RECENT ADVANCES IN THE RADIOLOGICAL EVALUATION OF PERIPHERALARTERIAL OCCLUSIVE DISEASE
R PANT*, SJ PUNAMIYA**
Department of Vascular/Interventional Radiology, Bombay Hospital and Medical Research Centre, Mumbai.
The first ever radiological demonstration of the vascular tree was carried out by Haschek and Liedenthal in Jan 1896, only a few months after the discovery of X-Rays. They displayed the blood vessels in a radiograph of an amputated hand by injecting bismuth salts into the artery. In 1923, Berberich and Hirsch visualized the femoral arteries radiographically using strontium bromide, and Brooks in 1924 did the same using sodium iodide. The development of catheter angiographic techniques by Forsmann in 1928 opened the gates to modern contrast angiography. 
Aetiology of peripheral vascular disease is varied. Large arteries may be obstructed by atherosclerosis, thromboembolism, trauma, arteritis of various types including Buergerís disease, and fibromuscular dysplasia, etc. Medium and small vessel occlusions may be related to diabetes, chronic recurrent trauma, multiple small emboli, collagen diseases, dysproteinaemias, polycythaemia vera, adverse drug reaction, pseudoxanthoma elasticum etc.
Few conditions which are specific to certain anatomical sites include cystic adventitial disease of the popliteal artery, popliteal artery entrapment, and various neurovascular compression syndromes affecting the upper limb, like cervical rib, costoclavicular syndrome, scalenus tunnel syndrome, hyperabduction syndrome and quadrangular space syndrome. These conditions are mentioned separately because they have peculiar presentations and require specific manoeuvres for diagnosis. 
EVALUATION OF ARTERIAL OCCLUSIVE DISEASE
The initial evaluation of peripheral occlusive arterial disease is clinical, with special emphasis on identifying risk factors, pulse palpability and ankle brachial index. Further assessment relies on functional non-invasive testing and radiological imaging to determine not only the anatomic, but also the physiological aberration of peripheral vascular flow.
Non invasive functional assessment includes pressure measurements, plethysmography, and continuous wave Doppler, percutaneous oxygen tension measurement and laser Doppler. All these methods are targeted towards evaluating the arterial flow dynamics in the affected limb, and are invariably supplemented with radiological depiction of anatomic abnormality. 
Radiological evaluation is aimed at providing the following information:
Conventional catheter angiography was the main modality available for the radiological evaluation of arterial tree in the past. It involved the injection of contrast into the vascular tree through an intra-arterial catheter/needle, and images were acquired on radiographs. Until the past decade or so, conventional catheter angiography was the mainstay in the diagnosis of peripheral vascular disease. Recent advances in imaging technology have changed the situation considerably, with the introduction of various modalities such as Duplex Doppler and Colour Doppler Imaging (CDI), Computed Tomographic Angiography (CTA), Magnetic Resonance Angiography (MRA) and Digital Subtraction Angiography (DSA). These modalities by and large have replaced conventional angiography in the diagnosis of peripheral vascular disease. Each modality has a unique role to play in the evaluation of these patients. An algorithm for the evaluation of peripheral arterial occlusive disease is given in Fig. 1.
- Presence or absence of significant obstruction to blood flow
- Site and anatomical extent of obstruction
- Condition of collateral flow and distal vasculature for planning treatment
- Results of therapy and disease progression
(i) Duplex Doppler and CDI
Doppler is a non-invasive method of evaluating the blood vessels using sound waves, similar to ultrasonography and echocardiography. With Doppler, one can obtain both anatomic and haemodynamic information. Anatomical detail of the vessel wall, intraluminal obstructive lesions such as plaques, and also images of perivascular compressive structures can be adequately assessed using routine gray-scale ultrasound. Haemodynamic information is acquired from Doppler, whereby ultrasound signals reflected by the moving RBCs are analysed and information is presented in two forms:
Blood flow velocity and velocity spectrum can be analysed to answer questions about the presence and degree of arterial obstruction. Duplex doppler has been shown to have sensitivity of 92.6% and specificity of 97% in localized femoral, popliteal of infra-popliteal occlusions, where angiography is used as the gold standard. However, it is shown to be inaccurate particularly in the adductor canal and the aorto-iliac regions. It has shown 95% accuracy in the detection of bypass graft stenosis, but it tends to overestimate the degree of stenosis. 
