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DOI: 10.1148/radiol.2401050772
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(Radiology 2006;240:222-229.)
© RSNA, 2006

Technical Developments

Contrast-enhanced MR Angiography of the Peripheral Vasculature with a Continuously Moving Table and Modified Elliptical Centric Acquisition1

Ananth J. Madhuranthakam, PhD, Houchun H. Hu, BS, David G. Kruger, PhD, James F. Glockner, MD, PhD and Stephen J. Riederer, PhD

1 From the MR Research Laboratory and Department of Radiology, Mayo Clinic College of Medicine, 200 First St SW, Rochester, MN 55905. Received May 5, 2005; revision requested June 28; revision received July 25; accepted September 1. Supported by National Institutes of Health grants HL70620, EB00212, and EB004281. Address correspondence to S.J.R. (e-mail: riederer{at}mayo.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
This study was approved by the institutional review board and was HIPAA compliant. All subjects provided written informed consent, and subject confidentiality was protected. The purpose of this study was to prospectively evaluate the feasibility of integrating a modified elliptical centric (EC) acquisition with a continuously moving table technique to acquire high-spatial-resolution contrast material–enhanced magnetic resonance (MR) angiograms of the peripheral vasculature. Incorporation of two-dimensional homodyne reconstruction modified the EC view order, allowing improved spatial resolution per unit time while retaining the advantage of venous suppression intrinsic to the EC technique. Spatial resolution was dynamically improved when the table reached the distal-most station. The modified view order provided improved spatial resolution in phantom examinations compared with that in standard examinations. Peripheral MR angiograms were generated in a group of 13 volunteers (eight women; five men; age range, 51–72 years; mean age, 58.5 years ± 7.9 [standard deviation]) at 1.5 T. Four arterial regions were evaluated on a five-point scale (scores ranged from 0 to 4; a score of 4 was considered excellent); venous suppression was also evaluated. The mean arterial scores exceeded 3.0 for all regions. There was no venous signal or only superficial venous signal in 10 of the 13 cases.

© RSNA, 2006


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Contrast material–enhanced magnetic resonance (MR) angiography of the peripheral vasculature is technically challenging because of the need to image a field of view (FOV) that typically extends from the renal artery origin to the feet. Various moving table methods have been developed to minimize residual background signal from multiple injections. Perhaps the most common method is a multiple-station approach, in which the table is moved to a discrete number of stations, with separate data acquisition at each station (1–5). An alternative approach in which the table is moved through the imager bore and data are acquired continuously has been developed (6). A potential major advantage of this approach over the multiple-station approach is that no additional time is needed to move between stations.

The demand for high spatial resolution in peripheral vascular studies often increases as the vessel diameter decreases, particularly in the lower regions of the legs. A number of techniques have been studied in an attempt to provide improved spatial resolution per unit time for contrast-enhanced MR angiography, including undersampled projection reconstruction (7–10) and parallel imaging (11–13). These acquisition schemes involve the use of some form of undersampling to expand the k-space coverage during the time that a given position is within the actively sampled FOV. Alternatively, a modified elliptical centric (EC) (14) technique with two-dimensional (2D) homodyne reconstruction has been proposed and shown to provide improved spatial resolution compared with a standard acquisition in a fixed FOV (15).

The purpose of our study was to prospectively evaluate the feasibility of integrating a modified EC acquisition scheme with a continuously moving table (CMT) technique to acquire high-spatial-resolution contrast-enhanced MR angiograms of the peripheral vasculature in the arterial phase.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
The portion of our study that involved volunteers was approved by the institutional review board, and written informed consent was obtained from all participants. All examinations were performed with a 1.5-T Signa MR imager (GE Healthcare, Milwaukee, Wis). GE Healthcare also provided software for gradient nonlinearity correction.

CMT Technique
The specific CMT technique used in this study enabled the acquisition of three-dimensional (3D) Fourier transform data with the readout direction (x direction) oriented in the direction of patient table motion (6). This direction is the same as the direction of the longitudinal axis of the patient. An anterior-to-posterior section-select direction (z direction) and a right-to-left phase-encoding direction (y direction) are also chosen. With this technique, every repetition of the 3D readout direction is applied to a volume that is fixed in the imager bore but continuously advances along the patient because of table motion. The length of the excitation volume along the long axis is defined as the FOV subvolume.

