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Published online before print February 28, 2007, 10.1148/radiol.2431052098
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(Radiology 2007;243:229-238.)
© RSNA, 2007

Technical Developments

Peripheral Vascular Disease: Comparison of Continuous MR Angiography and Conventional MR Angiography—Pilot Study1

Florian M. Vogt, MD, Michael O. Zenge, MSc, Mark E. Ladd, PhD, Christoph U. Herborn, MD, Katja Brauck, MD, Wolfgang Luboldt, MD, Jörg Barkhausen, MD and Harald H. Quick, PhD

1 From the Departments of Diagnostic and Interventional Radiology and Neuroradiology (F.M.V., M.O.Z., M.E.L., K.B., J.B., H.H.Q.) and Angiology (W.L.), University Hospital Essen, Hufelandstrasse 55, D-45122 Essen, Germany; and Department of Diagnostic and Interventional Radiology, University Hospital Hamburg-Eppendorf, Hamburg, Germany (C.U.H.). Received December 22, 2005; revision requested February 21, 2006; revision received March 16; accepted May 2; final version accepted August 4. Address correspondence to F.M.V. (e-mail: florian.vogt{at}uni-essen.de).


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 References
 
The aim of this study was to prospectively assess the accuracy of three-dimensional magnetic resonance (MR) angiography for evaluation of stenosis in the peripheral arterial system with a continuous moving table technique, with conventional MR angiography as reference. This study was approved by the local institutional review board; informed consent was obtained. Five healthy male volunteers (mean age, 27 years; range, 24–35 years) and four men and one woman (mean age, 63 years; range, 46–78 years) with peripheral arterial occlusive disease were examined. Images obtained with both techniques showed excellent concordance (Cohen {kappa} = 0.75). Images obtained with a conventional protocol had higher quality compared with those obtained with the continuous technique (mean, 1.07 ± 0.25 [standard deviation] vs 1.58 ± 0.6; P < .05); small vessels appeared sharper on them. For detection of significant stenosis and occlusion, accuracy, sensitivity, and specificity of the continuous technique were 92.8%, 100%, and 89.2%, respectively.

© RSNA, 2007

Bolus-chase techniques with the use of multistation table motion allow the stepwise assessment of the pelvic and runoff arteries within a single examination (1–7). Although various multistation approaches have been shown to be effective, these protocols have several inherent limitations. To cover extended anatomy with high image resolution and to stay within the arterial time window to prevent venous overlay, image time must be as short as possible (8). Repositioning of the table between discrete stations, however, leads to a reduction in image time efficiency because of the interruption of data acquisition during this process. In addition, gradient nonlinearities at the edges of individual fields of view (FOVs) have to be taken into account.

These limitations have been eliminated by the introduction of continuous moving table data acquisitions that provide seamless volume coverage and optimize image time efficiency (9). Since its application to three-dimensional (3D) contrast material–enhanced magnetic resonance (MR) angiography of the peripheral arteries by Kruger et al (10), several different acquisition and reconstruction methods for MR angiography during continuous table movement have been developed (10–13). In all these studies, the feasibility of such table movement during performance of 3D MR angiographic techniques has been demonstrated by using the body radiofrequency (RF) coil for signal reception. Limited sensitivity associated with body RF coil imaging, however, limits the inherently achievable signal-to-noise ratio (SNR) and, thus, spatial resolution, both of which are crucial for detailed assessment of the peripheral arterial system. The development of dedicated peripheral vascular RF coils has provided increased SNR and contrast-to-noise ratio (CNR) at the target vessels of the area of the leg between the knee and ankle (hereafter referred to as lower leg) for conventional multistation MR angiography of the peripheral arterial system. High spatial resolution combined with high SNR is the prerequisite for delineation of even small trifurcation arteries (14,15). Thus, our study aimed to prospectively assess the accuracy of 3D MR angiography with dedicated multichannel RF surface coils for evaluation of stenosis in the peripheral arterial system with a continuously moving table technique, with conventional multistation MR angiography as the reference standard. (Hereafter, these techniques are referred to as continuous MR angiography and conventional MR angiography, respectively.)


