The peripheral vesselsVascular imaging
Arterial system
The classic approach for imaging of the aorta and peripheral arterial system has been angiography performed by means of injection of contrast material through a direct needle puncture or an intraarterial catheter. However, non-invasive means of investigation of the peripheral arteries are being increasingly used before the more invasive angiographic procedures.
Clinical examination and non-invasive angiologic techniques such as oscillometry and Doppler pressure measurements are the first steps in the evaluation of the most common pathology of the vascular system: arterial occlusive disease.
For vascular imaging the new non-invasive modalities like ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) have partly replaced angiography for diagnostic purposes.
As a screening tool ultrasound has gained an important role in the workup of aneurysmal disease including dissections of the aorta, pelvic and peripheral vessels. The patent lumen, mural thrombus and dissection flap are well demonstrated (Fig. 1). The relatively new techniques of colour doppler and duplex scanning furthermore allow flow measurements and are used to assess stenosis and occlusions of peripheral arteries and for follow-up studies after bypass procedures. Colour doppler is the non-invasive method of choice for evaluation of atherosclerotic disease of the neck vessels and for peripheral AV-malformations and AV-fistulas. Duplex and colour doppler scanning is also used in diagnosing venous pathology such as deep vein thrombosis and venous valve incompetence.
Computed tomography is used to substantiate
ultrasound findings in greater detail. Since it is not affected by overlying air or bone
CT is an excellent method of demonstrating pathology of the
aorta and pelvic vessels such as aneurysms, dissections, rupture,
thrombus formation and
calcification as well as paravascular structures (Fig 2). Volume scanning and 3D reconstruction make
CT an important alternative to
invasive angiography as a preoperative diagnostic tool in occlusive and aneurysmal disease of the
aorta and pelvic vessels.
CT is also used to demonstrate pathology involving the vena cava and large central veins, particularly in connection with
thrombus formation or extrinsic compression by tumor etc.
MR-angiography (MRA) finally has the great advantage that vascular structures may be seen in great detail without using contrast material. Using special techniques blood flow can be quantified and flow directions can be determined. MRA has the advantage that any plane in all 3 dimensions may be selected and for example the aorta may by imaged in its entire saggital course. With fast sequences excellent images of the neck and head vessels as well as the peripheral arteries may be generated, however, the role of MRA in the periphery has not been established yet because of its limited availability and expense.
Although non-invasive evaluation and imaging play an important role for screening and assessment of aortic and peripheral vascular pathology, angiography is still the gold standard and is indispensible for percutaneous interventions. Recent advances using digital imaging and subtraction techniques render difficult recanalisation and embolisation procedures easier, faster and safer. Furthermore the amounts of contrast material required can be reduced for both diagnostic and interventional procedures.
Techniques for arteriography
Percutaneous puncture and catherization of the arterial system (Seidinger technique; Fig. 3): The approach to the arterial system is selected according to the clinical signs and location of the target organ respectively. Because of its superficial anatomic location the common femoral artery is the usual site for arterial puncture. Following local anesthesia and a small skin incision, the artery is punctured using a thin walled needle with a 1 - 1.2 mm outer diameter and a central mandril (Fig. 3). Some needles are additionally covered with a Teflon-sheath accepting a .038 guidewire. The needle is advanced into the artery at an angle of approximately 45 degrees. After removing the mandril the needle is pulled back till a pulsating backflow is seen. Then a guidewire with a flexible tip (usually a J-guidewire) is advanced into the vessel. Under manual compression the needle is withdrawn and an angiographic catheter is advanced over the guidewire into the artery and positioned at the desired location. The guidewire is then pulled back and the catheter is checked for backflow and carefully rinsed with saline. The position of the catheter is once more controlled under fluoroscopy using manual
 Figure 3.Schematic drawing of the SeIdinger technique.
1) Puncture of the artery.
2) Removal of the mandril.
3) Withdrawal of the needle until pulsating backflow of blood is seen.
4) Advance of the guidewire.
5) Withdrawal of needle while compressing artery and holding guidewire in place.
6) Advance catheter over the guidewire. |
injection of contrast material. For thoracic aortography 5 or 6 F catheters, 100 cm long which allow flow rates of up to 30 ml per second are used. For abdominal and peripheral angiography 4 and SF catheters of length of 60 cm, are used. Commonly a Pigtail configuration with multiple sideholes is preferred for flush injection in the aorta. The contrast material is commonly injected with a power injector. For selective femoral or carotid angiography a hand injection usually is sufficient.
If the femoral arteries cannot be punctured, e.g. post-operatively or in occlusive disease when the femoral artery itself is occluded or in obstruction of the distal aorta and/or the pelvic arteries, an alternative access route has to be used. Such access sites are the axillary and brachial artery or a translumbar approach to the abdominal aorta. Because of the higher complication rate of axillary or brachial approach, we prefer translumbar aortography.
