Correlative Imaging

36 Correlative Imaging





Arteriography


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 The rapid film changers of the 1970s and 1980s have been replaced by completely digital imaging technologies.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.


Imaging of vascular structures is dependent on producing x-rays, filling the vessels of interest with iodinated contrast, and recording the images generated (Figure 36-1).




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.




Digital Imaging


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.










Computed Tomography Angiography


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.



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.



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.






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.



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.



Mar 5, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on Correlative Imaging
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