Equipment and Principles

The basic physical principle behind diagnostic angiography is the production of x-rays that are attenuated in the body to different degrees, related roughly to the electron density in the soft tissues. Vascular structures must first be filled with an electron-dense substance, iodinated contrast material, before they can be clearly visualized. The amount injected and electron density of the material itself 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. 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 kilovoltage will improve the contrast between objects on the film, but it will deliver more radiation to the patient. Increasing the kilovoltage 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 to comfortably hold the equipment and personnel needed for patient care during the procedure. The room is usually based around the x-ray tube, a patient table, and an x-ray detector. Most digital imaging systems have a C-arm configuration with the x-ray tube coupled to the x-ray detector by a C-shaped bracket. The patient lies on a table situated between the tube and the detector (Figure 36-2). Imaging device configuration varies considerably in sophistication, from small portable systems to table-mounted systems equipped with moving tables for the performance of runoff arteriography.

FIGURE 36-2 Diagram showing the differences between traditional x-ray detectors and digital imaging technologies. Not only has the x-ray detector been replaced by solid state detectors, but the signals are fed directly to a computer for processing.

General Principles: Arterial

Vascular access sites are determined by the pattern of disease. Standard arterial access is the common femoral artery because it is a large vessel and has low complication rates historically. The axillary artery has a smaller caliber than the common femoral artery, but with the brachial plexus intimately wrapped around it in the brachial fascia, nerve-related complications are possible (0.4%-9.5%). Because of the risk for nerve injury, low brachial artery punctures, at midarm and performed under ultrasound guidance, are preferred. Direct popliteal artery punctures can also be performed but are limited by the increased rate of complications and are reserved for highly specific indications by experienced angiographers and not commonly used.

Puncture of vascular prosthetic grafts is commonly seen in the patient population coming to angiography. Graft puncture can be more difficult because of the fibrosis that surrounds the graft.

General Principles: Venous

Lower extremity venography is performed during continued fluoroscopic monitoring of contrast being injected into 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.

Common femoral vein punctures are performed when the iliac veins or the inferior vena cava has to be studied. Large volumes of contrast, typically 30 to 40 mL, 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. Most often, upper extremity venography is done during an intervention, typically venous thrombolysis to treat an acute deep venous thrombosis of the upper extremity veins.

Contrast Agent Administration

The amount and rate of contrast agent injection depends on the size of the vessel segment being studied and the amount of blood flow. The highest injection rates are used when injecting into the proximal ascending and descending aorta (70-80 mL injected at a rate of 30-40 mL/sec). Selective studies of the mesenteric vasculature can require large volumes of contrast (up to 50-60 mL at rates of 3-5 mL/sec) in order to opacify both the artery branches and mesenteric veins. Lower extremity runoff 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 mL at a rate of 6 to 10 mL/sec. Unilateral arterial studies of the arms and legs require 20 to 30 mL.

The general trend toward the use of nonionic contrast agents has been driven by the decrease in pain, discomfort, and contrast reactions seen when these agents are used. Low-osmolar ionic contrast agents are also available.

Administration of iodinated contrast material is not indicated when renal function is depressed. Below an estimated glomerular filtration rate (GFR) of 30 mL/min per 1.73 m², contrast is relatively contraindicated unless the patient is on dialysis. Contrast is administered cautiously and with a hydration protocol when the estimated GFR is between 30 and 60 mL/min per 1.73 m². There are no contraindications, except for possible allergic reactions, for estimated GFR above 60 mL/min per 1.73 m².

Accuracy and Reproducibility

The diagnostic accuracy of angiography has been verified by direct surgical correlations. It has been accepted as the 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 does not permit adequate evaluation of the diameter and quality of the artery. Direct surgical verification is needed to confirm suitability for endarterectomy.

In the lower extremity arteries, opacification of the peripheral runoff 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 has been suggested as a possible way of circumventing this problem in some patients.

Multiple projections are often needed to improve visualization of arterial lesions, related to the fact that arterial lesions tend to be eccentric. Rotational arteriography is another approach to more precisely estimating the degree of stenotic narrowing in an artery.

Complications

Complications are mostly linked to local access sites. Formation of pseudoaneurysms and hematomas increases with catheter size and procedure time as well as the duration of postprocedural groin compression of common femoral arteries. Arteriovenous fistulas are more likely with punctures made lower down the thigh.

Contrast allergy rates are much lower than rates seen for intravenous injections.

Local complications such as dissections and subintimal injuries are linked to poor catheter placement and can be caused by rapid contrast injection. Due to these potential complications, arteriography is not recommended for serial examinations. Recent concerns regarding radiation exposure, particularly in young and pregnant patients, also drive the tendency toward other less-invasive modalities.

Contraindications

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

Equipment

CT scanners have evolved continuously since their introduction in 1979. The first multidetector (four-slice) scanner was used in 1998. Currently, state-of-the-art scanners use a fan-shaped x-ray beam and an arc of x-ray detectors opposite the beam, both mounted on a ring surrounding the patient (Figure 36-4). This rotates to encompass a 360-degree circle around the patient. Rotation times are now shorter than half a second. Fast table speeds and thin slice width allow submillimeter (isotropic) resolution.