Radiologic documentation of the contrast injection is performed with digital substraction angiography if available, otherwise with 100 mm spotfilming or conventional film changing techniques (Figs. 11 - 14). The rate of contrast injections has to be tailored according to the filming technique (DSA requires lower injection rates) and the speed of blood flow. For thoracic aortic injections rates of 25 to 30 ml per second and for the abdominal aorta 18 to 25 ml per second are adequate. For peripheral run-off studies of the pelvic and femoral arteries, flow rates of 8 to 12 ml per second are sufficient using 80-100 ml of contrast for bilateral visualisation of the lower extremity arteries down to the feet. The frame rate for thoracic aortography is usually 2 to 4 frames per second; for abdominal aortography 2 per second. For the extremities, according to the speed of blood flow, one to two frames per second is generally used for the pelvis and from one film per second to one film every 3 seconds is used in imaging of the calves.
Complications
Complications can be caused by a) the puncture (bleeding, hematoma, pseudoaneurysms, A V-fistula, spasm, thrombus formation and peripheral embolisation), b) guidewire and catheter manipulation (perforation, dissection of the intima, spasm, embolization of plaque or air embolism) and c) contrast material: allergic or toxic systemic (cardiac, renal toxicity). With an adequate puncture and catheter manipulation technique and the use of non ionic or low osmolarity contrast materials, complication rates in angiography using a femoral approach are less than 1.8%.
Venous system
Ascending phlebography (venography)The most common method of visualization of the lower limb veins from the level of the foot to the lower cava is ascending phlebography (Fig. 4). Although in several ways now challenged by duplex ultrasound, this method is still considered the "gold standard" for an overview of the morphology of the lower extremity veins and their valves and flow patterns. Only veins draining blood mixed with contrast are visualized. The deep femoral and the internal iliac veins are often not opacified. Incompetent valves or venous obstruction may cause collateral circulation and drainage to superficial veins. This may prevent filling of the more important deep veins.
The examination is done in ordinary fluoroscopy rooms by manual exposures of films at various levels and in different projections during hand injection of 50 - 100 cc of a contrast medium via a cannula inserted into a dorsal foot vein. The medial vein of the big toe is usually preferred, since this is most often the easiest vein to puncture. More lateral dorsal foot veins may also be chosen where a retrograde insertion may then secure a better filling of the deep calf veins. The application of a hot, wet towel can help to locate the vein by inducing venodilatation. Tilting the x-ray table also increases the prominence of veins by raising hydrostatic pressure. In edematous feet, prolonged firm compression of the appropriate area of the dorsum of the foot is necessary. Nitroglycerine paste is employed by some to obtain local distension of subcutaneous foot veins for easier puncture. A rubber tourniquet is used at supramalleolar level to force the contrast medium into the deep veins. This may sometimes obstruct the filling of the anterior tibial vein. Tourniquets may also be employed at higher levels. The examination is done with the fluoroscopy table at 45 -60 in a semierect position, until the contrast column reaches the pelvis. The table is then lowered, and the extremity passively raised while the patient does a Valsalva manoeuvre to maximize contrast filling during radiography of the pelvic and lower caval veins. It is important to carry out the examination on a non-weight bearing, relaxed extremity, since the deep veins will otherwise be compressed by limb muscles. The calf veins should be image d in three projections, lateral oblique, anteroposterior and medial oblique, the veins above the knee require one or at most two projections.
Only water-soluble radiographic contrast media can be used for phlebography. Complications are rare, and are mostly related to the contrast media employed. One should be prepared for the immediate management of possible anaphylactoid reactions. Nausea, vasovagal reactions and injection pain may be encountered.
To avoid post-phlebographic thrombosis the contrast medium employed should be of low osmolality, preferably non-ionic. Ionic contrast media of higher osmolality are associated with an incidence contrast-induced thrombosis of 10-60 %. To lessen this risk, hyperosmolar contrast media should be diluted to a concentration of 200 mgI/ml, or alternatively heparin prophylaxis should be administered.
Extravasation of contrast medium at the puncture site may occur by displacement of the cannula or failed venous puncture. This is usually no problem if a low-osmolar contrast medium is employed. With high osmolarity, pain and even skin necrosis may occur.
While ascending phlebography of the lower limb is still the most important diagnostic test for deep venous thrombosis, it has a limited role in the diagnosis of primary venous insufficiency. Secondary venous insufficiency caused by post-thrombotic obstruction of inflow to the large veins in the pelvis, or venous anomalies like hypoplasia, valvular dys- or aplasia or Klippel-Trenaunay syndrome may require phlebography to demonstrate venous morphology.
Other phlebographic techniques of the lower limb
Retrograde phlebography
This technique is employed to demonstrate valvular incompetence at the upper femoral level. The common femoral vein is antegradely punctured with a plastic cannula and contrast medium injected during a Valsalva manoeuvre of the semi-erect patient. The degree of contrast back-flow down through incompetent valves into the veins of the thigh may then be visualized (Fig. 5).
 Figure 5.
Retrograde phlebography showing incompetent valves at the upper femoral level. Grade I incompetence is present if the contrast column passes retrograde and just below the inguinal ligament, grade Il if to the midline, grade III if to a level at the knee, and grade IV to a level below the knee. |
Isometric phlebographyVisualization of the long saphenous
vein for evaluation of its suitability as a graft in arterial reconstruction surgery, is achieved by contrast medium injection into a foot
vein in the supine patient during isometric tension of the limb muscles. This is achieved by pulling on a band passing beneath the soles of the feet.