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 References
 
Subjects
Our study included five healthy male volunteers (mean age, 27 years; range, 24–35 years) without a history of peripheral vascular disease (confirmed with clinical history, nonsmoking status, physical examination) and five consecutive patients (four men, one woman; mean age, 63 years; range, 46–78 years) who were referred for conventional four-station bolus-chase MR angiography from March 2005 to April 2005 because they were suspected of having peripheral vascular disease (Fig 1). On the basis of the Rutherford classification, peripheral vascular disease was determined: One patient had grade II category 3, three patients had grade II category 4, and one patient had grade III category 5 (16). This study was approved by the local institutional review board, and informed consent was obtained from all participants.


Figure 1
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  		Figure 1: Flowchart illustrates structure of the study and treatment of individuals enrolled. MRA = MR angiography, PAD = peripheral arterial disease.

 
MR Imaging
Conventional MR angiography and continuous MR angiography were performed with a 1.5-T system (Magnetom Avanto; Siemens Medical Solutions, Erlangen, Germany). The system was equipped with a full-body array of phased-array surface RF coils with up to 76 coil elements and 32 independent RF receiver channels (Tim Technology; Siemens Medical Solutions). All subjects were placed feet first in the imager and examined in a supine position. To cover the entire abdominal aorta, which included the renal arteries down to the pedal arteries, a dedicated peripheral vascular coil (CP Peripheral Angio Array Coil; Siemens Medical Solutions) was combined with two phased-array body surface coils; each provided six coil elements that were connected in sets of three to four independent RF receivers (dual-receive mode). Furthermore, 12 of 24 elements of the spine array coil of the imager were connected in sets of three to four receiver channels. Altogether, for both conventional MR angiography and continuous MR angiography, 32 coil elements that were connected to 16 of 32 available receiver channels were used for data acquisition; this configuration resulted in a total coverage of 152 cm. Finally, for both techniques the image reconstruction times were assessed. Both examinations could be performed in all 10 subjects without any adverse events and without technical problems.

Continuous MR Angiography
The image reconstruction algorithm of Kruger et al (10) was incorporated into a standard reconstruction program (Siemens Medical Solutions), which is part of the imager. The method enables the acquisition and reconstruction of 3D coronal or sagittal data during continuous table movement. The algorithm subdivides into a subpixel position correction of each acquired k-line before Fourier transformation and into a pixelwise correction after Fourier transformation in the read direction. Partial FOVs are then combined into one seamless extended FOV. The continuous MR angiographic protocol was adjusted to cover an identical volume of interest as was used with conventional MR angiography. The data were collected by using a 3D T1-weighted fast gradient-echo sequence (fast low-angle shot) with linear k-space sampling and partial Fourier acquisition. The volume of interest was defined by an FOV of 400 x 1380 mm2 and a constant slab width of 115 mm. The entire data set was acquired within 77 seconds by using a constant table velocity, or vtable, of 18 mm/sec. Determination of the table speed was based on the calculation recently introduced by Kruger et al for frequency encoding along the z direction: vtable = {Delta}lz, read/Nphase · TR, where {Delta}lz, read is the table translation during one partial measurement, Nphase is the number of phase-encoding directions, and TR is the repetition time of the gradient-echo sequence (10). Movement of the table started immediately with the beginning of the image acquisition and stopped simultaneously with the end of data acquisition. All subjects were asked to hold their breath at the beginning of the acquisition (abdominothoracic region) for as long as possible and subsequently to breathe shallowly.

The acquired voxel size was 1.6 x 1.3 x 2.5 mm3. The data set was zero interpolated in the section direction and was reconstructed to a voxel size of 1.6 x 1.3 x 1.6 mm3 (Table 1). All 16 receiver channels were active during the entire acquisition. The tuning and the shimming were performed for the entire continuously acquired data set only at the first location before commencement of data acquisition with the continuous table movement.


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  		Table 1. Parameters for Four Stations at Conventional MR Angiography and for Continuous MR Angiography

 
Volunteers and patients underwent continuous MR angiography of the aortoiliac artery and arteries in the area from the thigh to the ankle (hereafter referred to as lower extremity) 48 hours prior to or after the conventional MR examination in a randomized order.

Conventional MR Angiography
On the basis of multiplanar scout images, the acquisition of four overlapping 3D data sets was planned. The data were collected by using the same 3D T1-weighted fast gradient-echo sequence as was used with continuous MR angiography, as described previously. Commercially available floating table MR angiographic software ensured the motion of the table from one discrete imaging position to the next.

