DOI:10.2214/AJR.04.0945
AJR 2005; 185:1289-1293
© American Roentgen Ray Society
High-Resolution 16-MDCT Evaluation of Radial Artery for Potential Use as Coronary Artery Bypass Graft: A Feasibility Study
Gudrun M. Feuchtner1,
Alexander Smekal1,
Guy J. Friedrich2,
Thomas Schachner3,
Johannes Bonatti3,
Wolfgang Dichtl2,
Martin Deutschmann4 and
Dieter zur Nedden1
1 Department of Radiology II, Medical University Innsbruck, Anichstrasse 35,
A-6020 Innsbruck, Austria.
2 Department of Cardiology, Medical University Innsbruck, Innsbruck,
Austria.
3 Department of Cardiac Surgery, Medical University Innsbruck, Innsbruck,
Austria.
4 Siemens Medical Solutions Austria, Wien, Austria.
Received June 15, 2004;
accepted after revision November 23, 2004.
Address correspondence to G. M. Feuchtner
(gudrun.feuchtner{at}uibk.ac.at).
Abstract
OBJECTIVE. The objective of our study was to assess the feasibility
of using 16-MDCT angiography for the preoperative assessment of the radial and
ulnar arteries and the palmar arches in patients scheduled for radial artery
harvesting for coronary artery bypass graft (CABG) surgery.
CONCLUSION. Sixteen-MDCT angiography shows promise for the
noninvasive preoperative assessment of the radial artery as a CABG donor
site.
Introduction
Sixteen-MDCT angiography is a new imaging technique for the noninvasive
assessment of arteries in various vascular territories. It has been shown to
be useful for imaging the carotid arteries
[1] and coronary arteries
[2], including imaging for the
assessment of noncalcifying soft plaque
[3] and coronary vessel
calcifications [4]. Current
MDCT technology with simultaneous aquisition of 16 slices and a nearly
isotropic voxel calculation allows visualization of even small-caliber
vessels, such as antebrachial arteries and palmar arches. Moreover, high
spatial resolution can be achieved by the use of a specific high-resolution
convolution scanning method (ultra-high-resolution mode)
[5].
Arterial grafts have shown better short- and long-term clinical outcomes
than venous grafts for use as a coronary artery bypass graft (CABG)
[6]. Aside from the internal
mammary arteries, the radial artery may serve as suitable bypass vessel.
Reported advantages of radial artery harvesting include a high total arterial
revascularization rate of 80%
[7], excellent mid- and
long-term patency rates
[8-10],
and reduced morbidity in patients older than 65 years
[11]. However, the radial
artery can be harvested only if it is not severely stenosed and if it is
spared from extensive atherosclerotic involvement. Also, use of the radial
artery for a CABG is restricted to patients with proven collateral blood
supply from the ulnar artery via the palmar arches, because if such
collateralization is missing, patients are prone to develop hand ischemia.
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Fig. 2A —52-year-old man with severe (grade 3) atherosclerosis. MDCT
image obtained using volume-rendering technique (A) and CT angiography
image obtained using rotated volume-rendering technique (B) show course
of radial artery (RA) and ulnar artery (UA) are well depicted
(arrows). Proximal parts (P, arrowhead) of both vessels are
severely affected by atherosclerosis (grade 3), and vessel wall appears
irregular. Distal carpal (C) course of ulnar artery is less affected, and
fourth digital palmar artery (4DPA) is relatively spared from
atherosclerosis.
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Fig. 2B —52-year-old man with severe (grade 3) atherosclerosis. MDCT
image obtained using volume-rendering technique (A) and CT angiography
image obtained using rotated volume-rendering technique (B) show course
of radial artery (RA) and ulnar artery (UA) are well depicted
(arrows). Proximal parts (P, arrowhead) of both vessels are
severely affected by atherosclerosis (grade 3), and vessel wall appears
irregular. Distal carpal (C) course of ulnar artery is less affected, and
fourth digital palmar artery (4DPA) is relatively spared from
atherosclerosis.
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Fig. 3A —Two patients evaluated for atherosclerosis. RA = radial
artery, UA = ulnar artery. 66-year-old man with no (grade 0) atherosclerosis.
CT angiography image obtained using volume-rendering technique shows no
atherosclerosis. Vessel walls (arrows) of radial and ulnar arteries
are smooth.
