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Original Research |
1 All authors: Department of Diagnostic Radiology, University Hospital Basel, Petersgraben 4, Basel, Switzerland 4032.
Received June 12, 2005;
accepted after revision August 23, 2005.
Address correspondence to T. M. Gluecker
(glueckert{at}uhbs.ch).
Abstract
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MATERIALS AND METHODS. Ten healthy individuals (mean age, 24.4 years; range, 20-27 years) underwent 3D contrast-enhanced MR angiography of both hands. Starting 3 minutes before data acquisition, subsystolic upper arm cuff compression was applied unilaterally. A 1.5-T whole-body scanner with 3D gradient-echo sequence was used. Seven data sets (20 seconds) were obtained consecutively. IV contrast material of 0.1 mg/kg of body weight of gadobutrol was injected at rates of 0.5, 1.0, and 1.5 mL/s. For both hands, quantitative data evaluation was performed with contrast-to-noise ratio (CNR) in the radial, ulnar, palmar, and digital arteries and veins. Qualitative assessment of the arterial visualization score and venous contamination score was rated by two experienced radiologists using a 4-point scale.
RESULTS. The lowest venous contamination score (CNR and reviewers' assessment) was observed with an injection rate of 0.5 mL/s (p < 0.05). For the arterial signal, the reviewers' assessment was that an injection rate of 0.5 mL/s was best (p = 0.08). Compression yielded a significantly lower venous contamination score for the compressed side than for the noncompressed side for flow rates of 0.5 mL/s and 1.0 mL/s (p < 0.05).
CONCLUSION. Image quality of hand MR angiography was better with cuff compression. A flow rate of 0.5 mL/s yielded a good CNR and a significantly lower venous contamination score than the other flow rates.
Keywords: contrast media • hand • MR angiography • musculoskeletal imaging
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Thus, IV contrast-enhanced high-resolution 3D MR angiography of the hand was developed. It is an alternative imaging method to digital subtraction angiography and is being used increasingly in clinical practice. It is safe, minimally invasive, and avoids radiation exposure [1-5].
However, contrast-enhanced MR angiography is technically challenging for various reasons. First, the small caliber of the vessels of the hand requires high in-plane spatial resolution. Second, the short arteriovenous transit time results in early venous contamination [2, 5].
Recently, it has been shown that subsystolic continuous upper arm cuff compression can be used to increase arterial transit time and to delay the arrival of contrast material in the venous system [5]. Cuff compression causes a local axon reflex (venoarteriolar reflex). This regulation process results in reduced arteriolar inflow. This reflex likely protects the capillary bed from a high local pressure gradient as a result of vascular inflow and leads to a reduced signal in the arterial vessels. However, the flow of contrast material into the adjacent interstitium is reduced at the same time, which probably further delays the arrival of contrast medium in the venous vessels. Thus, undesired venous contamination is reduced and overall image quality is improved [6].
The effect of subsystolic continuous cuff compression was discovered and confirmed in other studies applying cuff compression at the level of the lower extremities [7-10]. This technique was then applied to the upper extremities [4, 5].
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The first study was performed with an IV flow injection rate of 0.5 mL/s, the second with 1.0 mL/s, and the third with 1.5 mL/s. Before each study, upper arm compression was applied unilaterally at the same side during all three studies, using a standard blood pressure cuff. Upper arm cuff compression was initiated 3 minutes before each study and was maintained during the entire period of data acquisition. Subsystolic compression was performed with a pressure value of 30% less than the brachial artery systolic blood pressure measured before the examination.
All volunteers were placed in the prone position with both upper extremities in pronation above the head. To avoid contact between the hands, the thumbs were separated with a soft cushion that further served as a support to avoid motion artifacts. This method was similar to a previously established method [5].
All MR angiography studies were performed on a 1.5-T whole-body MR scanner (Magnetom Sonata, Siemens Medical Solutions). A neck array coil was used for signal reception. All MR angiography studies of the hands were performed with the following imaging parameters: 3D gradient-echo sequences with matrix size, 512 x 176; field of view, 320 x 200 mm; 56 partitions per slab; partition thickness, 1 mm; TR/TE, 4.45/1.28; flip angle, 25°; acquisition time per slab, 20 seconds; bandwidth, 390 MHz; number of averages, 1; time of k-space center, 7.1 milliseconds; and zero filling.
Before contrast administration, one measurement was acquired that served as a mask for subtraction. Starting immediately after the initiation of the IV contrast administration, seven imaging series of measurements were acquired continuously. Each of the seven measurements was obtained during 20 seconds for a total scanning time of 140 seconds.
For all three studies, volunteers received a standard dose of gadobutrol (Gadovist, Schering) of 0.1 mg/kg of body weight, resulting in a total volume of 8 mL, into an antecubital vein of the noncompressed arm. This injection was followed by a flush of 30 mL of normal saline (0.9%). The total volume injected was 38 mL (8 mL of Gadovist and 30 mL of normal saline). A power injector (Spectris, Medrad) was used for contrast administration.
