Applied Radiology

Dr. Scott is Assistant Professor of Radiology and Dr. Letourneau is Professor of Radiology and Surgery at Louisiana State Univeristy Medical Center in New Orleans, LA.

The role of sonography in the evaluation of arterial vascular disease processes has continued to flourish and become more established over the past decade. Technological advances such as the improved acquisition and display of real-time vascular information, specialized computer software programs, and improvements in operator skill are highly responsible for the widespread application of diagnostic ultrasound to the arterial vascular system. Duplex sonography can provide the investigative foundation of vascular sonography; flow patterns, velocities, and direction can be determined by using duplex sonography to grade vessel stenosis and characterize vascular networks. Similarly, the application of color Doppler imaging (CDI), and most recently, color Doppler energy/power Doppler imaging (CDE), greatly facilitates duplex scanning by helping to identify vessel territories, optimize areas of vessel interrogation (by decreasing examination time), and characterize vascular lesions.

Principles of duplex sonography

Duplex sonography integrates real-time gray-scale images with pulsed Doppler velocity and directional information of vessel flow dynamics. Flow patterns are displayed graphically as velocity or frequency shifts and are indicative of flow vortices within the sample gate. Based on the Doppler equation F = 2 f v cosq/c, where F = observed frequency shift, f = emitted transducer frequency, v = velocity of moving reflectors, q = angle of incident beam, and c = speed of sound transmission, several details of the technique must be considered to understand the process. Choosing a pulse repetition frequency (PRF) or sampling transducer frequency great enough to minimize aliasing (Nyquist limit = 1/2 PRF) is one major factor that will affect the accuracy of the duplex sonogram. Optimizing q less than 60 degrees, maintaining a narrow Doppler gate (<3 mm), and placing the insonation gate within the center of the vessel lumen also are vital to ensuring the accuracy of velocity quantitation and laminar flow detection.

Color flow analysis is a display mode in which flow velocities are assigned to a color scale/hue that is indicative of the mean red-cell velocity and flow direction within each pixel of a chosen sample volume. Usually areas of the brightest hue (representing zones of highest velocities/turbulence) are insonated with duplex sonography to ascertain actual flow-pattern dynamics. The ability to survey large vascular territories and subsequently detect areas of potential pathology makes CDI a vital, possibly irreplaceable, component of vascular sonography. The expected vessel course should be kept oblique or perpendicular to the transducer face because a vessel that is imaged as parallel to the transducer will have artifactually reduced or nondemonstrable f1ow.

Color Doppler energy has recently been introduced as an adjunct to conventional CDI. Unlike CDI, where color assignment is based on the average velocity and/or flow direction, CDE assigns color based on the total number of moving blood reflectors (RBCs) contained within the sample volume. This method of analysis allows for greater flow sensitivity (detects lower flow velocities), and thus facilitates evaluation of high grade or critical stenoses. Directional information is sacrificed, but this can be obtained in conjunction with pulsed Doppler.

Flow patterns

Arterial vascular territories can essentially be divided into high- or low-resistance systems. Typical low-resistance territories include the internal carotid, vertebral, celiac, mesenteric, and renal arterial systems. These systems are displayed on Doppler spectral analysis

as continuous antegrade flow during diastole and systole, and usually have end-diastolic velocities that are significantly above baseline values (monophasic waveform) (figure lA). In contrast, high-resistance territories are represented by the majority of the peripheral arterial system of the upper and lower extremities, as well as the aortic and iliac arteries. Doppler waveforms reflect a non-continuous flow pattern, with antegrade systolic flow followed by early retrograde diastolic flow. The pattern usually ends with late antegrade diastolic flow (triphasic waveform) (figure lB). Loss or diminution of the late antegrade diastolic component is referred to as a biphasic waveform; it is often considered a normal variation, but may reflect minimal decreased wall compliance.

Flow velocities are somewhat variable and overlapping, depending on the vascular territory. Some reasonable parameters are as follows:1,2,3

ICA/VA: 40-120 cm/sec

Celiac/SMA: 111 cm/sec

Aorta: 76 cm/sec

CIA/EIA: 111 cm/sec

CFA/SFA: 90 cm/sec

POP A: 59 cm/sec

Renal A: < 100 cm/sec

Disease processes

Carotid/vertebral system-With the patient placed in a supine position and his neck extended, a 5 MHz to 10 MHz linear array transducer is utilized to scan the common carotid artery (CCA), external carotid artery (ECA), internal carotid artery (ICA), and often the vertebral arteries. Although many parameters and criteria exist, hemodynamica1ly significant stenosis (>50% diameter reduction) is generally recognized as an elevated peak systolic velocity (PSV >110 to 120 cm/sec) (figure 2).4 Increased PSV to greater than 270 cm/sec also will correspond to lesions with greater than 70% diameter reduction and, thus, can identify symptomatic patient subsets that have received beneficial outcome results with carotid endarterectomy.5