Parameters were adapted and optimized for each of the four stations and are summarized in detail in Table 1. Comparable to the voxel size with continuous MR angiography, an interpolated voxel size of 1.7 x 1.3 x 1.6 mm3 was achieved in the first two stations. Acquisition time was 16 seconds for each of these two data sets. For the two subsequent stations, the acquired voxel size was reduced to 1.5 x 1.3 x 2.1 mm3 for the thigh (hereafter referred to as upper legs) and 1.1 x 0.9 x 2.1 mm3 for the lower legs. Data acquisition time was 14 seconds for the upper leg station, and the time was increased to 31 seconds for the lower leg station. Although spatial resolution was increased in the two lower leg stations compared with the spatial resolution with continuous MR angiography, total acquisition time could be kept equal (77 seconds) because of tailoring of the two lower leg stations. All volunteers and patients were asked to hold their breath during the acquisition for the first two stations (abdominothoracic region), whereas they were allowed to breathe freely during the acquisition for the other two stations.

In contrast to continuous MR angiography, for which all 16 receiver channels were active during the entire acquisition, for conventional MR angiography only those four channels were activated that provided information from tissue within the active imaging volume.

Venous Compression Technique
To prevent venous overlay, at both conventional MR angiography and continuous MR angiography, a 30-cm-wide thigh cuff (Speidel & Keller, Jungingen, Germany) was placed at the midfemoral level of both legs prior to the examination. The cuffs were inflated prior to acquisition of the precontrast image and were manually adjusted to a pressure of 50 mm Hg by using a nonferromagnetic pressure gauge (Speidel & Keller). The cuffs remained inflated until the end of the examination. The pressure of 50 mm Hg for compression was selected on the basis of the results of a study by Vogt et al (17).

Contrast Agent Injection Protocol
A paramagnetic contrast agent (gadoterate dimeglumine, Dotarem; Guerbet, Paris, France) was injected automatically (Spectris; Medrad, Pittsburgh, Pa) through a 19-gauge needle placed in the antecubital vein. Contrast agent transit time was calculated on the basis of the results published by Prince et al (8). A weight-adjusted dose of 0.2 mmol per kilogram of body weight (total dose range, 20–39 mL) was injected by using a biphasic protocol for both angiographic techniques: The first third of the volume was injected at a rate of 1.5 mL/sec, whereas the remaining amount was administered at a rate of 1.0 mL/sec, and the injection was followed by a 20-mL saline flush administered at a rate of 1.0 mL/sec. Thus, the entire injection duration ranged between 38 and 54 seconds. A test-bolus technique was used for synchronization of the contrast agent timing. To enable image subtraction, each 3D data set was collected twice: before contrast agent administration and during the arterial phase. The precontrast data set was subtracted from the postcontrast data by using imaging software (Syngo, version VA 2002 C; Siemens Medical Solutions). Subtracted data sets were not used for image evaluation but to render rotated maximum intensity projections over a 180° sector with 45 reconstructions.

Image Evaluation
For qualitative analysis, the arterial tree was classified into the following 35 segments: segments 1 and 2, right and left renal arteries, respectively; segment 3, infrarenal aorta; segments 4 and 5, right and left common iliac arteries, respectively; segments 6 and 7, right and left external iliac arteries, respectively; segments 8 and 9, right and left common femoral arteries, respectively; segments 10 and 11, proximal half of right and left superficial femoral arteries, respectively; segments 12 and 13, distal half of right and left superficial femoral arteries, respectively; segments 14 and 15, right and left popliteal arteries, respectively; segments 16 and 17, right and left tibioperoneal trunk, respectively; segments 18 and 19, proximal half of right and left anterior tibial arteries, respectively; segments 20 and 21, distal half of right and left anterior tibial arteries, respectively; segments 22 and 23, proximal half of right and left peroneal arteries, respectively; segments 24 and 25, distal half of right and left peroneal arteries, respectively; segments 26 and 27, proximal half of right and left posterior tibial arteries, respectively; segments 28 and 29, distal half of right and left posterior tibial arteries, respectively; segments 30 and 31, right and left dorsal pedal arteries, respectively; segments 32 and 33, right and left medial plantar arteries, respectively; and segments 34 and 35, rightand left lateral plantar arteries, respectively.