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Fig. 3B —Two patients evaluated for atherosclerosis. RA = radial
artery, UA = ulnar artery. 79-year-old man. CT angiography image obtained
using volume-rendering technique shows atherosclerosis: calcified plaques in
combination with vessel wall irregularity of ulnar artery and distal
tortuosity of ulnar artery (arrows). In contrast, radial artery is
sharply delineated with smooth margins without apparent atherosclerosis.
Volume-rendering technique permits rotation of up to 360° around
longitudinal plane and allows optimal visualization of both vessels.
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The purpose of this preliminary study was to assess the feasibility of
using 16-MDCT angiography for the preoperative assessment of the radial artery
as a potential CABG vessel. We were interested in assessing whether the radial
artery could be well visualized with MDCT, whether atherosclerotic involvement
could be imaged, and whether MDCT could be used to evaluate the
collateralization of the radial artery with the ulnar artery via the deep and
superficial palmar arches in the hand.
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Fig. 4A —Two patients evaluated for palmar arch. RA = radial artery.
74-year-old man. CT angiography image obtained using thin-slab
maximum-intensity-projection technique shows complete superficial palmar arch
(PA) fed by volar branch (ramus volaris, RV) of radial artery. UA = ulnar
artery.
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Fig. 4B —Two patients evaluated for palmar arch. RA = radial artery.
65-year-old man. CT angiography image obtained using thin-slab
maximum-intensity-projection technique shows incomplete superficial palmar
arch supplied via ulnar artery. AP = arcus palmaris (palmar arch).
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Materials and Methods
Study Population
We examined the antebrachial arteries and palmar arches with 16-MDCT
angiography in 21 forearms (11 right, 10 left) in 11 consecutive patients (10
men, one woman; age range, 52-83 years; mean age, 69 years) scheduled for CABG
surgery. Written informed consent was obtained from all patients.
MDCT Technique
Contrast-enhanced high-resolution 16-MDCT angiography (Somatom Sensation
16, Siemens Medical Solutions) with spatial resolution equivalent to 24 lp/cm
at a 2% setting for the modulation transfer function (MTF) was performed. The
collimation was 16 x 0.75 mm and table translation speed, 2.7 cm/sec.
The tube parameters were 50 mAs and 120 kV, with a tube rotation time of 0.75
sec. The field of view was 250 x 250 mm and the imaging matrix was 512
x 512, resulting in a pixel size of 0.48 x 0.48 mm2 and
a voxel size of 0.17 mm3.
Approximately 560 images with a slice width of 0.75 mm and at a 0.5-mm
increment were reconstructed for each volume data set. For image
reconstruction, a smooth convolution kernel (U 30 U, Siemens Medical
Solutions) was used as recommended for the ultra-high-resolution mode.
Patients were examined in the prone position with both arms elevated above
their head. The forearms were positioned parallel on the tabletop in a neutral
position with the hand palms facing each other while grasping a rolled-up
towel ("praying" position)
(Fig. 1). The scanning
direction was from the fingertips toward the elbow.
A bolus of 100 mL of iodine contrast agent with a concentration of 370 mg
I/mL of iopromide (Ultravist, Schering) was injected IV into an antecubital
vein using a 20-gauge cannula connected to a power injector with a continuous
flow rate set at 3 mL/sec. Data acquisition started after a fixed delay of 25
sec after the start of injection. The image acquisition time was 17 sec, and
the overall inroom examination time was approximately 20-25 min per
patient.
Image Analysis
Data were viewed on a dedicated workstation (Leonardo, Siemens Medical
Solutions) as axial slices and as 3D reconstructed images as maximum intensity
projection (MIP), multiplanar reformation (MPR), and volume-rendering
technique (VRT) images. First, image quality was graded subjectively by two
experienced radiologists independently. Image quality was scored on the
following 5-point scale: 1, excellent visualization of the arteries; 2, good;
3, mediocre; 4, poor; and 5, no visualization of the arteries. To quantify
image quality, we calculated the intraluminal attenuation and SDs of the
radial artery and ulnar artery within a region of interest (ROI) on axial
images. In addition, the contrast-to-noise ratio (CNR) was computed as
follows: attenuation (intraluminal) / SD (intraluminal).
Then, the presence and severity of atherosclerotic involvement of the
radial and ulnar arteries were graded on a 4-point scale (Figs.
2A,
2B,
3A, and
3B): 0, no atherosclerosis; 1,
little; 2, medium; and 3, severe atherosclerosis. The scale was based on
predetermined criteria of vessel wall calcifications, vessel wall
irregularity, and the presence of atherosclerotic soft plaque in consensus
review by two observers: grade 1, presence of vessel wall irregularity and
soft-tissue plaque and less than 25% of vessel affected by calcified plaque;
grade 2, presence of vessel wall irregularity and soft-tissue plaque and
25-50% of vessel affected by calcified plaque; and grade 3, presence of vessel
wall irregularity and soft-tissue plaque and more than 50% of vessel affected
by calcified plaque.
