Correlative Imaging

Equipment and principles

Diagnostic angiography relies on the attenuation of x-rays that have entered and then exited the body. The exiting x-rays are attenuated by the amount of soft tissue traversed and their electron density. When vascular structures are filled with iodine-containing contrast agent they become much denser than the surrounding soft tissues. The amount injected must be sufficient to permit preferential visualization by the x-ray imaging device.

The penetrability of x-rays in soft tissues relates directly to the energy of the x-ray photons and is described by the kilovoltage (kV). For angiography, it is usually set between 70 and 80 kV but can be adjusted based on the amount of contrast seen on the final films. Lowering the kV will improve the contrast between objects on the film, but it will deliver more radiation to the patient. Increasing the kV increases penetration of the x-ray beam but reduces image contrast.

The x-ray tube defines the area from which x-rays are produced and the x-ray beam is further focused by collimators (lead screens that reduce the size of the beam to fit the body part being imaged as closely as possible).

A large space is required for the equipment and personnel needed to perform the procedure. The room is usually based around the x-ray tube, a patient table, and an x-ray detector. Many digital imaging systems have a C arm configuration with the x-ray tube coupled to the x-ray detector by a C-shaped bracket so that they can rotate together. The patient lies on a table situated between the tube and the detector ( Fig. 34.1 ). Imaging device configuration varies considerably in sophistication, from small portable systems to table-mounted systems equipped with moving tables for the performance of run-off arteriography.

Digital imaging

The resolution of digital images depends on the size of the imaging detector and the matrix size of the digital image. Large image fields that cover up to 16 inches need a large matrix size such as 1024 by 1024 pixels to give 1.3 line pairs per millimeter of spatial resolution.

Most units allow for single-station imaging at a fixed location over an arterial segment or venous segment. More expensive configurations with a moving table can provide multiple station bolus-chasing images for evaluation of the lower extremity run-off arteries.

Digital subtraction builds on digital imaging by obtaining one or several mask images prior to radiographic contrast agent arrival in the imaging field. The mask image is then subtracted digitally from the images with contrast to display the areas containing contrast ( Fig. 34.2 ). Patient movement during injection causes an imaging mismatch, as does respiratory excursion. Full patient cooperation remains necessary for high-quality imaging. Digital subtraction techniques reduce the amount of contrast agent needed to visualize a vessel properly.

General principles: arterial

Vascular access sites are determined by the pattern of disease. The common femoral artery is the preferred arterial access site because it is a large vessel and its use is associated with low complication rates. The axillary artery is of comparable caliber to the common femoral artery, but because the brachial plexus is intimately wrapped around it in the brachial fascia, nerve-related complications are possible (0.4% to 9.5%). Low brachial artery punctures, when performed under ultrasound guidance, are safer. Radial artery access can also be used. Direct popliteal artery punctures have an increased rate of complications and are reserved for highly specific indications. Direct puncture of vascular prosthetic grafts can be used when other access sites are not available.

General principles: venous

Lower extremity venography is performed during continuous fluoroscopic monitoring of iodinated contrast agent being injected in a pedal vein. Normally the patient is placed on a tilt-table so that gravity can help distend the veins. Full opacification of the veins is needed to exclude deep vein thrombosis. Images are acquired at the different levels of the leg.

Common femoral vein punctures are performed when the iliac veins and the inferior vena cava have to be studied. Large volumes of contrast contrast, typically 30 to 40 cc, are then rapidly injected while digital venography is performed.

Direct ultrasound guided punctures into the popliteal or tibial veins are occasionally done during venous interventions.

Upper extremity venography is occasionally performed to map the anatomy of the upper extremity veins ( Fig. 34.3 ). More commonly, upper extremity venography is done during an intervention, typically venous thrombolysis to treat an acute deep vein thrombosis of the upper extremity veins, or as part of dialysis access graft evaluation.

Iodinated contrast agent administration

The amount and rate of contrast agent injection during angiography depends on the size of the vessel and the amount of blood flow in the vessel segment being studied. The highest injection rates are used in the proximal ascending and descending aorta (70 to 80 cc injected at a rate of 30 to 40 cc/s). Selective studies of the mesenteric vasculature can require large volumes of iodinated contrast agent (up to between 50 and 60 cc at rates of 3 to 5 cc/s) in order to opacify both the artery branches and mesenteric veins. Lower extremity run-off studies are conducted by sequentially shifting the imaging field-of-view over the leg arteries during the injection of a bolus of 60 to 90 cc at a rate of 6 to 10 cc/s. Unilateral arterial studies of the arms and legs require 20 to 30 cc.

The general trend toward the use of nonionic and low-osmolar iodine contrast agents has been driven by the decrease in morbidity such as pain, discomfort, and allergic reactions when these iodinated contrast agents are used.

Administration of iodinated contrast material may not be appropriate when renal function is depressed. Below an estimated glomerular filtration rate (eGFR) of 30 mL/min per 1.73 m ² , use of an iodinated contrast agent is relatively contraindicated unless the patient is on dialysis. Contrast is administered cautiously and with a hydration protocol when the eGFR is between 30 and 60 mL/min per 1.73 m ² . There are no contraindications, except for possible allergic reactions, for eGFR above 60 mL/min per 1.73 m ² .

Accuracy and reproducibility

The diagnostic accuracy of angiography has been verified by direct surgical correlations. It has been accepted as a gold standard. There are possible limitations in the carotid artery and peripheral arterial systems. In cases of near-occluded carotid arteries, the small amount of contrast that enters the distal internal carotid artery may not permit adequate evaluation of the diameter and quality of the artery.

In the lower extremity arteries, opacification of the peripheral run-off vessels in the leg and foot is sometimes not possible. The use of on-table arteriograms in the operative suite while the patient is under anesthesia can circumvent this problem.

Multiple projections are often needed to improve visualization of arterial lesions, a reflection of the eccentric nature of atherosclerotic lesions. Rotational arteriography is another approach to estimate more precisely the degree of stenotic narrowing in an artery.

Complications

Complications are common at the artery access site. The incidence of pseudoaneurysms and hematomas increases with increasing catheter size and shorter compression times after catheter withdrawal. Arteriovenous fistulas are more likely to occur with punctures made lower down the thigh. Complications of catheter entry such as dissections and subintimal injuries are linked to poor catheter manipulation and can occur at the access site or further along the arterial tree.

The incidence of renal failure following angiography is contested but can be as high as 10%. Allergic reactions to iodinated contrast agents are much lower than for intravenous injections yet still occur.

Recent concerns regarding radiation exposure, particularly in young or pregnant patients, also drive the preference for other less-invasive imaging modalities.

Because of these potential complications, arteriography should be used judiciously when serial examinations are contemplated.

Contraindications

Contrast allergy is a relative contraindication. In general, premedication with steroids and the selection of a different contrast material significantly decrease the risk of severe allergic reaction. Poor renal function is also a relative contraindication.