For measurements in the five volunteers, each arterial segment was assessed for image quality with both conventional MR angiography and continuous MR angiography on the nonsubtracted source images in consensus by two radiologists with 4 (F.M.V.) and 8 (J.B.) years of experience in MR angiography. Assessment was conducted on the basis of the following five-point scale: score 1, excellent diagnostic arterial display and excellent differentiation of arterial vasculature from background tissue; score 2, good diagnostic arterial display and no impairment of the delineation of vascular structures; score 3, fair diagnostic arterial display and delineation of vessel structures and exclusion or detection of vascular lesion still possible; score 4, poor diagnostic arterial display and inadequate vessel enhancement or severe blurring of vessel segment; and score 5, nondiagnostic arterial display.

For the data acquired in five patients, continuous MR angiographic image sets and conventional MR angiographic image sets were analyzed in a blinded order for the presence of vascular disease. The degree of stenosis for each vascular segment was characterized by using a four-point scale (grade 0, normal; grade 1, luminal narrowing ≤50%; grade 2, luminal narrowing >50% but no occlusion; or grade 3, occlusion). When two or more stenoses were present in one segment, the most severe lesion was used for subsequent assignment of a grade and analysis. Finally, all continuous MR angiographic data sets (in volunteers and patients) were evaluated for the presence of possible motion and breathing artifacts.

Quantitative Evaluation
In each volunteer, SNR and CNR values were calculated on the basis of the source images of the nonsubtracted data sets for 16 arterial segments as follows: suprarenal aorta, infrarenal aorta, bilateral external iliac arteries, bilateral common femoral arteries, bilateral superficial femoral arteries, proximal half of the bilateral popliteal arteries, proximal half of the bilateral anterior tibial arteries, middle part of the bilateral peroneal arteries, and distal half of the bilateral posterior tibial arteries. Signal intensity measurements were performed by using circular regions of interest (1.6–100 mm2) placed by one author (F.M.V.) within the center of the arterial vessel segments, defined as a region with the highest but also homogeneous signal intensity (SIvessel). Signal intensity determination was also performed in a reference area with regions of interest of identical sizes in adjacent muscle tissue (SItiss). These measurements were obtained close to the vessel to minimize the error that might occur caused by inhomogeneities of the signal-receiving RF surface coils. Absolute signal intensity measurements were related to noise, which was defined as the standard deviation of signal intensity measurements (SDnoise) collected in the background outside the body (air). SNR and CNR were calculated from signal intensity and standard deviation as follows: SNR = SIvessel/SDnoise, and CNR = (SIvessel – SItiss)/SDnoise.

For determination of overall image reconstruction times for conventional MR angiography and continuous MR angiography, the time between the end of data acquisition and the end of image calculation was recorded.

Statistical Analysis
A statistical software package (SPSS, version 12.1.0 for Windows; SPSS, Chicago, Ill) was used for subsequent statistical analysis. For comparison between conventional MR angiography and continuous MR angiography on the basis of the semiquantitive data, vessel segment image quality, and degree of stenosis, a Wilcoxon signed rank test was performed. Because many vessels were evaluated in each participant to assess image quality, Genmod repeated measures (generalized estimating equation) were performed additionally to account for data clustering. Therefore, total scores for the entire data sets were evaluated without prior classification of the arterial tree into segments. Comparison between measured SNR and CNR was accomplished by using the paired-samples t test. P values less than .05 were considered to indicate a significant difference. To measure concordance between conventional MR angiography and continuous MR angiography, the Cohen {kappa} value was calculated. Sensitivity, specificity, and accuracy values for continuous MR angiography were determined by using the results of conventional MR angiography as the standard of reference.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCE IN KNOWLEDGE
 References
 
Quantitative Evaluation
Both conventional MR angiography and continuous MR angiography permitted assessment of the entire vascular tree from the renal arteries down to the pedal arteries (Figs 2, 3). Results of quantitative analysis indicated a statistically significant difference between SNR and CNR values for arterial segments imaged with conventional MR angiography and those imaged with continuous MR angiography (P = .016 and .018, respectively) (Table 2). Mean SNR values amounted to 127.2 (range, 55.4–203.4) by using conventional MR angiography and 64.9 (range, 38.3–98.9) with continuous MR angiography; mean CNR values amounted to 108.1 (range, 46.9–172.1) by using conventional MR angiography and 55.5 (range, 30.2–89.2) with continuous MR angiography. The time that the MR system required for image data reconstruction for each examination was significantly increased from approximately 1 minute 40 seconds to 5 minutes 50 seconds when continuous MR angiography was used.