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Volunteers were continuously monitored during data acquisition and contrast administration. The study protocol required the recording of all adverse effects. Volunteers were questioned after each study regarding adverse effects and discomfort during the study. All volunteers were monitored for 1 hour after the study.
For each measurement, maximum intensity projections (MIPs) were reconstructed in the anteroposterior projection for further data evaluation. The measurement before contrast administration served as a mask for subtraction. Data assessment was performed both qualitatively and quantitatively.
Quantitative data evaluation consisted of contrast-to-noise ratio (CNR) measurements. For this purpose, regions of interest (ROIs) were defined in the source images. The ROIs had a diameter of 3 mm and a surface of 7 mm2. Measurements were obtained in both the arteries and the veins. The ROIs were located both on the compressed and at the corresponding positions of the noncompressed side at the following arterial locations: the distal radial artery, the distal ulnar artery, the superficial palmar arterial arch, one proper palmar digital artery, and the ulnar branch of the digital artery of the index finger. Concerning the venous vessels, measurements were obtained in the vena cephalica and the vena basilica (both at the level of the wrist). Additional ROIs were positioned in the ulnar vein of the thumb and the ulnar vein of the index finger. For all three flow rates, signal intensity measurements were obtained at identical anatomic levels.
Noise was defined as the SD of signal intensity (SI) outside the body.
CNR was calculated as
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Qualitative data evaluation was performed by two experienced radiologists. Each reviewer was unaware of the flow rate used and the other reviewer's assessment. The arterial visualization score and venous contamination score were assessed for all three flow rates. This assessment was performed on MIP reconstructions for all seven acquisitions, separately for the compressed and noncompressed sides.
The arterial visualization score was rated on a 4-point scale as 0, study nondiagnostic; 1, poor data quality, diagnostic impairment; 2, suboptimal arterial signal, no diagnostic impairment; and 3, good arterial signal. The venous contamination score was rated on a 4-point scale as 0, no venous contamination; 1, minimal venous contamination, no reduction of diagnostic value; 2, major venous contamination, important diagnostic impairment; and 3, nondiagnostic study.
Statistical analysis was performed using a paired Student's t test for both qualitative and quantitative data. A calculated p value of less than 0.05 was considered statistically significant.
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To illustrate the impact of cuff compression, one MIP reconstruction of a contrast-enhanced MR angiogram of the hand is displayed in Figure 1. Compression was applied on the right side. The left side was noncompressed and major venous contamination (venous contamination score, 2) was observed. This example illustrates the diminished venous contamination on the compressed side and the improved image quality related to compression.
Figure 2 illustrates the
impact of various flow rates on image quality. Subsequent data
acquisitions—on the compressed side only— are displayed in various
rows. The top row was obtained using a flow rate of 0.5 mL/s, the center row
using a flow rate of 1.0 mL/s, and the bottom row using a flow rate of 1.5
mL/s. The least venous contamination was observed using a flow rate of 0.5
mL/s. During late sequences (i.e., starting 80 seconds after IV contrast
administration), major venous contamination was observed for all three flow
rates (venous contamination score 
2). Using an injection rate of 1.5
mL/s, important venous overlay was observed after 60 seconds.
Quantitative Data Assessment
CNR of arterial signal—The mean CNR values of the arterial
vessels at the compressed side are shown in Figures
3A and
3B. For all three flow rates, a
CNR > 10 was obtained. The highest flow rate of 1.5 mL/s provided the
highest CNR values (40 seconds) during early acquisitions. During the
subsequent data acquisitions (i.e., 80-140 seconds), similar CNR values for
all flow rates were observed. Minimally higher CNR values were seen with an
injection rate of 0.5 mL/s. However, comparing the various injection rates, no
significant difference in arterial signal was observed (0.5 vs 1.0 mL/s:
p = 0.65; 0.5 vs 1.5 mL/s: p = 0.20; 1.0 vs 1.5 mL/s:
p = 0.65).
CNR values in the venous vessels—The mean CNR values of the venous signal at the compressed side are shown in Figures 3A and 3B. Comparing various injection flow rates, the significantly lowest venous signal (i.e., least venous contamination) was measured with a flow rate of 0.5 mL/s (0.5 vs 1.0 mL/s: p = 0.05; 0.5 vs 1.5 mL/s: p = 0.005; 1.0 vs 1.5 mL/s: p = 0.004).
Qualitative Data Assessment
Qualitative assessment of image quality was based on blinded, independent
assessment of arterial image quality and venous contamination. Reviewers'
assessments of arterial (arterial visualization score) data quality are
displayed in Figure 4A. Flow
rates of 0.5 mL/s were scored best (i.e., highest score) starting at 60
seconds. However, no statistically significant difference concerning arterial
data quality was observed (0.5 vs 1.0 mL/s: p = 0.71; 0.5 vs 1.5
mL/s: p = 0.88; 1.0 vs 1.5 mL/s: p = 0.52).
