Time-Resolved Undersampled Projection ReconstructionMagnetic Resonance Imaging of the Peripheral VesselsUsing Multi-Echo Acquisition

Jiang Du,1* Aiming Lu,2 Walter F. Block,1,2 Francis J. Thornton,3 Thomas M. Grist,1,3

and Charles A. Mistretta1,3

The hybrid projection reconstruction (PR) imaging provideshigh temporal resolution through an undersampled PR acquisi-tion for the in-plane dimensions and Cartesian slice encodingfor the through-plane dimension. The undersampling of projec-tion data introduces streak artifact, which may severely com-promise image quality. This study reports on a combination ofmulti-echo acquisition with time-resolved undersampled PRimaging and its application to peripheral magnetic resonanceangiography. Multi-echo acquisition improved imaging speedeffectively, thereby reducing the undersampling streak artifactand improving the temporal resolution. The gradient distortionwas reduced through gradient calibration and accurate k-spacetrajectory measurement. Magn Reson Med 53:730–734, 2005.© 2005 Wiley-Liss, Inc.

Key words: undersampled projection reconstruction; multi-echo; gradient distortion; peripheral MRA; contrast-enhancedMRA

Recently undersampled projection reconstruction (PR) im-aging has been investigated extensively in magnetic reso-nance angiography (MRA) (1–3). Hybrid undersampledprojection reconstruction imaging integrates Cartesianslice-encoding for the through-plane dimensions and PRacquisition for the in-plane dimension. The in-plane im-age spatial resolution is determined primarily by the read-out resolution. The number of acquired projections can besignificantly reduced from the Nyquist limit in regions ofhigh contrast, such as those available in contrast-enhancedMRA.

Both high spatial resolution and high temporal resolu-tion have been achieved for peripheral MRA using hybridundersampled projection reconstruction imaging (3). Thetime-resolved undersampled hybrid PR acquisition ade-quately depicts the complex contrast dynamics, such asasymmetric filling of the contrast material in the lowerextremities (3). This approach provides an advantage overthe conventional non-time-resolved technique when im-aging the lower extremities (4–7).

There is no spatial resolution tradeoff for temporal res-olution in time-resolved undersampled PR acquisition (1–3). However, angular undersampling results in star-likestreak artifact outside a local area of supported field ofview (FOV), the severity of which depends on the degreeof undersampling (1). This undersampling streak artifact issuperimposed on blood vessels and can severely compro-mise image quality. Streak artifact can be suppressed byacquiring more projections. However, this approach leadsto lower temporal resolution.

The conventional radiofrequency (RF) spoiled gradientecho (SPGR) sequence employs one RF excitation pulse foreach readout. However, multi-echo imaging techniques,such as echo-planar imaging (8), fast spin echo (9), andgradient and spin echo (10), acquire many readouts foreach RF excitation pulse, therefore greatly enhancing theacquisition efficiency. Multi-echo acquisition can be com-bined with 2D PR for MR fluoroscopy (11). It is highlydesirable to combine multi-echo acquisition with a time-resolved hybrid 3D PR sequence to further improve theacquisition efficiency, therefore suppressing the streak ar-tifact and further allowing increases in the temporal reso-lution.

In this study, a novel multi-echo acquisition strategyemploying a multigradient-echo sequence was combinedwith undersampled hybrid projection reconstruction. Fourhalf echoes were acquired in each TR. Dephaser/rephaserand ramp sampling were used to further improve the ac-quisition speed. A gradient distortion correction was per-formed to improve the image quality. Both phantom andpatient studies were carried out to test the performance ofthe multi-echo time-resolved undersampled hybrid PR ac-quisition method.

MATERIALS AND METHODS

The multi-echo hybrid PR sequence was developed basedon a previously reported projection reconstruction-hypertime-resolved imaging of contrast kinetics (HyperTRICKS)sequence, where a single echo was sampled during theflattop readout gradient (no data were sampled during theramps and dephaser/rephaser segments in this earlier im-plementation) (3). Figure 1 shows the multi-echo pulsesequence timing diagram. The excitation pulse employedwas a slab-selective asymmetrically truncated sinc RFpulse of 0.8 ms duration. The logical Gz-gradient was usedfor slice encoding in the anterior/posterior (A/P) directionin a coronal scan. The logical Gx-gradient was appliedduring signal readout and was also used for slab selectionin the superior/inferior (S/I) direction. Therefore, two dif-

