36 Correlative Imaging
The current trend in cardiovascular imaging is to increase reliance on noninvasive approaches, to minimize complications linked to invasive imaging, and to reserve invasive techniques for therapeutic interventions.
Arteriography, still the gold standard, is now being performed with greater speed and lower complication rates than in previous decades. This has been achieved with the use of smaller catheters and decreased procedure times linked to digital technologies.
A new facet to the use of traditional x-rays has been the development of computed tomographic angiographic methods. This partly noninvasive approach relies on the intravenous administration of iodinated contrast material. In addition to rapidly obtaining traditional cross-sectional images, the image sets can be reprojected and rendered as three-dimensional (3D) data sets.
Magnetic resonance imaging displays soft tissue images similar to those made with computed tomography (CT) but without the need for ionizing radiation. In addition, specially designed magnetic pulse sequences can be used to create magnetic resonance angiograms. Magnetic resonance angiography (MRA) can be performed with or without the administration of intravenous contrast material containing gadolinium.
Doppler ultrasound imaging has strengths and weaknesses when compared to these other imaging modalities. This chapter reviews different aspects of each of these imaging approaches and compares their diagnostic efficacy.
Arteriography remains the gold standard examination for the evaluation of patients before any vascular intervention. Current techniques allow for outpatient diagnostic studies to be performed with extremely low morbidity and mortality. The rapid film changers of the 1970s and 1980s have been replaced by completely digital imaging technologies. The iodinated contrast media have lower associated complications with the adoption of nonionic and low-osmolar compounds.
FIGURE 36-1 Venogram of the upper thigh using traditional cut-film venography. Cut-film technique has been replaced by digital imaging techniques. Note the filling defect (arrowheads) in the femoral vein (FV) due to acute thrombus. PFV, profunda femoral vein; GS, great saphenous vein.
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.
The resolution of digital images is dependent on the size of the imaging detector, either a phosphor tube intensifier or increasingly, a solid state 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 imaging of the lower extremity runoff arteries.
Digital subtraction builds on digital imaging by obtaining one or several “mask” images before radiographic contrast injection. The mask image is then subtracted digitally from the images with contrast to display the areas containing contrast only (Figure 36-3). Patient movement during injection causes an imaging “mismatch,”, as does respiratory excursion. Full patient cooperation remains necessary for high-quality imaging. Digital subtraction techniques can result in reduction of the amount of contrast agent needed by twofold to threefold.
FIGURE 36-3 Right upper extremity venogram using digital technique. Note the absence of the bones. A faint outline of the ribs can be seen as a result of mild respiratory motion. AV, axillary vein; BV, brachiocephalic vein; CV, cephalic vein; SV, subclavian vein.
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.
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.
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.
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.
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 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.
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.
Computed tomographic angiography (CTA) has multiple vascular applications. Essentially any arterial bed that can be studied with arteriography, including the pulmonary arteries, can be examined with CTA. Advances in multidetector technologies and imaging processing have made these examinations very cost-effective.
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.
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.
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.
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.