FIGURE 36-4 Diagram showing a typical computed tomographic detector setup. An x-ray source projects x-rays through the body as it rotates. The x-ray beam creates a “fan beam,” thereby increasing the efficiency for capturing imaging information.

CT was originally performed as a “step-and-shoot” approach: one image was obtained during a 360-degree rotation of the detector, and then the patient table moved to the next position for the next image acquisition. The current approach is to use helical or spiral imaging (Figure 36-5). Rather than sequentially acquiring data at fixed distances, these devices continuously acquire data as the patient is moved at a constant rate through the scanner. The addition of multiple detectors, currently 16 and often 64 or more, permits coverage of larger distances with each rotation of the detector (Figure 36-6). For example, if a 64-detector array scans over a 40-mm-long segment, each rotation of the gantry can cover a slice thickness of 40 mm with an effective slice thickness less than 1 mm.

FIGURE 36-5 Diagram showing the difference between “step-and-shoot” computed tomographic imaging (left) and spiral imaging (right). With step and shoot, the table with the patient on it is moved by a given increment, a picture is taken, and the table is moved again by the same increment before another picture is taken. This is repeated as often as needed to cover the region of interest. With spiral imaging, the table is fed in a continuous fashion during the rotation of the x-ray tube and detectors. This covers the region of interest more rapidly than the step-and-shoot approach.

FIGURE 36-6 Diagram showing the difference between single-detector (left) and multiple-detector (right) computed tomographic scanners. Since the detector array is wider on the right, it takes a lower number of rotations to cover the region of interest. Acquisition times are dramatically shortened.

Physical Principles

The x-ray detectors are used to measure the attenuated x-ray beam after it has passed through the patient. These data are digitally manipulated to calculate a cross-sectional image. Images are displayed with a gray scale of -1000 Hounsfield units (air) to over 1000 Hounsfield units (bone), on a television monitor, normally with a format of 512 × 512 pixels.

Technique

In CTA, the x-ray data are continuously acquired during a single breath hold while the patient is moved through the x-ray beam of the gantry. Iodinated contrast is simultaneously injected intravenously to enhance vessels at a rate of 3 to 5 mL/sec depending on the application. The acquired data are reconstructed to produce multiple slices of preselected thicknesses. The data are then reconstructed and displayed as axial slices or rendered in a 3D format. A display of vascular structures can be achieved when a maximum intensity projection (MIP) algorithm is applied. This projects only the brightest pixel along each ray path. The image data set can also be manipulated and displayed in a format resembling the multiple projections used in conventional angiography or in selected coronal and sagittal planes.

CTA can be used to carefully evaluate the soft tissues. In this, it is superior to angiography. A simple example is the evaluation of abdominal aortic aneurysm. The CT angiogram can depict the extent of thrombus deposition in the aorta, an evaluation that is not possible by arteriography. CT spiral angiograms can record abdominal vessels to the third-order branches from the aorta (Figure 36-7). The limited spatial resolution restricts visualizing smaller arterial branches. Constraints in the amount of contrast that can be injected make it difficult to follow long vessels such as those found in the extremities.

FIGURE 36-7 Volume-rendered computed tomographic angiogram with partial visualization of the bones. This type of display is useful since it can be reoriented in three-dimensional space in order to visualize vessels of interest on different projections.

Timing of the arrival of contrast in the target arterial segment is critical. Optimal performance of the 3D formatting programs requires a maximal amount of contrast agent in the artery segment while minimizing venous opacification. Selecting the appropriate timing interval is key to optimizing image quality. A simple strategy follows: a small bolus of contrast is administered first. Images are taken and used to time the appearance of contrast material in the vessels of interest. The time from injection to visualization of contrast on the CT image is used to protocol the study for optimal data acquisition. For imaging of the pulmonary arteries, the quality is limited by patient motion, and acquisition must take place during a breath hold, typically 15 to 20 seconds. Bolus-tracking algorithms are also available on modern CT scanners that automatically detect the arrival of contrast material in the target vessel and initiate scanning through the area of interest.

With spiral CTA, the operator selects the speed of acquisition or the effective rate at which the patient is moved through the gantry. Final slice thickness and the distance between slices depend on these parameters but can be further modified by the operator at the time of image reconstruction.

Accuracy and Reproducibility

CT pulmonary angiography has essentially replaced pulmonary arteriography. The validation studies have mostly been done on the basis of outcomes study. The basic outcome used in pulmonary embolism is the absence of recurrent thromboembolic disease over a 3-month period if the study is called negative.

The overall accuracy of the technique for the detection and grading of arterial stenoses is competitive with arteriography in all arterial beds. The basic limitations for CTA are characterizing lesions in arterial segments with diameters smaller than1 mm, difficulty visualizing the smaller visceral branches, and heavy arterial calcification that can hinder visualization of luminal patency.

Complications

Related posts:

Normal Cerebrovascular Anatomy and Collateral Pathways Controversies in Venous Ultrasound Nonimaging Physiologic Tests for Assessment of Lower Extremity Arterial Disease Arterial Anatomy of the Extremities Duplex Ultrasound Evaluation of the Uterus and Ovaries Extremity Venous Anatomy and Technique for Ultrasound Examination

Stay updated, free articles. Join our Telegram channel

Join