1Department of Medical Physics, University of Wisconsin, Madison, Wiscon-sin.2Department of Biomedical Engineering, University of Wisconsin, Madison,Wisconsin.3Department of Radiology, University of Wisconsin, Madison, Wisconsin.Grant sponsor: NIH; Grant numbers: R01 HL51370 and R01 HL62425.*Correspondence to: Jiang Du, University of Wisconsin–Madison, Departmentof Radiology, E3/311 Clinical Science Center, 600 Highland Avenue, Madison,WI 53792-3252. E-mail: jiang@mr.radiology.wisc.eduReceived 3 November 2003; revised 10 June 2004; accepted 11 June 2004.DOI 10.1002/mrm.20404Published online in Wiley InterScience (www.interscience.wiley.com).

Magnetic Resonance in Medicine 53:730–734 (2005)

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ferent gradients were applied for slab selection (Gx) andslice encoding (Gz). Compared to the conventional A/Pslab selection pulse, where all the spins within the thickA/P slab are excited regardless of their S/I location, the S/Islab selection pulse limits the excitation spins within thecoronal imaging FOV, therefore effectively suppressingstreak artifact originating from the anatomy either superioror inferior to the imaging FOV (12). A relatively highreceiver bandwidth of �125 kHz was used to keep TRshort.

Four dual half echoes or two full echoes were acquiredwithin each TR. The data acquisition starts from the centerof k-space, proceeds to the edge, rotates slightly to the nextangle and goes through the whole projection, and thenrotates back to the initial angle and returns to the k-spaceorigin. Figure 2b shows the trajectory of the four halfechoes in each TR. Two small blips were added in thereadout gradient to rotate the projection angle. Dephaser/rephaser and ramp sampling were applied to furthershorten TR. Figure 2a shows the k-space partition strategy,where the whole k-space was divided into four regions: A,B, C, and D. A full echo partial slice encoding scheme wasadapted to further improve the acquisition efficiency (12),where all the slice encoding from region A was sampled,while half of the slice encoding was sampled for the B, C,and D regions, respectively. To maintain the spatial reso-lution, homodyne reconstruction was used along kz. Thisfull echo partial slice encoding strategy required the fourdual half echoes to line up into two full echoes, as shownin Fig. 2b. Figure 2c shows the sampling order of k-space

FIG. 1. Pulse sequence used for the continuous acquisition, asindicated by the data acquisition window (DAW), of four dual halfechoes in hybrid projection reconstruction, where PR was applied inthe kx–ky plane, and Cartesian slice encoding was applied along thekz direction. In addition to the standard encoding gradients, twosmall rotation gradients were applied to Gx and Gy, respectively, torotate the next readout projection. S/I slab selection was achievedusing the logical x-gradient and RF excitation pulse in a coronalscan. This figure stands for two continuous angles of trajectories,which were rotated for each excitation.

FIG. 2. (a) k-space partition formulti-echo partial number-of-excitation hybrid undersampledprojection reconstruction ac-quisition. (b) k-space trajectoryfor the four dual half echoes ac-quired in one echo train, wherethe dashed arrow correspondsto the rotation gradients to ro-tate the readout projection. (c)The conventional PR-Hyper-TRICKS acquisition strategy.

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data from the four regions, where the mask data wereacquired first, followed by the acquisition of the dynamicdata from low spatial frequency A, B, and C regions. Highspatial frequency D region was acquired during thepostvenous phase to further increase the slice resolution.The mask data were used for subtraction of the dynamicdata to generate time-resolved images (3).

Both PR acquisition and multi-echo acquisition are verysensitive to gradient distortion effects, which cause blur-ring or shifts that may severely compromise the diagnosticvalue of the MR images. Therefore, gradient distortioncorrection is especially important for multi-echo PR acqui-sition. In this study two previously reported gradient dis-tortion correction methods were applied consecutively.First, the physical gradient delay times along each readoutgradient were calibrated to center the projection data prop-erly so that gradient delay errors were reduced (13). Thenthe small deviation of k-space trajectory of the multi-echoPR sequence was measured using an eddy current correc-tion method proposed by Duyn et al. (14), where the truek-space trajectories were found from the phase differencebetween two measurements with and without spatial en-coding gradients. Typically, 40 projections were acquiredfor trajectory measurement along the kx and ky directions,respectively, before the 3D scan of each patient. This lattermethod proved to be very effective in gradient distortioncorrection in vastly undersampled isotropic 3D projectionreconstruction imaging (15). A linear density correctionwas performed for the data points sampled during ramps(15), followed by a Ram–Lak filter applied to all the pro-jection data to compensate for the nonuniform densitydistribution (3). The corrected k-space trajectories werethen regridded onto a Cartesian grid for fast Fourier trans-form to generate the final images.