Several factors are used to determine the spatial resolution of CMT imaging. Spatial resolution along the readout direction is calculated by dividing the FOV subvolume (FOVs) by the number of points in the readout, identical with the calculation for readout resolution in conventional MR imaging. Spatial resolution in the transverse plane is generated with the phase-encoding process and is ultimately determined by the time that the plane lies within the excitation volume. This is shown in the following equation (6):

Formula
where Ntotal is the total number of phase-encoding steps applied along the two phase-encoding directions (y and z), Vt is the velocity of the patient table, and TR is the repetition time of the pulse sequence. Thus, if many phase-encoding steps and a high lateral spatial resolution are desired, the FOV subvolume should be large, the patient table should be moved slowly, and a short repetition time should be used. For the specific application of CMT techniques to contrast-enhanced MR angiography, it is desirable for the advancing table position to match the peak arterial phase of the contrast material bolus as it moves along the vasculature (16). This factor typically drives the selection of the patient table velocity, which then dictates the spatial resolution that is available.

Modified EC Sampling
The previously mentioned equation indicates the relationship between the number of individual phase-encoding measurements and the other parameters of the CMT acquisition. The lateral resolution of the final image is determined by the k-space extent, with which the number of individual phase-encoding measurements is applied in the two phase-encoding directions. Spatial resolution is better with more k-space coverage. For standard 3D Fourier transform acquisition, the total number of phase-encoding steps is often set equal to the product of Ny times Nz, where Ny and Nz are the number of phase-encoding steps along the y and z directions, respectively. Another way to apply the same total number of phase-encoding steps is to use an elliptical pattern (Fig 1, A). This allows more extensive sampling along the ky and kz axes within the k-space, with improved apparent resolution.


Figure 1
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  		Figure 1: Schematic illustrations show k-space sampling of, A, conventional, and, B and C, modified EC view orders. The rectangular bounding boxes show the 2D phase-encoding plane, with axes ky and kz, and with a size of 256 x 80 points. Each repetition of the acquisition is identified as a small black dot; the shaded regions are groups of these dots. One phase-encoding step is measured per repetition. The total number of phase-encoding steps is 2560 for both sampling schemes (A and B); however, because of undersampling, the k-space coverage is extended farther in B. The central region is fully sampled to obtain the true phase of the object. The sampled views in the outer region are antisymmetrically oriented such that the 2D homodyne process can be performed. In C, high-spatial-resolution mode k-space sampling was performed at the lower station. The areas covered by conventional EC in A and modified EC in B are shown by the dashed line and the solid line, respectively.

 
The modified EC acquisition provides more extensive k-space coverage while using the same number of phase-encoding steps. This is done by defining an outer annular region, selecting radial sectors or "vanes" within this annulus, and sampling every other sector. The time freed up by not sampling half of these sectors is essentially reassigned for sampling a greater radius of the sectors actually sampled (Fig 1, B). This, in turn, provides improved spatial resolution with the same total acquisition time. The homodyne process is used to estimate data in the nonsampled sectors in Figure 1, B (17,18). This is similar to partial echo (17) or partial Fourier (19) methods, both of which are commonly available on many commercial MR imagers and are used clinically. The modified EC method, which is equivalent to a 0.55 fractional phase-encoding acquisition, has been studied technically and shown to provide improved spatial resolution with contrast-enhanced MR angiography of a fixed FOV (15).

CMT Acquisition and Reconstruction
The modified EC acquisition scheme was integrated with the CMT technique (6). Cartesian sampling enabled standard Fourier transformation to be used for image reconstruction. Each group of images (typically 16–64 images) was Fourier transformed in the x direction and placed into the hybrid x-ky-kz space. This was designated the hybrid space because Fourier transformation was performed along only one of the three k-space axes. Two separate reconstructions were performed at this point: Each reconstruction included Fourier transformation in the y and z directions and was followed by gradient nonlinearity correction (20). This was repeated for each group. In the first reconstruction, only the fully sampled central views were included to generate a low-pass 2D image for each x location. In the second reconstruction, a high-pass filtered 2D image was generated for each x location by multiplying all data points comprising the central views by one and data points within the outer-sample views by two to account for undersampling. The standard homodyne technique (17) and the phase of the low-pass 2D image were used to correct the phase of the high-pass 2D image and thus yield the final result.

