Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

ground anatomy causes geometric tomography images to be inherently low in contrast,so this modality is used only for very high-contrast imaging situations~forexample, when contrast agent is present in the kidney (IVPs), for air-bone contrast ofthe inner ear, or for imaging tissue-air contrast in the lungs. In the past, specializedradiographic units (dedicated tomography systems) were produced by several manufacturersand were capable of a variety of motions. In addition to the linear motionthat is commonly available on standard radiographic rooms (as an option) today, dedicatedtomographic systems had circular, tri-spiral, and other complicated motions.These nonlinear motions produced blurring patterns that were thought to be moreuseful in some circumstances. However, dedicated tomography systems have limiteduse in today's radiology departments and are no longer sold commercially.The notion of slice thickness in geometric tomography is not as tangible as itis in computed tomography. The object blurring that was illustrated in Fig. 12-2 isreally a continuous effect-there is only a minute amount of blurring of objects thatare located precisely at the focal plane (related to the size of the object), and theamount of blur for out-of-plane objects increases as the distance from the object tothe focal plane increases. For larger tomographic angles, the blurring that an outof-planeobject experiences is greater than for smaller tomographic angles. Figure12-3 illustrates the slice sensitivity profiles for two different tomographic angles.Notice that the amplitude of the slice sensitivity profile for objects even 20 cm awayfrom the focal plane is still non-zero, meaning that ghost shadows from such objectsremain in the image. By comparison, the slice sensitivity profiles in computedtomography become zero just outside the collimated slice thickness.Geometric tomography requires an x-ray exposure of the entire thickness of thepatient to acquire just one tomographic image. If a series of tomographic images atdifferent focal depths are to be acquired, the tissue volume in the patient is exposedrepeatedly for each acquisition. If the dose from one tomographic acquisition is 5mGy (0.5 rad) and ten tomographic slices are acquired, then the dose to the patientincreases to 50 mGy (5 rad). By comparison, if the dose from one computed tomographicslice is 30 mGy (3 rad), then the dose from ten contiguous slices is still 30mGy (3 rad), excluding the dose resulting from scattered radiation. However, a10090C 801ii 70~l: 60o() 50~•• 40••~ 302010o -10 -8 -6 -4 -2 0 2tFIGURE 12-3. The slicesensitivity profiles for geometrictomography areillustrated. Thinner slices(sharper peaked profiles)occur when the tomographicangle is greatest.Tomograms with a 10-degree tomographic angleresult in very thick slicesand are called zonograms.Notice that for geometrictomography, the slice sensitivityprofiles never go completelyto zero, unlike thosethat are characteristic incomputed tomography.