A phantom study was performed to compare single-echoand multi-echo PR-HyperTRICKS acquisitions. A GeneralElectric phantom was put at the center of the imaging FOV.Then another uniform cylindrical phantom was put at theedge of the nominal FOV to further generate undersam-pling streak artifact. Four patient studies were performedusing multi-echo time-resolved undersampled PR-Hyper-TRICKS acquisition on a standard 1.5-T MR scanner (SignaCV/i; GE, Waukesha, WI) with a multi-channel quadra-ture/phased array peripheral vascular coil (Medical Ad-vances, Milwaukee, WI). Written informed consent in ac-cordance with regulations set forth by the institutional

review board was obtained before imaging procedures. Aninjection of 14 mL of gadolinium-based contrast agent(Omniscan, Nycomed Amersham, Princeton, NJ) was fol-lowed by a flush of 14 mL of saline. A computer-controlledpower injector (Spectris, Medrad, Indianola, PA) was usedto ensure a precise injection rate of 1.5 mL/s for the first7-mL contrast agent and 0.5 mL/s for the remainder.

RESULTS

Figure 3 shows the phantom study using both single-echoand multi-echo non-time-resolved acquisition with thesame spatial resolution and acquisition time. Figure 3awas imaged using a single-echo technique, and Fig. 3b wasimaged using the multi-echo technique. The imaging pa-rameters for single-echo acquisition are TR/TE/flip an-gle � 5.8 ms/1.9 ms/30°, FOV/matrix � 40 cm/384, ac-quired slices � 72, slice thickness � 1.5 mm, number ofprojections � 72. The imaging parameters for multi-echoacquisition are TR/flip angle � 7.5 ms/30°, TE for the fourdual half echoes were 0.8/2.584/2.584/4.368 ms, FOV/matrix � 40 cm/892, acquired slices � 72, slice thick-ness � 1.5 mm, number of projections � 224 dual halfecho projections or 112 effective full echo projections. Intotal, 892 points were continuously sampled, resulting inan effective matrix size of 384, where the first/sond/thirdreadout � 222/414/222. During each of the rotation gradi-ents, 17 points were sampled and disregarded during thereconstruction. Signal weighting due to T2* effect was notconsidered in the multi-echo study. Within the same scantime of 18 s, the undersampling factor was decreased from8.4 to 5.4 due to the multi-echo acquisition strategy, whicheffectively reduced the undersampling streak artifact asshown in Fig. 3b.

Figure 4 demonstrates the effect of the gradient distor-tion correction method. Figure 4a shows the original un-corrected image, which was severely blurred (marked by athick arrow) and distorted (marked by a thin arrow) due togradient distortion. The corrected image, shown in Fig. 4b,demonstrates effective removal of the blurring.

The time-series of images of the lower extremity fromanother patient study, shown in Fig. 5, depicts four timeframes corresponding to time frames 3, 5, 7, and 9. With aframe rate of 3.8 s/frame, the study clearly demonstratesthe complex contrast dynamics in this patient, especiallythe asymmetric filling of the contrast agent in the two legs.

FIG. 3. Phantom images scanned with(a) single-echo hybrid undersampled PRsequence and (b) multi-echo hybrid un-dersampled PR sequence. The samescan time of 18 s was used to acquire 72projections for single-echo acquisitionand 112 projections for multi-echo acqui-sition. Streak artifact was effectively sup-pressed (arrows) due to the improved ac-quisition efficiency in multi-echo acquisi-tion.

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Both the arterial and the venous systems in the left legwere enhanced about 20 s earlier than the arterial systemin the right leg. It is impossible to demonstrate this com-plex dynamics information using a conventional non-time-resolved bolus chase technique.

