JUULit

1.06 Ankle/Brachial Index 1.07

7/1/2005

Exercise

Rest

1

2

3

4

5

6

7 8 9 10

R Ankle (PT):

152

163

157

143

147

145

L Ankle (PT):

153

159

156

155

153

151

R Brachial:

143

148

152

144

140

130

R ABI

1.06

1.10

1.03

0.99

1.05

1.12

L ABI

1.07

1.07

1.03

1.08

1.09

1.16

150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

Rest

Minutes

Pt walked for 5 min at 1.3 mph. He had no claudication at this time. Figure 27-24 A, Normal pulse volume recording demonstrates triphasic waveforms with normal segmental pressures. No pressures were obtained in the left thigh because of previously placed stent. B, Normal ankle-brachial indexes at rest, with a normal response to exercise. The red lines and green lines are essentially flat throughout exercise.

lura angiography (CTA), or magnetic resonance angiography (MRA) are more suited to proven detailed anatomic information and complement the NIVS data.

Doppler Ultrasound

Arterial duplex Doppler sonography utilizes high-frequency sound waves (typically 5.0-7.5 MHz) to provide real-time vascular images that can accurately localize atherosclerotic disease. Color-flow encoding is useful to localize vessels quickly and determine the presence or absence of blood flow. Doppler technology uses the physical principles of the reflected sound wave frequency in relation to the transmitted frequency to determine the velocity of blood flow. As the severity of stenosis increases, the peak systolic velocity (PSV) of flow increases. By using established criteria of the absolute

Table 27-15 Ankle-Brachial Index (ABI) and Severity of Disease

ABI

Disease Classification

>1.30

Noncompressible

0.90-1.30

Normal

0.80-0.89

Mild

0.60-0.79

Moderate

0.40-0.59

Severe

<0.40

Critical ischemia

PSV and the ratio of PSV in the normal reference segment compared to the PSV in the diseased segment, the overall range of stenosis can be determined.

Ultrasound is particularly useful for assessing stent or graft patency after a revascularization procedure. Potential pitfalls of ultrasound include visualizing the tibial vessels, which are relatively small and deep in the calf, and visualizing highly calcified vessels, which are acoustically shadowed by the calcium.

Computed Tomography and Magnetic Resonance Angiography

The noninvasive imaging modalities CTA and MRA have essentially replaced traditional invasive diagnostic angiog-raphy. Both CTA and MRA produce similarly accurate anatomic information and provide highly detailed images that can be used to plan revascularization procedures, assess the size and location of aneurysms, and occasionally find incidental pathology such as occult malignancy. These two technologies are fundamentally very different.

Traditional CT has evolved by the addition of multiple detectors, now up to 64 per machine. MDCT allows much shorter acquisition times and submillimeter resolution; thus an entire body scan can be performed in seconds to minutes with excellent spatial resolution. By adding three-dimensional reconstruction software, the bone, soft tissue, and organs can be virtually removed, and the vasculature reconstructed and viewed from multiple projections. Figure 27-26 provides examples of normal and abnormal CT angiograms. CTA utilizes ionizing radiation and iodinated contrast. Therefore, multiple scans (cumulative radiation dose)

Rest

1

2

3

4

5

6

7

8

9

10

Brachial BP

126

152

151

146

148

141

138

142

142

145

141

R ankle BP

117

22

55

71

88

75

75

75

80

86

86

L ankle BP

137

75

87

88

95

83

104

104

100

116

116

R ABI

0.93

0.14

0.36

0.49

0.59

0.53

0.54

0.53

0.56

0.59

0.61

L ABI

1.09

0.49

0.58

0.60

0.64

0.59

0.75

0.73

0.70

0.80

Rest 1

E 150 re

r

) (

• c

3 C

J f

) c

-, O O c

\

\

i

i-3

í-i

y"

- 1 -i

r

i-■

\

/

9 10

9 10

□ = R ankle BP ISCHEMIC WINDOW; 444 X = L ankle BP ISCHEMIC WINDOW; 393 O = Brachial BP

Figure 27-25 Normal noninvasive vascular study at rest. The right ankle-brachial index (ABI) is 0.93, and the left ABI is 1.09. However, there is a marked ischemic response to exercise. Note the pronounced drop of the red and blue lines with exercise and the severe drop of the right ABI to 0.14 and the left ABI to 0.49. This patient had bilateral, focal, 95% to 99% stenoses in the superficial femoral arteries, which were successfully stented. This example clearly demonstrates the importance of obtaining ABIs at rest and with exercise. Failure to obtain ABIs with exercise, particularly when the resting values are normal, often fails to diagnose the disease. BP, Blood pressure.

and renal insufficiency are relative contraindications to CTA. Other limitations include severe vascular calcifications and prosthetic joints, which cause scatter artifact. The lumen of a stented vessel can be visualized with CTA, although a mild to moderate degree of scatter artifact makes accurate assessment of in-stent restenosis difficult.

Traditional MRI has also evolved through the development of more powerful magnets and improved scanning algorithms. MRA can also produce submillimeter spatial resolution. In contrast to CTA, however, the scanning time for MRA can be up to an hour per patient. In a busy practice or hospital, this can lead to problems with patient throughput. Software can also reconstruct MRA images into three dimensions. MRA utilizes magnetic fields and variable-frequency radio waves to detect changes in alignment and distribution of protons in a given tissue. The obvious advantage over CTA is that MRA does not use ionizing radiation or iodin-ated contrast. Thus, there is practically no risk of stochastic injury or contrast nephropathy. Patients with pacemakers or defibrillators cannot undergo MRA because the magnet can interfere with device function. Severely calcified vessels can be adequately imaged with MRA, but stents appear as voids and thus falsely give the impression of a totally occluded artery even if the stent is widely patent.

The false-positive rate for detecting a hemodynamically significant stenosis is considerably higher with MRA than CTA. With CTA the lumen of the vessel visualized is essentially a column of contrast; thus a precise assessment of the

RUNOFF ALS

S Radiology Gruup.Davenpurt. IA

■ Ex(rtirnilies"UJ Aurla with Runofr

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