- Spectral Waveforms
- Colour Doppler Imaging (CDI)
The main advantages of Doppler are the availability, economy and the ability to be repeated. It can be even carried out at the bedside on all kinds of patients and has no known contraindications.
Unfortunately, Doppler is a very operator dependent and time consuming procedure. Presence of gas and calcification degrades image quality severely. Its sensitivity to slow flow is low, making detection of small collaterals difficult.
(ii) Computed Tomography and CT Angiography (CTA)
CT scans, used traditionally for head and body cross-sectional imaging, have now rapidly evolved as an excellent modality for studying vascular patho-anatomy, with the development of spiral/helical CT. The spiral/helical CT scanner is a specialised CT machine, whereby images are rapidly acquired during injection of iodinated contrast media enabling display of vasculature. The images are reconstructed using various formats such as Shaded Surface Display (SSD) and Maximum Intensity Projection (MIP) to display arterial anatomy at levels of interest.
CTA is a non-invasive method of evaluating blood vessels, requiring only an IV injection of contrast. The images can be seen in 2-D as well as 3-D, providing excellent pictures almost on par with the gold standard catheter angiograms. In fact, CTA can at times display eccentric stenoses more accurately than angiography. It can be invaluable for evaluation of stent deployment and post procedure complications in iliac arteries. 
Cross sectional CT has been used to display extra-luminal structures causing arterial obstruction, as in popliteal artery entrapment.
In spite of these advantages, CTA is still not preferred for routine evaluation of PVD, as it requires injection of large volume of contrast media, subjecting the patient to risks of contrast induced renal failure. Additionally, the study is limited by the short segments of scanning, and examining the entire arterial tree from the aorta to the pedal arteries is generally not possible. The recent development of multislice scanners had redressed this problem to some extent. These scanners are able to acquire data at multiple levels simultaneously, thus allowing coverage of a much largerarea with a single contrast injection. CTA may fail to demonstrate short stenoses and horizontally oriented branches are poorly visualized, thus significant lesions may be missed. No functional assessment is possible. The specialised CT scanners are not yet universally available. High quality imaging requires a tailored study with precise flow rates of contrast and timing of image acquisition, failing which the study is useless.
(iii) Magnetic Resonance Imaging and MR Angiography (MRA)
Magnetic Resonance Imaging employs the magnetic properties of hydrogen nuclei in the body to image anatomy with superb contrast and detail. MRI is also sensitive to motion and this property has led to the development of MR Angiography. MRA displays the anatomic details of vasculature without using contrast material. Two basic techniques are used in MRA, Time of Flight (TOF) and Phase Contrast Angiography (PCA). The initial promise of totally non-invasive visualisation of vasculature has not been fulfilled, as adequate visualisation of anatomic detail in the peripheral vasculature is only attainable through the use of paramagnetic contrast (Gadolinium DTPA).
Along with vascular anatomy, MRI has the advantage of being able to resolve perivascular structures exquisitely. Acquired images can be displayed in 2-D or 3-D as in CTA. MRA with gadolinium has been shown to give results equivalent to contrast angiography in the assessment of peripheral vascular disease. It has been shown in a few studies using surgery as gold standard that MRA with gadolinium is more sensitive than angiography in displaying runoff vessels in severe arterial occlusive disease.  MRA using PCA also has the potential of obtaining haemodynamic information like blood flow rates and velocities. MRA suffers from its prolonged examination time and extreme sensitivity to motion. Arterial pulsations, peristalsis and respiratory motion can all cause image degradation and loss of information. Reasonable scan times and good anatomic detail require the use of high field strength scanners and surface coils which are expensive and not available everywhere. MRA is contraindicated in certain patients who have pacemakers or other metallic prostheses installed. The presence of metallic vascular stents can cause serious artifacts and image degradation. The monitoring of ill patients can be very difficult during MRA, and requires specialized equipment. (iv) Digital Subtraction Angiography (DSA)
DSA is the gold standard of arterial imaging, and has almost totally replaced conventional cut film angiography. DSA uses images intensifiers and digital image processing software to provide display of vascular anatomy. Subtraction is carried out by comparing a pre contrast image with a post contrast image using a computer, and "subtracting" elements that are common to both. This prevents images of objects like bones etc from obscuring vascular details. Contrast resolution is improved through use of image enhancement software. Radiation exposure is lower than with conventional contrast angiography, and lower volumes of contrast medium can be used. Image acquisition is rapid, and acquired images are immediately available for review. Images are stored in digital format on computerized data storage media like hard discs or compact discs (CDs). Films can be printed if required.