Phantom Studies
A body coil was used for signal reception. A radiofrequency-spoiled gradient-echo sequence was modified with the intended EC k-space sampling. A resolution phantom and a long tube filled with gadolinium-doped water were used. The resolution phantom was placed so the resolution bars were oriented along the z direction for acquisition of coronal images. The number of phase-encoding steps for modified EC sampling was set (Ny = 128 [along the y direction], Nz = 70 [along the z direction], and Ntotal = 4100). For comparison, the same phantom was also imaged with sequential sampling, with 128 phase-encoding steps along the y direction, 32 phase-encoding steps along the z direction, and 4096 total phase-encoding steps. The FOVs along the x, y, and z directions were set to 24.6 cm, 24.0 cm, and 16.0 cm, respectively, and the extended superior-to-inferior FOV was 120 cm. The other parameters for the phantom study were as follows: repetition time msec/echo time msec, 5.9/2.1; full echo; number of readout points along the x direction, 256; flip angle, 30°; and receiver bandwidth, ±62.5 kHz. With these parameters, the table velocity was 1.0 cm/sec, and the total imaging time was 2 minutes for both acquisitions. A full echo was acquired for all examinations because of the 2D homodyne process along the phase-encoding directions.

Volunteer Studies
Contrast-enhanced MR angiography was performed in 13 consecutive healthy volunteers. The study group included eight women and five men (mean age, 58.5 years ± 7.9 [standard deviation]; age range, 51–72 years).

Volunteers were placed in the supine position, with feet first in the MR imager and pads under the legs to raise the lower leg vasculature to the level of the abdominal vasculature. Three sets of stationary scout images were obtained at the levels of the abdomen, thighs, and calves and used to prescribe volume locations for the CMT examination. A modified EC acquisition was used, with 128 phase-encoding steps along the y direction, 40 phase-encoding steps along the z direction, and 2560 total phase-encoding steps. The other imaging parameters were as follows: FOV subvolume, 36–40 cm; FOV along the y direction, 28–32 cm; FOV along the z direction, 9–12 cm; 5.9/2.1; full echo; number of phase-encoding steps along the x direction, 256; flip angle, 30°; receiver bandwidth, ±62.5 kHz; and table velocity, 2.35–2.70 cm/sec.

To corroborate the resolution improvement provided by the modified EC technique versus that obtained by using a fully sampled k-space with an equal total number of phase-encoding steps applied along the phase-encoding directions, a volunteer examination was performed with a sequential sampling scheme, with 128 phase-encoding steps along the y direction, 20 phase-encoding steps along the z direction, and 2560 total phase-encoding steps; all other parameters remained the same. For each volunteer study, 20–30 mL of gadodiamide (Omniscan; Nycomed, Princeton, NJ) was injected at a rate of 2 mL/sec; this was followed by injection of 25 mL of saline with a power injector (Spectris Solaris; Medrad, Indianola, Pa). For each contrast-enhanced examination, precontrast mask acquisition was performed before contrast material was injected with the CMT technique. At the outset of the contrast-enhanced examination, the same proximal excitation slab used to initiate the subsequent angiographic sequence was repeatedly excited at a reduced phase-encoding resolution of 128 x 4, without table motion, in a fluoroscopic triggering mode. Each 3D data set was reconstructed in real time, and a coronal maximum intensity projection (MIP) was formed and presented to the operator (A.J.M.) at a rate of 0.8 second per image. The time delay between central k-space acquisition and the display of real-time MIPs was 400 msec and included a reconstruction time of 300 msec. This enabled the operator to observe contrast material as it entered the abdominal aorta.

After the abdominal aorta was completely filled with contrast material, CMT imaging was triggered. This simultaneously caused the pulse sequence to switch to the high-spatial-resolution angiographic acquisition and initiated table motion. The table moved with the prescribed velocity and stopped at the distal-most FOV, covering the lower legs. After the table was stopped, data acquisition continued until it was stopped by the operator. The absolute central k-space was sampled one time for every FOV subvolume (36–40 cm) covered during the table motion and two or three times at the last station. In seven of the volunteers, a CMT time-resolved examination (16) was performed prior to the modified EC run to assess bolus velocity. On the basis of the findings of this study, a table velocity in the range of 2.3–2.7 cm/sec was selected. For the six studies that did not use a prior time-resolved run, a table velocity of approximately 2.5 cm/sec was used.

During CMT angiography, the sampled data were simultaneously directed along two parallel channels. In the first channel, 3D MIP images were reconstructed and displayed in real time; this permitted visualization of the passage of contrast material along the vasculature. Reconstruction was performed by only selecting the central-most 128 x 16 phase-encoding steps during each cycle of 128 x 40 phase-encoding steps. With this capability, it was possible to visualize the contrast material as it moved along the major blood vessels. In particular, this allowed us to visualize when the vasculature in the distal-most station was filled with contrast material. This permitted the operator (A.J.M.) to switch to the high-spatial-resolution mode of 256 x 80 phase-encoding steps, where k-space was sampled further in an undersampled manner (Fig 1, C). This method of interactive switching to a high-spatial-resolution mode was developed midway through the volunteer examinations and used in five studies.