The patency of the deep and superficial palmar arches was assessed by two
observers who were blinded to Allen's test results.
Allen's Test
Allen's test, a clinical test, was performed by an experienced
cardiothoracic surgeon who was blinded to the MDCT angiography results. The
ulnar and radial arteries were compressed at the wrist for more than 30 sec to
induce ischemia. Then, the pressure on the ulnar artery was released and if
hyperemic reperfusion of the thumb occurred within 5 sec, the test was
regarded as normal. A reperfusion time of more than 5 sec was regarded as
abnormal and indicative of insufficient collateralization because of
incomplete deep and superficial palmar arches
[7,
12].
Statistical Analysis
We quantified the interobserver agreement for the subjective grading of
image quality of MDCT angiography and of the extent of atherosclerosis using
Cohen's kappa statistic analysis. The McNemar test was performed to compare
MDCT-based results with the results of Allen's test with regard to the
presence of collateralization.
Results
MDCT Angiography
A complete superficial palmar arch was shown with MDCT angiography in 14
(67%) of 21 forearms: nine right and five left forearms
(Fig. 4A). An incomplete
superficial palmar arch was depicted in seven of 21 forearms. A complete deep
palmar arch was visible in 14 (67%) of 21 forearms: nine right-sided and five
left-sided. An incomplete deep palmar arch was seen in seven of 21 forearms.
The ulnar artery was occluded in two of 21 patients. The presence of
discontinuity of the palmar arches (Fig.
4B), occlusion of the ulnar artery, or both correlated
significantly to an abnormal Allen's test (p < 0.0001, McNemar
test). Overall, there was good agreement (94%) between MDCT angiography and
Allen's test. In one patient who had a borderline Allen's test (reperfusion
time was exactly 5 sec and thus borderline), MDCT angiography showed an
incomplete deep palmar arch but a complete superficial palmar arch supplied
predominately by the radial artery.
Atherosclerosis was present in the radial artery in 13 (62%) of 21
forearms. Grade 1 atherosclerosis (little) was found in 23.8%, grade 2
atherosclerosis (medium) in 4.8%, and grade 3 atherosclerosis (severe) in
23.8% (Table 1 and Figs.
2A and
2B). There was good
interobserver agreement between the MDCT observers with a kappa value of
1.
Follow-Up
In patients with an incomplete palmar arch, atherosclerosis, or both, the
radial artery was not harvested. Considering the severity of arteriosclerosis
and the display of incomplete palmar arches, the use of the radial artery as
the CABG donor was recommended in seven of 21 forearms. The radial artery was
harvested after performing an intraoperative oxygen saturation measurement to
confirm collateralization of the radial artery and ulnar artery in one of
seven patients. This patient did not develop hand ischemia after a follow-up
period of 12 months. In five of seven patients with multi-vessel disease,
harvesting the radial artery was considered preoperatively, but concerns were
given intraoperatively regarding procedure risk (comorbidity such as previous
myocardial infarction [n = 2], combined aortic valve replacement
[n = 1], severe atherosclerosis of ascending aorta [n = 1],
intraaortic counterpulsator implantation [n = 1], and high patient
age of more than 70 years) because harvesting radial artery in addition to
saphenous vein grafts would have delayed CABG procedure time. In one of seven
patients, CABG was cancelled due to comorbidity.
Image Quality
Both observers rated the overall image quality as excellent (score of 1) in
nine of 11 patients (17 forearms, 81.9%), good (score of 2) in one patient
(two forearms), and insufficient (score of 5) in one patient (two forearms).
In the latter patient, the insufficient image quality was presumably due to
technical problems with the power injector that resulted in venous filling
during image acquisition with venous overlay. The primary contrast bolus
injection was stopped automatically due to air trapping, but little contrast
agent was injected and remained IV while a new contrast bolus injection was
started. Subsequently, venous contrast agent contamination occurred during
image aquisition during the arterial phase. There was good interobserver
agreement between the MDCT observers with a kappa value of 1. Quantification
of image quality by measurements of intraluminal attenuation and SDs of the
radial and ulnar arteries showed an average intraluminal attenuation of 358.3
H. The mean calculated CNR was 20.64 (range, 4.7-51.9), respectively. Images
with a quality score of excellent (score of 1) had a lower SD (mean SD, 25.17)
and higher CNR ratio (19.09) than images with a score of 2 for image quality
(mean SD, 41.1; CNR, 7.0), respectively.