DSA provides better anatomic detail than any other modality. Haemodynamic information regarding pressure gradients across stenoses can be obtained simultaneously. This is important in deciding on the significance of observed anatomic abnormalities. Most importantly, interventions using techniques of angioplasty and stenting, thrombolysis, thromboaspiration etc. are possible at the same time. The unique ability of DSA to function as a diagnostic as well as a therapeutic tool makes it the premium modality in the management of peripheral arterial obstructive disease.
DSA requires intra-arterial insertion of catheters and injection of contrast medium. These procedures have a small incidence of morbidity. The risks of use of contrast media are all present. These are the main drawbacks to the use of DSA, and preclude its use as a screening modality. The issue of contrast nephrotoxicity assumes significance particularly in patients with renal compromise, and in the renal transplant patient. In these cases, the use of Carbon Dioxide (CO2) as a contrast medium for DSA is being increasingly advocated. CO2 is reported to be totally free from nephrotoxicity, but it is recommended that its use be restricted to areas below the diaphragm. An alternative is the use of Gadolinium as contrast agent in these cases.
TABLE 1 Factor CDI/Doppler CT Angiography MR Angiography DSA HaemodynamicAssessment Yes No Yes (with PCA) Yes Anatomic Detail Not Well Seen. Tends to stenoses. Poor in aortoiliac disease. Error prone with horizontal branches, but shows eccentric stenosis well Good anatomic detail with use of, gadolinium Excellent Display 2D 2D and 3D 2D and 3D 2D Reproducibility Operator dependent and painstaking Good Good Good Patient Comfort No known adverse effects or contraindications, Can be used at the bedside. All adverse effects of IV Contrast and radiation Contraindicated in patients with certain prosthetic implants, pacemakers etc. Very difficult to use in patients requiring intensive monitoring. Adverse effects are related to contrast injection and arterial access. Intervention Not Possible Not Possible Not Possible Not Possible
Patients with peripheral arterial disease are frequently also suffering from other diseases. It is important that the limiting disability is identified and treatment planned accordingly, so that benefits of treatment outweigh the risks. 
Modalities available for assessment must be selected on basis of a thorough clinical evaluation, so that required information can be obtained with the least risk and at reasonable expense. The relative merits of the different modalities have been displayed in Table 1. Although DSA remains the gold standard for evaluation of PVD, newer modalities that match its accuracy are rapidly evolving; it would be a matter of time before imaging replaces DSA, with the invasive angiographic techniques being used at the time of interventional radiological treatment.
1.Abrams HL. Historical Notes. In : Abramís Angiography : Vascular and Interventional Radiology, 4th edition. Baum S Ed. Little, Brown and Company, Boston. 1997; 3-11.
2.Johnsrude IS, Jackson DC, Dunnick NR. In A practical approach to angiography, 2nd Ed. Little, Brown and Company. 1987; 163-81.
3.Zierler RE, Sumner DS. Physiological Assessment of peripheral arterial occlusive disease. In : Vascular Surgery (4th Ed.) Rutherford RB Ed, WB Saunders, Philadelphia. 1995; 65-112.
4.Polack JF. Duplex Doppler in peripheral arterial disease. Radiol Clin N Amer 1995; 33 : 71-88.
5.Rubin GD, Dake MD, Semba CP. Current status of 3D spiral CT scanning for imaging the vasculature. Radiol Clin N Amer 1995; 33 : 51-70.
6.Nelemans PJ, Leiner T, de Vet HCW, Van Engelshoven JMA. Peripheral arterial disease : Meta analysis of the diagnostic performance of magnetic resonance angiography. Radiology 2000; 217 : 105-14.