DISCUSSION

A fast multi-echo SPGR sequence integrated with hybridundersampled time-resolved projection reconstruction ac-quisition is described in this study. In the new acquisitionstrategy, four dual half echoes were acquired after each RFpulse, therefore significantly improving the speed perfor-mance and efficiency of the sequence. Sampling data dur-ing the dephaser/rephaser and gradient ramp further im-proved the speed performance and efficiency. Comparedto the single-echo acquisition strategy, the new multi-echoacquisition increased speed by 56%. The undersamplingstreak artifact was effectively suppressed through thephantom study using the multi-echo acquisition method(Fig. 3). The conventional single-echo acquisition couldalso benefit significantly from the dephaser/rephaser andramp sampling.

Compared to the single-echo acquisition, the multi-echoacquisition proved to be more sensitive to gradient distor-tion related factors, such as gradient anisotropy (16) andeddy current effect from the fast switching gradients (14).This gradient distortion effect becomes much more severewhen the multi-echo acquisition is combined with projec-tion reconstruction. As shown in Fig. 4a, severe imageblurring and distortion were caused by the gradient dis-tortion effect. The two-step correction method almost fullyresolves the gradient distortion resulted echo shift errorand resulted in excellent recovery of image quality asshown in Fig. 4b.

Another problem associated with multi-echo acquisitionis the T2* decay effect. Increasing the echo-train length ofa multi-echo acquisition will increase the acquisition effi-ciency. However, when the echo train length becomeslarge, T2* decay begins to dominate, resulting in a de-crease in SNR. Reeder et al. suggested the optimal durationof the echo train should be approximately T2* (17). In theperipheral lower extremity study, images from each halfecho were reconstructed. Not much T2* decay was foundwith an echo train length of four dual half echoes. Further

FIG. 4. (a) Gradient distortion related im-age blurring (thick arrow) and distortion(thin error) are shown in the original image.(b) The image blurring and distortion werecorrected through the gradient calibrationand k-space trajectory measurement.

FIG. 5. Asymmetric filling of thecontrast agent is demonstratedwith the time-resolved multi-echo PR-HyperTRICKS acquisi-tion through the coronal MIP offour frames: (a) frame 3, (b)frame 5, (c) frame 7, and (d)frame 9. The temporal resolu-tion is 0.26 frames/s. The ac-quired voxel size is 1.1 � 1.1 �1.5 mm3, which was zero pad-ded to 0.86 � 0.86 � 0.86 mm3

in the reconstruction.

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increasing the echo train length resulted in limited SNRimprovement.

In the current patient study, the multi-echo acquisitionmethod was mainly used to suppress the streak artifact byacquiring more projections, while slightly improving thetemporal resolution to 0.26 frames/s (fps) from 0.21 fps inthe single-echo acquisition (12). It seems that a frame rateof 0.26 fps provides enough temporal resolution to catchthe contrast dynamics in the lower extremities. In all pa-tient studies to date, the contrast dynamics were welldepicted, even in the case with asymmetric filling of thecontrast material in the tibial and popliteal arteries shownin Fig. 5, where the arterial system in the right leg wasenhanced 20 s later than that in the left leg. The dynamicimage series also provides a high spatial resolution of1.1 � 1.1 � 1.5 mm3, which was zero padded to 0.86 �0.86 � 0.86 mm3 in the reconstruction. Therefore, thismethod provides a true 4D acquisition, with both hightemporal resolution and high 3D spatial resolution, whichis a big benefit for clinical diagnosis.

This multi-echo acquisition strategy can be combinedwith any hybrid undersampled PR imaging to improve theacquisition efficiency and to suppress streak artifacts, es-pecially in contrast-enhanced MRA and interventional MRwhere both spatial resolution and temporal resolution arevery important.

CONCLUSIONS

A multi-echo sequence combined with time-resolved un-dersampled hybrid projection reconstruction acquisitionwas presented in this study. The gradient distortion effectswere corrected through gradient calibration and k-spacetrajectory measurement. The undersampling streak artifactwas effectively reduced, and the temporal resolution wasslightly improved. Asymmetric filling of the contrast ma-terial was well depicted using this novel sequence with ahigh temporal resolution of 0.26 fps and a high acquiredspatial resolution of 1.1 � 1.1 � 1.5 mm3.

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