Figure 2A
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  		Figure 2a: Three-dimensional MR angiograms of peripheral vascular system in 30-year-old healthy male volunteer. (a) Continuous angiogram (2.06/0.86; flip angle, 19°; bandwidth, 1420 Hz/pixel) and (b) conventional angiogram (2.36–3.43/0.96–1.28; flip angle, 20°; bandwidth, 400–780 Hz/pixel). Increase in spatial resolution in two lower stations with conventional protocol compared with spatial resolution with continuous technique leads to slightly improved delineation of infrapopliteal vessels, especially the smallest intramuscular branches.

 

Figure 2B
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  		Figure 2b: Three-dimensional MR angiograms of peripheral vascular system in 30-year-old healthy male volunteer. (a) Continuous angiogram (2.06/0.86; flip angle, 19°; bandwidth, 1420 Hz/pixel) and (b) conventional angiogram (2.36–3.43/0.96–1.28; flip angle, 20°; bandwidth, 400–780 Hz/pixel). Increase in spatial resolution in two lower stations with conventional protocol compared with spatial resolution with continuous technique leads to slightly improved delineation of infrapopliteal vessels, especially the smallest intramuscular branches.

 

Figure 3A
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  		Figure 3a: Three-dimensional MR angiograms of the peripheral vasculature in 63-year-old male patient with peripheral arterial occlusive disease (Rutherford grade II category 4). (a) Continuous angiogram (2.06/0.86; flip angle, 19°; bandwidth, 1420 Hz/pixel) and (b) conventional angiogram (2.36–3.43/0.96–1.28; flip angle, 20°; bandwidth, 400–780 Hz/pixel). Note multiple stenoses (arrows) and occlusions, especially in pelvic and upper leg region. Better delineation of distal vessels at conventional MR angiography is seen because of inherent higher spatial resolution.

 

Figure 3B
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  		Figure 3b: Three-dimensional MR angiograms of the peripheral vasculature in 63-year-old male patient with peripheral arterial occlusive disease (Rutherford grade II category 4). (a) Continuous angiogram (2.06/0.86; flip angle, 19°; bandwidth, 1420 Hz/pixel) and (b) conventional angiogram (2.36–3.43/0.96–1.28; flip angle, 20°; bandwidth, 400–780 Hz/pixel). Note multiple stenoses (arrows) and occlusions, especially in pelvic and upper leg region. Better delineation of distal vessels at conventional MR angiography is seen because of inherent higher spatial resolution.

 

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  		Table 2. Mean SNR and CNR Values for Arterial Segments in Volunteers

 
Arterial Image Quality
Assessment of arterial image quality on the basis of the segment-to-segment comparison in the five volunteers revealed better mean grades for extremities imaged with conventional MR angiography compared with those imaged with continuous MR angiography: 1.07 ± 0.25 (standard deviation) versus 1.58 ± 0.6 (P < .05). Differences were even more apparent when the comparison was limited to only the distal segments (including the popliteal artery down to the pedal arteries): 1.1 ± 0.30 versus 1.75 ± 0.6 (P < .05) (Fig 4). None of the segments, however, was rated poor or nondiagnostic in either of the two imaging series. Assessment of arterial image quality on the basis of the entire data sets showed approximately similar results compared with the results of the segment-to-segment analysis: 1.2 ± 0.45 for conventional MR angiography and 1.6 ± 0.55 (P < .05) for continuous MR angiography.


Figure 4A
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  		Figure 4a: Selective view of runoff vessels with intraindividual comparison in a 46-year-old man. (a) Maximum intensity projection of the lower legs acquired with continuous MR angiography fails to show smaller branches of the distal arteries. Because all coils are in receive mode during image acquisition (as shown in diagram at left) and thus contribute to image noise detection, the SNR is negatively affected with this technique, compared with (b) conventional MR angiogram. Only the coil elements that are in the isocenter are switched on during acquisition (as shown in diagram at left) and contribute to the desired signal. Increased spatial resolution of lower stations with conventional MR angiography leads to better delineation of small distal arteries.