The second channel was simply directed toward digital storage for off-line reconstruction, as described previously. It was not possible to perform this reconstruction in real time because of the complexity of the gradient nonlinearity correction (20) and the high spatial resolution of the data.

Image Evaluation
The MIP images obtained in the volunteers were qualitatively evaluated along different projections. The peripheral vasculature was divided into the following arterial segments: abdominal aorta, iliac arteries, combination of femoral and popliteal arteries, and distal tibial arteries. Arterial scores were based on crisp edges, high arterial signal intensity, and background suppression and were assigned with a five-point subjective scale, as follows: 4, excellent quality; 3, good quality; 2, acceptable quality for diagnosis; 1, marginally acceptable quality for diagnosis; and 0, uninterpretable. Images were also evaluated for overall venous overlay on a four-point scale, as follows: 0, no venous overlay; 1, superficial venous overlay; 2, deep but interpretable venous overlay; and 3, deep and uninterpretable venous overlay. Ratings were assigned in consensus by all authors, including an experienced MR vascular radiologist (J.F.G.). The authors had 3–20 years of experience.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Phantom Studies
The concept of CMT reconstruction with modified EC and 2D homodyne was demonstrated with phantom studies. Extended FOV images of good quality were reconstructed without any undersampling or motion-related artifacts (Fig 2, A). Images acquired with the modified EC technique provided improved spatial resolution compared with images acquired with sequential sampling and the same imaging time and table velocity (Figs 2, B, C). The improvement in spatial resolution along the section direction ({Delta}z = 2.3 mm) is clearly evident in Figure 2, B compared with the spatial resolution seen in Figure 2, C ({Delta}z = 5.0 mm).


Figure 2
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  		Figure 2: Phantom study results obtained with the 3D gradient-echo sequence (6.0/2.3; FOV subvolume, 24.6 cm; right-to-left FOV, 24 cm; slab thickness, 16 cm; and total FOV, 120 cm). A, Coronal MIP obtained with CMT and modified EC acquisition. B, Transverse reformat from the coronal image at the location shown by the dashed line in A acquired with 70 contiguous sections along the z direction. C, Corresponding transverse reformat acquired with sequential sampling and 40 sections. The resolution in B is better than that in C (2.3 vs 5.0 mm). Both images were acquired with the same acquisition time, table velocity, and number of views.

 
Volunteer Studies
Peripheral angiograms acquired with the CMT technique and modified EC sampling consistently were high-quality images (Table). The average arterial scores for all vascular regions were more than 3.0, which indicated diagnostic quality for the entire vasculature. Also, venous contamination was not considered a problem.


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  		Volunteer Study Evaluation

 
Compared with peripheral MR angiography performed with sequential sampling, the current technique provided improved spatial resolution and retained good image quality for the entire vasculature with use of the same examination time and table velocity (Fig 3). The improved spatial resolution was observed in the sagittal orientation (Figs 3, B, C). Because of the smaller section thickness acquired with modified EC ({Delta}z = 2.2 mm), the spatial resolution of the vessels is better along the section direction in Figure 3, B than in Figure 3, C ({Delta}z = 4.4 mm). Overall good image quality was observed in all images along different orientations (Figs 3–5).


Figure 3
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  		Figure 3: Contrast-enhanced MR angiograms obtained in a volunteer were used to compare modified EC acquisition and sequential sampling with the 3D gradient-echo sequence. Imaging parameters were as follows: 5.9/2.1; FOV subvolume, 40 cm; FOVy, 28 cm; slab thickness, 9 cm; and total FOV, 114 cm. A, Coronal MIP obtained with modified EC sampling in volunteer 10 shows good quality for depiction of the entire vasculature. B, Sagittal MIP of the left side of the body, and, C, corresponding sagittal MIP obtained with sequential sampling. Improved spatial resolution along the section direction is evident (arrows).

 

Figure 4
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  		Figure 4: A, Coronal, and, B and C, sagittal MIPs of the right (B) and left (C) sides of volunteer 5 show high-quality angiograms. Imaging parameters were as follows: 5.9/2.2; FOV subvolume, 36 cm; FOVy, 30 cm; slab thickness, 10.4 cm; and total FOV, 131 cm.