Three-Dimensional Image Reconstruction
Of the various 3D reconstruction techniques, MIP and multiplanar
reconstruction appeared more useful for assessing the continuity of the palmar
arches (Fig. 5). We preferred
the volume-rendered technique reconstructions for imaging the complete course
of the antebrachial arteries, especially for giving an overview of the overall
extent of calcified plaques. The time required for image reconstruction and
analysis ranged between 15 and 25 min.
Discussion
Our results indicate that 16-MDCT angiography may be a useful technique to
accurately image the antebrachial arteries. Assessment of even smaller
vessels, such as the palmar arteries, is feasible, a feature that is enhanced
by the recent introduction of ultra-high-resolution convolution scheme with
dedicated kernel (ultra-high-resolution [UHR mode]) with voxel size of 0.17
mm3 [5].
To the best of our knowledge, the feasibility of using 16-MDCT for the
assessment of antebrachial arteries has not been reported yet. MDCT
angiography, our results suggest, shows the relevant luminal stenoses in the
antebrachial arteries and may be helpful for grading the severity of
atherosclerosis. This additional decision-making information is relevant to
the cardiothoracic surgeon when seeking an appropriate donor vessel as a
potential CABG. With MDCT, one can show or refuse the continuity of the palmar
arches to determine the presence of collateralization between the radial and
ulnar arteries.
Compared with currently used standard clinical tests, MDCT angiography
provides more comprehensive information. Conventional angiography can show
luminal stenoses but will insufficiently display the extent of atherosclerotic
plaque formation along the arterial wall. Allen's test, a clinical occlusion
test, may indicate collateralization but does not provide any additional
information about the suitability of the radial artery as a CABG vessel.
Besides, Allen's test has been shown to have a relatively low sensitivity
ranging between 50% and 79.5%
[12]. Despite a normal Allen's
test, patients may therefore present with hand ischemia after radial artery
harvesting [13]. By the way,
we should mention that a modified Allen's test can be performed with
accompanying ultrasound wave flow evaluation in clinical practice
[13]. Accurate preoperative
evaluation of collateralization is crucial: In patients in whom the radial
artery had been harvested for CABG (and in whom preoperative assessment had
been limited to Allen's test only), numbness and pain of the hand (as vascular
or neurologic complications) occurred in 12.9% and 8.4% of patients,
respectively, at 12 months of follow-up
[14]. Therefore, a more
accurate examination technique than the Allen's test is desirable.
Multiplanar reconstruction and thin-slab MIP were superior for depicting
palmar vasculature and thus distinguishing between a complete or incomplete
palmar arch, whereas the volume-rendered technique failed to display palmar
arches in 89.4% forearms. In contrast, we preferred the volume-rendered
technique reconstructed images for visualization of the complete forearm
course of the ulnar and radial arteries and for display of the extent of
calcified plaques: With the volume-rendered technique, the severity of
atherosclerosis in vessels can be shown quickly and precisely for the
cardiothoracic surgeons within one image that can be rotated 360° on a
dedicated workstation (Leonardo, Siemens Medical Solutions) with predetermined
settings within a few seconds. Although multiplanar reconstruction and MIP
images were reviewed as axial slices, additional 3D reconstructions of the
targeted vessel usually took more than several minutes. Image quality was
optimal in a majority of forearms (81.9%). Limitations occurred in only one
patient due to technical problems with the power injector during image
acquisition.
One limitation of our study is the lack of comparison with diagnostic
interventional angiography, the currently accepted standard. With respect to
ethical considerations concerning invasiveness and various associated risks,
we did not perform interventional angiography of the forearm arteries. Thus,
we consider our work to be a "proof of concept" rather than an
exhaustive validation of the method.
In summary, the use 16-MDCT angiography for the noninvasive preoperative
assessment of the radial artery as a CABG donor vessel seems feasible. MDCT
angiography shows relevant lumen stenoses, the extent of atherosclerosis of
the radial artery including calcified and soft plaque, and collateralization
between the radial artery and ulnar artery via the deep and superficial palmar
arches. Compared with the current standard preoperative assessment, MDCT
angiography may provide the surgeon with additional relevant information that
may influence the decision about which vessel is most appropriate for CABG
surgery.
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Abstract
Introduction