ICA occlusion can be recognized by a lack of pulsed Doppler flow within the vessel, and usually is suspected when an abnormal high-resistance flow pattern is seen in the CCA (figure 3). CDI facilitates lesion detection by outlining anechoic plaque and targeting areas of turbulence which may be indicators of stenotic zones. Recently, the addition of CDE to color flow analysis has been shown to provide greater discrimination between high-grade stenoses and complete occlusion, as well as improved characterization of plaque surface morphology.6 Also, in regions of Ca ++ plaque, CDE provides a better assessment of luminal area and diameter reduction.7

Duplex sonography also can indirectly detect areas of hemodynamically significant stenosis by documenting reversed flow within vessels. Flow reversal will be manifested by spectral display below the baseline (away from the transducer), or by reversed color assignment on CDI. Such recognition has proven quite useful in CCA occlusion, where reversed flow in ECA branches supply forward flow to the ICA territory and brain. In this setting, the ECA will show an abnormal

low-resistance monophasic pattern rather than the normal higher-resistance waveform. Reversed flow also may be detected in the vertebral artery (VA), indicating ipsilateral subclavian artery stenosis or occlusion (subclavian steal).

Abdominal system-Applications of Duplex sonography in the abdomen include the evaluation of mesenteric ischemia, detection of renal vascular and parenchymal disease, and characterization of aortic atherosclerotic disease. Most duplex studies require 2 MHz to 7 MHz sector or curved-linear array transducers, and generally are performed after a fasting period. As can be predicted, the success of duplex sonography of the abdomen is highly dependent on the patient's body habitus (obese versus thin), presence of obscuring bowel gas, and the patient's internal anatomy, as well as his or her overall clinical condition and tolerance.

In the appropriately selected patient, mesenteric duplex scanning can be useful as a screening exam to detect stenosis of the superior mesenteric (SMA) and celiac (CA) arteries. By demonstrating a CA peak systolic velocity (PSV) of greater than 200 cm/sec or a SMA PSV of greater than 275 cm/sec, Monetta et al achieved a sensitivity/

specificity of 87%/80% and 92%/96%, respectively, for the detection of greater than 70% stenosis in the fasting patient.8 Monetta et al also have suggested that failure to achieve an increased PSV of the SMA approximately 20 to 30 minutes following a meal may indicate a significant stenosis.

Hemodynamically significant stenosis in the renal arteries also can be detected by utilizing duplex sonography. Although many criteria exist, an elevated PSV greater than 180 cm/sec in the main renal artery (MRA), or an elevated RAR (renal artery PSV/aorta PSV ratio) greater than 3.5 are direct indicators of stenotic disease (figure 4A).3 However, as there are potential limitations of visualizing the MRA (body habitus, presence of bowel gas, multiple renal arteries), spectral waveform analysis of the low resistive interlobar/arcuate arteries can be used to provide additional diagnostic information which may be more reproducible and accurate for the detection of hemodynamic stenosis.

In such cases, a "pulsus parvus et tardus" waveform, in which there is a delayed systolic upstroke and diminished systolic peak, has been shown as suggestive of more proximal MRA or segmental renal artery stenosis (figure 4B).9 Another application of duplex renal sonography is the evaluation of renal parenchymal resistance by determination of the resistive index (RI) or Pourcelot index (PI). Though it is nonspecific, an elevated RI of greater than 0.7 or PI of greater than 1.0 suggests renal parenchymal disease from medical renal disease processes (i.e. ATN, glomerulonephritis) or postrenal obstruction.10,11

When used as an aid to CDE, pulsed Doppler evaluation of the aorta may provide information regarding aorto-iliac occlusive disease (figure 5). In the setting of complete aortic occlusion (Leriche syndrome) or high-grade stenosis, the normal triphasic waveform may convert to a high-resistive monophasic (preocclusive) waveform with loss of early reversed and late antegrade diastolic flow. Detection of such disease process can be particularly helpful when an occluding thrombus is relatively anechoic/hypoechoic, and thus not easily recognized by gray-scale techniques alone.

Peripheral vascular system-Duplex sonography of the peripheral arterial system often is performed with linear array transducers ranging from 5 MHz to 10 MHz. Indications for this technique primarily involve the detection of stenoses in atherosclerotic patients, but also may include the detection of pseudoaneurysms or A-V fistula that result from complications of previous interventions or trauma.

In the peripheral artery system, a hemodynamically significant stenosis is most directly recognized as a doubling of the PSV at the area of stenosis compared to a region approximately 2 cm to 4 cm proximal to the stenosis (PSV ratio >2) (figure 6).12 Other morphological changes in the spectral waveform that suggest significant disease are conversion of the normal triphasic waveform to a high-resistive monophasic pattern (distal preocclusive disease) or a low-resistive monophasic pattern (poststenotic vasodilitation (figure 7). Significant discrepancy or asymmetry in ipsilateral-to-contralateral spectral waveforms also would be indicative of significant stenotic disease processes. Color flow analysis can aid greatly in the identification of areas of turbulence or higher velocities, which may be reflective of hemodynamic disturbance.