 

Figure 4B
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  		Figure 4b: Selective view of runoff vessels with intraindividual comparison in a 46-year-old man. (a) Maximum intensity projection of the lower legs acquired with continuous MR angiography fails to show smaller branches of the distal arteries. Because all coils are in receive mode during image acquisition (as shown in diagram at left) and thus contribute to image noise detection, the SNR is negatively affected with this technique, compared with (b) conventional MR angiogram. Only the coil elements that are in the isocenter are switched on during acquisition (as shown in diagram at left) and contribute to the desired signal. Increased spatial resolution of lower stations with conventional MR angiography leads to better delineation of small distal arteries.

 
Continuous table movement, in general, did not result in any noticeable motion artifacts. Of 175 possible arterial segments in five examined patients, three segments (1.7%) were not adequately depicted with both conventional MR angiography and continuous MR angiography because of severe venous overlay. At conventional MR angiography, 172 segments were well seen. Of these, 82 segments were characterized as normal. Disease was identified in 90 segments: vessel irregularities with 50% or less stenosis (n = 47), stenosis of more than 50% (n = 18), and occlusion (n = 25). By using continuous MR angiography, stenosis with 50% or less luminal narrowing was identified in 55 segments, severe stenosis of more than 50% was observed in 27 segments, and occlusion was seen in 30 segments (Table 3).


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  		Table 3. Scores Assigned by Two Observers in Consensus for Vessel Segments Depicted at Conventional and Continuous MR Angiography

 
Stenosis Evaluation
Continuous MR angiography led to overgrading in 29 segments (renal artery, n = 1; external femoral artery, n = 1; superficial femoral artery, n = 1; popliteal artery, n = 1; anterior and posterior tibial arteries, n = 11; peroneal artery, n = 6; plantar arteries, n = 5; dorsal pedal arteries, n = 3) and undergrading in one segment (tibial artery).

Significantly better agreement (P < .05) was found between both conventional MR angiography and continuous MR angiography for the evaluation of only vessels with a larger vessel lumen (infrarenal aorta down to popliteal artery): Nineteen segments were classified with mild stenosis, 10 were classified with severe stenosis, and 17 were classified as occluded by using both techniques. Continuous MR angiography led to overgrading in only three segments.

By combining results for severe stenosis and occlusion, the overall sensitivity and specificity of continuous MR angiography for lesions associated with hemodynamically significant arterial disease were 100% and 89.2%, respectively, with conventional MR angiography as the standard of reference. For the evaluation of only the infrapopliteal segments for lesions associated with hemodynamically significant stenosis or occlusion, the overall specificity value decreased to 86.2%, whereas the sensitivity value remained constant (100%). For analysis of all segments and of segments limited to the infrapopliteal region, the calculated accuracy was 92.8% and 87.6%, respectively. The Cohen {kappa} value, calculated to measure concordance between conventional MR angiography and continuous MR angiography, was 0.75.


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
 References
 
Implementation of a continuously moving table in MR angiography of the peripheral vascular system has recently also been presented by other investigators who have demonstrated good image quality in volunteers (11,13,18). Because of the limited number of receiver channels in the available MR systems, these protocols were evaluated by using the body RF coil for data collection. Sensitivity and specificity values for the detection of stenotic disease by using the multistation technique, as reported in the literature, range between 85% and 98% for femoral and popliteal vessels. When even smaller arteries are evaluated, considerably lower sensitivity and specificity values have been reported (3,4,19,20). The use of surface RF coils ensures high SNR, which is of particular importance to improve diagnostic accuracy when small arteries, such as those of the renal system or the pedal arteries, are imaged. In consideration of the increased achievable SNR that surface RF coils potentially can deliver, Fain et al (21) combined a moving table technique with the use of surface RF coils. However, because of a limited number of receiver channels available, only the imaging range from the pelvis to the pedal arteries could be covered with surface coils. The remaining parts (ie, the abdomen including the renal arteries) were imaged with the RF body coil (21). In our study, 32 surface coil elements were connected to 16 receivers and thus enabled full coverage of the imaging range with surface coils. Furthermore, beyond imaging healthy volunteers, we demonstrated high sensitivity and specificity values for detection of significant stenosis and occlusion in patients.

