radiologists(�1.13 � 0.76 mm) (P � .0001) (Table 3). The repro-ducibility of the measurements in the phantom betweenthe group of three observers and QA software was verygood, as revealed by a global intraclass correlation co-efficient (ICC) of 0.83.

Stenosis CalculationsPairing the diameters generated 880 measures of steno-

ses. Of a total of 3,520 potential stenoses for four observ-ers, 341 were not valid because one of the two diameterswas nonmeasurable.

Stenosis overestimation was observed with both man-ual and automated measurements (Table 3). Mean errorson stenosis calculation were also higher for the three radi-

Table 3Mean Error and Precision of Diameter Measurements and Sten

Observers

Diameters

MeanError (mm)

Precision(1 SD)

QA software �0.083 �0.827Average 3 radiologists �1.130 �0.764ICC for 4 observers 0.8304Difference between software

and radiologistsMean � 1.21 � 0.

P �.0001

SD: standard deviation; QA: quantitative angiography; ICC: intra-

ologists (12.3% � 8.2%) than for QA software (11.4% �

58

15.1%) (P � .0342). Reliability among the three observ-ers and QA software was good, with a global ICC of0.76.

Stenosis quantification by the radiologists was moreaccurate for larger than for smaller vessels, with an over-all accuracy of 8.8% � 6.3% (SD) for 6- to 8-mm ves-sels, 15.5% � 8.2% for 4- to 5-mm vessels, and18.9% � 7.5% for 1- to 3-mm vessels (Table 4).

Effect of Spatial Resolution in the PhaseDirection

Regression analysis of the mean errors on stenosis mea-surement and spatial resolution in the phase direction (pixelwidth) is detailed for the three different vessel sizes in Fig 3.Our results show that the correlation between mean errors onstenosis measurement and spatial resolution in the phasedirection was significant for all vessel sizes, but higher forsmaller vessels: r � 0.538 for 1- to 3-mm vessels (P �.0001); r � 0.471 for 4- to 5-mm vessels (P � .0001); and

Table 4Stenosis Quantification by Radiologists/by Vessel Size for AllAcquisitions

Vessel Size

Stenoses

MeanError (%) Precision (1 SD)

ConfidenceInterval

1–3 mm 19.0 �7.6 17.5/20.54–5 mm 15.6 �8.2 14.5/16.66–8 mm 8.8 �6.3 8.1/9.4ICC for stenoses

measurements1–3 mm � 0.57614–5 mm � 0.66976–8 mm � 0.7755

SD: standard deviation; ICC: intraclass correlation coefficient.

Estimation for All Sequences

Stenoses

ConfidenceInterval

MeanError (%)

Precision(1 SD)

ConfidenceInterval

0.138/�0.030 11.4 �15.1 10.3/12.51.081/�1.180 12.3 �8.2 11.7/12.9

0.7632m Mean � 0.9 � 0.12%

P � .034

correlation coefficient.

osis

�

�

89 m

r � 0.365 for 6- to 8-mm vessels (P � .001).


Effect of Pixel SizeSlightly lower but still highly significant correlation

coefficients were found by regression analysis betweenthe mean errors on stenosis measurement and pixel size(pixel surface): r � 0.4666 for 1- to 3-mm vessels (P �.0001); r � 0.3730 for 4- to 5-mm vessels (P � .0001);and r � 0.2877 for 6- to 8-mm vessels (P � .001).

Effect of Slice ThicknessLinear regression analysis between the mean error on

stenosis and slice thickness is presented in Fig 4. Thecorrelation was also stronger for small vessel diametersand significant for all vessel sizes: for 1- to 3-mm ves-sels, r � 0.4501 (P � .0001); for 4- to 5-mm vessels,r � 0.4816 (P � .0001); and for 6- to 8-mm vessels, r �0.3843 (P � .001).

According to these statistical results, a mean stenosismeasurement error below 10% was obtained for 6- to

Figure 4. Regression analysis of mean errors on the measureddegrees of stenosis (%) versus slice thickness (mm) for the threevessel size ranges. Bars represent standard error of the means.

Figure 3. Regression analysis of mean errors of the measureddegrees of stenosis versus pixel width (mm) for the three vesselsize ranges. The pixel width is the spatial resolution defined bythe matrix size in the phase direction. Bars represent standarderror of the means.

8-mm vessels by using a maximal pixel size in the phase

direction of 1.34 mm, pixel surface of 1.56 mm2, andslice thickness of 1.70 mm (voxel size 2.65 mm3); for 4-to 5-mm vessels, a maximum pixel size in the phase di-rection of 1.10 mm, pixel surface of 1.31 mm2, and slicethickness of 1.34 mm (voxel size 1.76 mm3). To achievethe same level of accuracy with 1- to 3-mm vessels, itwas extrapolated from the same regression analyses equa-tions that a maximum pixel size in the phase direction of0.90 mm, a pixel surface of 1.17 mm2, and a slice thick-ness of 0.90 mm would be required (voxel size1.05 mm3).

Improvement in the accuracy and precision of stenosisevaluations was observed when only sequences having aspatial resolution equivalent to or higher than the thresh-olds previously defined were chosen (1.5% � 7.0% forQA software and 6.7% � 4.5% for the three radiologists)with a very good reproducibility between the group ofthree observers and QA software (ICC of 0.84) (Table 5).