 

Figure 5
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  		Figure 5: A, Coronal, and, B and C, oblique MIPs obtained in volunteer 9. The k-space sampling for this study was extended from 128 x 40 at the upper station to 256 x 80 at the lower station after the table had stopped. D, Oblique MIP of the lower region identified in C, as reconstructed with 128 x 40 views, and E, the corresponding 256 x 80 reconstruction. Note the improved sharpness and better visualization of the vessels in E. Imaging parameters were as follows: 5.9/2.1; FOV subvolume, 40 cm; FOVy, 30 cm; slab thickness, 11.2 cm; and total FOV, 114 cm.

 
The method of extending k-space sampling at the last station provided further spatial resolution improvement (Fig 5). The true number of acquired phase-encoding steps was extended from Ny x Nz = 128 x 40 (Fig 5, D) to Ny x Nz = 256 x 80 (Fig 5, E), which resulted in increased spatial resolution by a factor of two along the right-to-left and anterior-to-posterior directions.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
We have presented a CMT technique with a modified EC acquisition scheme and 2D homodyne (15) in an attempt to acquire high-quality peripheral angiograms. Performing k-space sampling in an EC manner provides intrinsic venous suppression, and undersampling the higher k-space allows improved spatial resolution in a given time. Selecting the sampled views in the higher k-space to be antisymmetrically oriented permits a 2D homodyne process to compensate for the nonsampled views.

The k-space was undersampled in the phase-encoding plane to improve the spatial resolution; hence, the undersampled artifacts were restricted to the transverse plane in a coronal acquisition. Contrast-enhanced MR angiographic images are typically obtained in the coronal orientation and are most commonly visualized in the coronal and sagittal planes for diagnosis. With the current acquisition technique, the data are fully sampled along the frequency-encoding direction and maintain full spatial resolution along the superior-to-inferior direction.

The k-space is traversed faster with the current modified EC technique because of undersampling; thus, the modified EC technique provides additional venous suppression compared with the standard EC technique. Tourniquets were not used to delay venous onset (21); however, they may provide additional venous suppression and better spatial resolution.

The signal-to-noise ratio of the images presented in this article was somewhat limited owing to the use of a body coil for signal reception. The need for a higher signal-to-noise ratio in the lower legs becomes particularly important as the vessel diameter becomes smaller. In all of the images, however, the three major vessels in the lower legs were clearly visible, and the images depicting them were of diagnostic quality. In general, integration of surface coils with CMT techniques is expected to improve the signal-to-noise ratio. Also, additional benefits can be gained by using surface coils, which will allow the implementation of parallel imaging techniques like sensitivity encoding (22) with modified EC sampling owing to its cartesian sampling.

For all volunteer studies, a constant table velocity of approximately 2.5 cm/sec was used to acquire data. In two of the 13 studies, this table velocity was too fast for adequate depiction of the lower vasculature (16); hence, the popliteal vessels just above the trifurcation were not completely filled with contrast material before the table reached the last station. However, continuing data acquisition at the last station after the table stopped allowed capture of the contrast material in the tibial arteries. On the other hand, three of the 13 examinations had a venous score of 2 (ie, deep, but interpretable) because the table velocity was too slow. This will be more problematic in the patient population because of rapid venous return and asymmetric flow in different legs. In the future, performing data acquisition with multiple table velocities, similar to the approach used in the dual table velocity technique, (23) may address this limitation.

The major limitation of this study was the use of a body coil for signal reception. Use of surface coils is especially desirable in the lower legs because surface coils provide a higher signal-to-noise ratio. The other limitation was the lack of images, such as multistation peripheral MR angiograms, for comparison; hence, the arterial scores were assigned on a station basis as opposed to a vessel segment basis.

In conclusion, we have presented a CMT technique with a modified EC acquisition scheme and 2D homodyne reconstruction for acquisition of high-spatial-resolution peripheral MR angiograms in a given acquisition time and with a given table velocity.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   ACKNOWLEDGMENTS
 
The authors thank Kelly Dunagan for her efforts in coordinating the volunteer studies.


   FOOTNOTES
 

Abbreviations: CMT = continuously moving table • EC = elliptical centric • FOV = field of view • MIP = maximum intensity projection • 3D = three-dimensional • 2D = two-dimensional

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, A.J.M., D.G.K., S.J.R.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, A.J.M., S.J.R.; clinical studies, A.J.M., D.G.K., J.F.G., S.J.R.; experimental studies, all authors; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

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