Although surface RF coil combinations were used while data were collected for both conventional MR angiography and continuous MR angiography, significantly lower SNR and CNR values were measured in the intraindividual comparison for the latter. This reduction may be related to the fact that, for continuous MR angiography, all 16 coil elements were in the receive mode during image acquisition and thus contributed to the sampling of noise, whereas for conventional MR angiography, only the four coil elements that were in the isocenter contributed to the desired signal and, thus, also to the sampling of noise. Implementation of a technique for actively switching on only the RF coil elements that are in the isocenter would potentially provide a solution to this artificial SNR reduction in continuous MR angiography.

In our study, 32 coil elements were combined and connected to 16 receivers only. When more than 16 receivers are activated, the amount of data that is obtained during an acquisition with continuous MR angiography exceeds the data reconstruction capabilities of the current MR system setup. This limitation also prevented the use of in-plane zero-filling interpolation in continuous MR angiography, and implementation of zero-filling interpolation potentially would have led to even better display of small arteries and, thus, to higher sensitivity and specificity values.

Not only because of increased SNR and CNR values but also because of the higher spatial resolution of the two lower stations (above the knee to the ankle), small vessels could be better delineated by using conventional MR angiography. Although optimization of image parameters and image resolution in conventional MR angiography was possible independently for each location, this flexibility was not available for continuous MR angiography. The latter protocol only allowed data acquisition with constant resolution; thus, the ability to assign a grade to the stenosis in smaller arteries with greater accuracy was impaired. Several attempts have been made to reduce image time or to improve spatial resolution in the setting of conventional high-resolution contrast-enhanced MR angiography by using parallel acquisition techniques (simultaneous acquisition of spatial harmonics, or SMASH; sensitivity encoding, or SENSE; generalized autocalibrating partially parallel acquisition, or GRAPPA), and these techniques can at least in part compensate for the lengthening in data acquisition times associated with high-spatial-resolution imaging (22–24). In recently performed studies, Zenge et al (25) and Hu et al (26) have shown that the combination of a continuous moving table technique with parallel imaging enabled the acquisition of seamless peripheral 3D contrast-enhanced MR angiograms with a more detailed display of the arterial vessel system when compared with the display with MR angiograms obtained with the continuously moving table as in our study.

The present study had several limitations. Active coil switching "on the move" has been identified as a major limitation in MR angiography with a surface coil and the continuously moving table and resulted in lower SNR and CNR values. This current limitation also is the reason for not completely exploiting the full capabilities of the MR imaging system. The MR imaging system potentially would have allowed connection of the 32 independent coil elements, which covered the anatomy of interest, to 32 independent receivers. In the technique with the moving table that we presented, compensation of gradient nonlinearities has not yet been implemented. Because compensation has not been implemented, the effective length of the sub-FOV (FOV in readout direction of individual k-lines) to the most linear region of the gradients is limited. By implementing a gradient nonlinearity correction, a larger portion of the imaging FOV in the z direction can be included in the data for reconstruction (27). Furthermore, with our technique for continuous table movement, only one constant table speed has been available for the full lengths of the FOV (28).

In summary, we demonstrated the capability for acquisition of high-quality seamless large-FOV 3D MR angiograms of the peripheral vascular system in patients during continuous table movement by using dedicated surface coils. We believe our results justify further clinical investigation in patients. Further spatial resolution increases and an extension of the FOV to whole-body MR angiographic applications can potentially be implemented in MR angiography with the continuously moving table. Therefore, a technique for actively switching the RF coil elements on and off during table movement, as well as a gradient nonlinearity correction, have to be investigated to further optimize the acquisition.


   ADVANCE IN KNOWLEDGE
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
References
 


   ACKNOWLEDGMENTS
 
We thank Sandra Massing, RT, and Michaela Jökel, RT, Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, Germany, for assisting with the volunteer and clinical studies.


   FOOTNOTES
 

Abbreviations: CNR = contrast-to-noise ratio • FOV = field of view • RF = radiofrequency • SNR = signal-to-noise ratio • 3D = three-dimensional

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, F.M.V., M.E.L., J.B., H.H.Q.; 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, F.M.V., M.O.Z., M.E.L., H.H.Q.; clinical studies, F.M.V., M.O.Z., C.U.H., K.B., W.L., J.B., H.H.Q.; statistical analysis, F.M.V., M.E.L., C.U.H., H.H.Q.; and manuscript editing, F.M.V., M.O.Z., M.E.L., C.U.H., W.L., J.B., H.H.Q.


   References
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 References
 

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