Videophlebography
This is used for functional studies of venous flow patterns.
Intraosseus phlebography
With this technique a cannula is directly inserted into the medullary space of the greater femoral trochanter or lateral malleolus, in order to visualize the pelvic veins or lower limb veins. The procedure requires general anaesthesia, and is rarely used.
Upper extremity venography
The veins of the upper extremity and the communication with the superior vena cava are phlebographically examined on ordinary fluoroscopy tables by hand injection of contrast medium. First the basilical vein is cannulated. The indication of the examination will often be suspected axillary or subclavian vein thrombosis or tumorous obstruction. Images of the extremity, shoulder and mediastinum are then taken. No tourniquet is applied. The risk of postphlebography thrombosis is probably less than in the lower extremities possibly due to a higher thrombolytic activity in the venous intima of the arms. The volume of contrast medium required is variable. 20-50 ml is usually sufficient. If a pump injector is employed, a flow rate of 8 mI/s may be used. Images of the brachial, axillary, subclavian and brachiocephalic veins and superior vena cava are then taken. In suspected superior vena cava obstruction the veins of both arms may be simultaneously injected with contrast.
Cavography
The Seldinger technique is usually used during angiography of the inferior vena cava. The common femoral vein is punctured at the groin, and a Pigtail catheter is inserted with its tip placed at the bifurcation. Serial exposures during injection of 40-80 ml of contrast medium at a flow rate of 15-20 ml are employed. The patient should perform a Valsalva manoeuvre, by forced expiration against a closed glottis. Using digital subtraction angiography (DSA), the concentration of the contrast medium, as opposed to the volume, can be substantially lowered. Combined pelvic and caval vein angiography may be performed by concomitant injection of contrast medium through plastic cannulae inserted into both common femoral veins at the groin.
Ultrasound
Ultrasound imaging may be used for the non- invasive examination of veins and their surrounding tissues, either by grey-scale real-time imaging of the morphology, or by combining the image with pulsed Doppler determination of vascular flow. In this so-called duplex scanning, the two-dimensional grey-scale image is combined with the flow velocity signal depicted as superimposed colour information, as a spectral curve, or as sound.
Ultrasound is operator-dependent, however in skilled hands it is very useful for venous examinations. In some centres it is the most frequently used diagnostic tool for the depiction of deep venous thrombosis. There is a wide range of indications for ultrasound mapping of peripheral as well as central veins.
Low frequency transducers (2.3 MHz) are used to examine the iliac veins and inferior vena cava and high-frequency transducers (7.5-10 MHz) are used for superficial veins. Other veins are interrogated by midrange transducers of 5-7.5 MHz. Colour-Doppler is us ed to localize the paired deep veins below the knee, to diagnose venous back-flow and valvular incompetence. The ultrasound image will also reveal surrounding soft tissue processes which affect the veins, like hematomas, Baker cysts of the knee, or tumours.
Duplex ultrasound scanning of the lower extremities may be used in patients with a clinical suspicion of deep venous thrombosis, in those with suspected or proven pulmonary emboli, in unexplained leg swelling after orthopedic, pelvic or vascular surgery, and in those with unexplained, chronic leg swelling. The two limbs should be compared at the same sites. It is preferable to start in the groin and move sequentially down to the posterior tibial veins at the ankle. The popliteal veins are best studied in the prone position with the feet supported by a pillow. For the external iliac to the popliteal veins, normal venous flow should be spontaneous and vary with respiration. During inspiration, the flow will diminish as the intra-abdominal pressure exceeds that of the inferior vena cava. The posterior tibial veins may be undetectable. Compression of the foot should then be employed, resulting in an easily detectable surge. While deep venous thrombosis in the veins between the knee and inguinal ligament may be reliably diagnosed using ultrasound, the calf is rather more difficult and even more dependent on the skill and experience of the sonographer. There is a high false positive rate below the knee.
Duplex ultrasound is useful in following-up of patients undergoing anticoagulant or fibrinolytic therapy. Assessing venous patency or caval placements is done in a rapid non-invasive way. Venous duplex scanning is also effective in revealing the presence and anatomical distribution of valves in chronic venous insufficiency.
Magnetic Resonance Imaging (MR/), Computed tomography (CT)
These two imaging modalities have a role in venous imaging similar to their value in imaging arteries. In particular, involvement of the vena cava by intraluminal masses like thrombi or in growing tumours, and caval displacement, compression or obstruction from surrounding processes is well shown. CT examinations require the use of contrast media for venous opacification.
In magnetic resonance angiography (MRA) the vessels may be imaged and the flow rate determined directly, without the use of paramagnetic
 Figure 6.
Schematic drawing of normal anatomy of the aorta and peripheral arteries. |
contrast media. This technology is likely to develop further and become considerably more important and useful than today.
Christoph L. Zollikofer and Frode Laerum