Reviewers' assessments of venous contamination (venous contamination score)
are displayed in Figure 4B. The
venous contamination score corresponds to the least amount of venous signal.
During all series, the significantly lowest venous contamination score was
with a flow rate of 0.5 mL/s (p < 0.05). The imaging studies
performed with an injection rate of 1.5 mL/s received the worst venous
contamination score. Starting at 40 seconds, venous contamination was observed
and starting at 60 seconds, major venous overlay (venous contamination score
2) was present.
Comparison of the Compressed and Noncompressed Sides
CNR values in the venous system were measured on both the compressed and
noncompressed sides. Data of this quantitative assessment are shown in
Table 1. Diminished venous
signal (i.e., venous contamination) is observed on the compressed side for all
flow rates. These values were statistically significant for flow rates of 0.5
and 1.0 mL/s.
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Tables 2 and 3 display the data comparing reviewers' qualitative assessment of the compressed and noncompressed sides. Both reviewers rated less venous contamination (i.e., a lower venous contamination score) on the compressed side. This difference was statistically significant for flow rates of 0.5 and 1.0 mL/s (p < 0.05). When a flow rate of 1.5 mL/s was used, image quality was again rated better on the compressed side than on the opposite, noncompressed side. However, this finding was not statistically significant.
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Various techniques can be applied to ensure good resolution in peripheral vessels. Dedicated peripheral surface coils afford excellent CNR and signal-to-noise ratio (SNR) in peripheral vessels [11]. Upper arm subsystolic cuff compression significantly reduces venous contamination and thus ensures good imaging quality [5]. When cuff compression is applied, physiologic parameters should be considered and assessed.
The injection rate of the IV contrast medium proved to have considerable impact on image quality. At higher flow rates, initially a higher CNR can be obtained. However, with higher injection rates, venous contamination becomes dominant, image quality is subsequently reduced, and the diagnostic outcome is possibly impaired. In this study, the highest flow rate—1.5 mL/s—initially afforded (during the first data acquisitions) the best arterial CNR (Figs. 3A and 3B). However, this difference was not statistically significant. On the contrary, higher flow rates are compromised by early filling of the venous vessels and venous contamination. The highest injection rate of 1.5 mL/s obtained initially (i.e., at 60 seconds) the highest CNR value. However, because major venous overlay was observed, reviewers scored the imaging data obtained with the highest injection rate as having significantly reduced image quality.
On the other hand, when a lower flow rate of 0.5 mL/s is used, the initial arterial signal is decreased. This is reflected in the lowest CNR during the first data acquisitions with a flow rate of 0.5 mL/s. However, venous contamination proved to be by far less dominant. This finding is reflected in the fact that reviewers rated higher arterial image quality and lower venous contamination at the lowest flow rate tested. A possible physiologic explanation for this observation might be the venoarteriolar reflex, which has been described in earlier studies [6, 12]. This venoarteriolar reflex consists of a precapillary arteriolar vasoconstriction that probably plays a protective role in capillary hemostasis. Impairment of the autonomic nervous system due to various conditions, among them chronic venous insufficiency, diabetic autonomic neuropathy, and aging, results in obliteration of this reflex. This can be seen on laser Doppler flowmetry [6, 12].
For all flow rates, venous contamination was less on the compressed side than on the contralateral noncompressed side. This fact was observed in the quantitative data (i.e., lower CNR values, less venous filling in the venous structures on the compressed side) and in the qualitative data (lower venous contamination rated by both observers on the compressed side). These data are statistically significant for flow rates of 0.5 and 1.0 mL/s for both quantitative (CNR values) and qualitative (reviewers' assessment) data sets.
Others groups applied a technique similar to venous compression in the lower extremities [7, 8]. They reported good arterial signal and less venous contamination in the lower extremity with cuff compression when compared with the side without venous compression. Another group that applied cuff compression at the level of the thigh reported not only reduced venous contamination but also increased arterial SNR [13].
A limitation of this study is the small size of the study group, which consisted of 10 healthy individuals. Furthermore, the ROIs were positioned in the source images in the same five defined arterial and venous vessels in all imaging studies. Sampling errors could occur when positioning the ROIs in small distal vessels. This study was designed to assess injection parameters of MR angiography of the hand. Only healthy volunteers were examined. Further controlled studies are warranted to assess the diagnostic outcome of MR angiography of the hand in clinical settings, such as in patients with vasospasm, vascular stenosis, or trauma.
In conclusion, 3D contrast-enhanced MR angiography of the hand allows good visualization of the hand vessels. The best data quality is achieved with subsystolic cuff compression of the upper arm. When various flow rates of IV injection are compared, a flow rate of 0.5 mL/s affords the best arterial signal and the least venous contamination.
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