DISCUSSION

Lower extremity peripheral vessels are commonly ac-quired in the coronal (XZ) rather than the axial (XY)plane to maximize the spatial resolution in the coronalplane, which is important in imaging lower limb vessels.Because stenoses are perpendicular to the vessel axis,stenosis severity would be better evaluated in the XYplane for most of the peripheral vessels.

In this study, we have found a proportional relation-ship between mean stenosis error versus spatial resolutionin the phase direction, pixel surface, and slice thickness.This work demonstrates also that higher spatial resolution

Table 5Stenosis Quantification by Radiologists/by Vessel Size AfterSequence Selection

Vessel size

Stenoses

MeanError (%) Precision (1 SD)

ConfidenceInterval

1–3 mm 8.7 �4.0 2.3/15.04–5 mm 8.4 �5.6 5.9/10.96–8 mm 6.2 �4.1 5.3/7.1ICC for stenoses 1–3 mm � 0.7241

4–5 mm � 0.77216–8 mm � 0.8584

SD: standard deviation; ICC: intraclass correlation coefficient.

than presently used in clinical practice is necessary to

59


properly image lower limb arteries in peripheral MRA,even at the level of iliac and femoral arteries. Pixel di-mensions currently used for peripheral runoff studies aretoo large for the vessel diameter to be measured espe-cially at the infrapopliteal level. This study also showedthat an optimal combination of smaller pixel dimensionsin the x, y, and z axes improved the accuracy and preci-sion of stenosis measurements.

Our phantom study revealed a slight overestimation ofvessel diameter when assessed by the radiologists. Steno-ses were overestimated by all three observers and by theQA software. This effect was more pronounced on smallthan large vessel diameters, as reported previously (24).In the literature, stenosis overestimation has been oftenreported with Gd-enhanced MRA (7,11,14).

We demonstrated better accuracy and interobservervariability on measurement with QA software over man-ual measurement by an independent observer, both fordiameter and stenosis assessments. Overestimation of di-ameters by the radiologists can be explained by percep-tion issues. Conversely, quantitative automated measure-ments are window-independent and rely on plot profilesof source magnetic resonance imaging signals (22). How-ever, the accuracy of this method can be influenced bysaturation effects.

Obtaining the level of spatial resolution recommendedby our phantom study during a three-station examinationwhile avoiding venous contamination requires MRAequipment with high gradient capabilities, time-resolvedimaging, and a dedicated phased array coil. Recently, ad-vances in gradient technology have shortened the minimaltime of repetition. However, even with last-generationequipment, it is not possible to get this level of spatialresolution using conventional 3D-Flash sequences.

The 3D “shoot-and-scoot” techniques involving centralk-space acquisition during bolus injection followed byperipheral k-space acquisition during the venous phasecould be another alternative to obtain this level of spatialresolution while minimizing venous superposition (25).

Another alternative is to use parallel acquisition tech-niques [SiMultaneous Acquisition of Spatial Harmonics(SMASH), SENSitivity Encoding (SENSE), GeneRalizedAutocalibrating Partially Parallel Acquisition (GRAPPA)]and to shorten scan time by a factor proportional to theparallel imaging factor. De Vries et al and Bezooijen et alhave used a parallel imaging factor of 2 with satisfactoryclinical results (8,9). However, the voxel dimension usedin their studies was larger than the maximum voxel size
defined in our study (5.3 mm3, 4.3–3.9 mm3, and 1.4–
60

0.98 mm3 at the pelvic, upper, and lower leg stations,respectively). In a clinical setting, we tried to implementon an Avanto unit (Siemens, Erlangen) acquisitions pa-rameters for peripheral runoff examinations to get a spa-tial resolution higher or equivalent to the thresholds de-fined in our study. To fit these parameters within an ac-quisition time inferior to 70 seconds, we had to apply aGRAPPA factor of 3. This gain in time of acquisition andspatial resolution is made at the price of additional arti-facts and loss of signal-to-noise ratio, which can deterio-rate image quality (26,27).

Study LimitationsThis study was performed in ideal conditions with a

vessel phantom parallel to the magnetic field (B0) that didnot take into account the obliquity of the iliac bifurcationand popliteal trifurcation. Also, the effects of flow andpulsatility were not included. We have not measured ste-nosis within vessels, but we have simulated stenoses bycombining diameter ratios. Atherosclerotic stenoses areoften irregular, eccentric, and more difficult to evaluate.At the time this phantom study was conducted, parallelimaging was not available in our institution; therefore,optimized pixel dimension have been set only with con-ventional 3D-Flash sequences.

Based on our phantom study, we can conclude thathigher spatial resolution than currently used in clinicalroutine should be selected to image peripheral arteries andthat quantitative automated analysis of vessel diameterand stenosis is more accurate than manual measurements.

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The Association of University Radiologists wishes to thank the followingcompanies for their generous support of the AUR Junior MembershipProgram:

4th YearRadiologyResidents

SiemensMedical Systems, Inc.

3rd YearRadiologyResidents

BerlexLaboratories

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optimization of spatial resolution for peripheral magnetic resonance angiography

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