Beyond the detection of stenosis, duplex sonography can be used to aid in the identification of arterial vascular complications such as pseudoaneurysms. These are primarily recognized by their gray-scale appearance of a sonolucent mass adjacent to a vascular structure; however, spectral analysis can show the classic "to-and-fro" or systolic antegrade/diastolic retrograde waveform documenting flow dynamics within the neck or vascular pedicle of the pseudoaneurysm (figure 8). Once the neck has been identified, sonographically-guided compression can be applied, with successful, noninvasive therapeutic outcomes achieved in the majority of cases.13 A-V fistula, another arterial vascular complication, often is detected on duplex examination by demonstrating an abnormal low-resistive monophasic arterial waveform proximal to the arteriovenous communication, and a normal triphasic/biphasic high resistive waveform distal to the communication (figure 9). Likewise, interrogation of the recipient draining vein will show an exaggerated and abnormal pulsatile arterial waveform superimposed over the normal phasic venous waveform (arterialization) (figure 10). Actual visualization of the fistula usually will require the aid of color flow analysis with "fill in" of the communicating channel.

Arterial graft patency or dysfunction may be evaluated by duplex sonography. Depending on the graft purpose, composition, and its subsequent elasticity/recoil properties, normal spectral waveforms may range from triphasic/biphasic to high-resistance monophasic patterns (arterio-arterial communications) or to low-resistance monophasic patterns (arteriovenous communications/dialysis grafts). Complete graft occlusion would present without any spectral waveform signal (the sample gate should traverse the entire lumen), while areas of significant graft stenosis would be recognized by a one and a half- to two-fold increase in PSV (PSV ratio >1.5 to 2).14 PSVs within the graft of less than or equal to 45 cm/sec would indicate impending graft failure (figure 11).14

Conclusion

As has been shown, duplex sonography has widespread applications in the noninvasive assessment of arterial disease processes. The ability to characterize and quantify arterial flow patterns makes duplex sonography a vital and likely irreplaceable diagnostic modality. In fact, when presented with the evaluation of arterial disease, many institutions may consider this modality the primary diagnostic method. AR

References

1. Robinson ML: Duplex sonography of the carotid arteries. Semin Roentgenol 27:17-27, 1992.

2. Sacks D: Peripheral arterial duplex ultrasonography. Semin Roentgenol 27:28-38, 1992.

3. Taylor KJW, Rosenfield AT: US of the kidney. RSNA Ultrasound 1991 (Syllabus: special course): 225-236, 1991.

4. Carroll BA: Carotid sonography. Radiology 178:303-313, 1991.

5. Neale ML, Chambers JL, Kelly AT, et al: Reappraisal of duplex criteria to assess significant carotid stenosis with special reference to reports from the North American Symptomatic Carotid Endarterectomy Trial and the European Carotid Surgery Trial. J Vasc Surg 20(4):642-649, 1994.

6. Griewing B, Morgenstern C, Driesner F, et al: Cerebrovascular disease assessed by color-flow and power Doppler ultrasonography. Stroke 27:95- 100, 1996.

7. Steinke W, Meairs S, Ries S, et al: Sonographic assessment of carotid artery stenosis. Stroke 27:91-94, 1996.

8. Moneta GL, Lee RW, Yeager RA, et al: Mesenteric duplex scanning: A blinded prospective study. J Vasc Surg 17:79-86, 1993.

9. Stavros AT, Parker SH, Yakes WF, et al:

Segmental stenosis of the renal artery: Pattern recognition of tardus and parvus abnormalities

with duplex sonography. Radiology 184:487-492, 1992.

10. Platt JF, Rubin JM, Ellis JH, et al: Intrarenal arterial Doppler sonography in patients with nonobstructive renal disease: Correlation of resistive index with biopsy findings. AJR 154:1223-1228, 1990.

11. Platt JF, Rubin JM, Ellis JH, et al: Duplex Doppler ultrasound of the kidney: Differentiation of obstructive from non-obstructive dilatation. Radiology 171:515-517, 1989.

12. Polak JF: Peripheral arterial sonography. RSNA Ultrasound 1991 (Syllabus: special course):

211-223, 1991.

13. Mooney MJ, Tollefson DF, Andersen CA, et al: Duplex-guided compression of iotrogenic femoral pseudoaneurysms. J Am Coll Surg 181 (2):155-159, 1995.

14. Beidle TR, Brom-Ferral R, Letourneau JG: Surveillance of infrainguinal vein grafts with duplex sonography. AJR 162:443-448, 1994.