Medical Imaging Systems PDF

Medical Imaging Systems Part 2 X-ray beam scattering Analysis & X-ray Modern Film Detectors  Radiography Geometry of Projection Radiography >1  Magnification in Radiography Screen-Film Radiography Characteristic Curve  Digital Radiography Computed...

Medical Imaging Systems Part 2 X-ray beam scattering Analysis & X-ray Modern Film Detectors  Radiography Geometry of Projection Radiography >1  Magnification in Radiography Screen-Film Radiography Characteristic Curve  Digital Radiography Computed Radiography (CR) ❑ Computed radiography (CR) refers to photostimulable phosphor detector (PSP) systems. ❑ The CR cassette is exposed to x-rays during the radiographic examination and is subsequently placed in a CR reader. Once placed in the CR reader, the following steps take place: 1. The cassette is moved into the reader unit, and the imaging plate is mechanically removed from the cassette. 2. The imaging plate is translated vertically in the (y) direction by rollers across a moving stage and is scanned horizontally in the (x) direction by a laser beam of approximately 700 nm wavelength. 3. Red laser light stimulates the emission of trapped energy in a tiny area (x, y location) of the imaging plate, and blue-green visible light is emitted from the storage phosphor as energetic electrons drop down to their ground state. 4. The light emitted through photostimulated luminescence is collected by a fiber optic light guide and strikes a photomultiplier tube (PMT), where it produces an electronic signal. 5. The electronic signal is digitized and stored as a pixel value. For every spatial location (x, y) on the imaging plate, a corresponding gray scale value is determined that is proportional to the locally absorbed x-ray energy. 6. The plate is exposed to bright (strong) white light to erase any residual trapped energy. 7. The imaging plate is returned to the cassette and is ready for reuse. Computed Radiography (CR) Computed Radiography (CR)  Charge-Coupled Device and Complementary Metal-Oxide Semiconductor detectors FIGURE 7-11 A. A photograph of a high-resolution CCD chip is shown. B. The readout procedure in a CCD chip is illustrated. After exposure, electrodes in the chip shift the charge packets for each detector element by switching voltages, allowing the charge packets to move down by one detector element at a time. Charge from the bottom element of each column spills onto the readout row, which is rapidly read out horizontally. This process repeats itself until all rows in each column are read out. C. This illustration shows the shift of a given pattern of exposure down one column in a CCD chip in four (t1–t4) successive clock cycles. Linear arrays are configured with single or multiple rows in a wide selection, such as 1 x 2048 detector elements (dexels), or 96 x 4096 dexels in a rectangular format for line scan detection (e.g, movement of a slot scan detector with a narrow fan-beam collimation)  Flat Panel (TFT) Detectors FIGURE 7-14 This diagram shows circuitry for a TFT flat panel detector system. FIGURE 7-13 A. flat panel detector systems are pixelated discrete detector systems. The detector array is comprised of a large number of individual detector elements (dexels). Each dexel has a light sensitive region and a light-insensitive area where the electronic components are located. B. A photomicrograph of an actual TFT system is shown. The electronics component can be seen in the upper left corner of each dexel (Image courtesy John Sabol and Bill Hennessy, GE Healthcare).  Direct vs Indirect Detection TFT Arrays FIGURE 7-15 Indirect and direct detector TFT–based x-ray detectors are shown. A. Photons in the indirect system propagate laterally, compromising resolution. The detected signal shown for the indirect detector shows this lateral spread in the signal from one x-ray photons interaction. B. For the direct detector system, the ion pairs liberated by x-ray interaction follow the electric field lines (electron holes travel upwards, electrons travel downwards) and have negligible lateral spread. Here, the detected electronic signal from one x-ray photons interaction Is collected almost entirely in one detector element, and therefore better spatial resolution is achieved.  Scatter to Primary beam Ratio (SPR) & Scatter Fraction (F) The detection of scattered photons causes film darkening but does not add useful information content to the image. The SPR depends on the area of the x- ray field (field of view), the thickness of the patient, and the energies of the x- rays. The scatter-to-primary ratio (SPR) as a function of the side dimension of the x-ray field ❖ As the field of view is reduced, the scatter is reduced. Therefore, an easy way to reduce the amount of x-ray scatter is by collimating the x-ray field to include only the anatomy of interest and no more. For example, in radiography of the thoracic spine, collimation to just the spine itself, and not the entire abdomen, results in better radiographic contrast of the spine. The Anti-Scatter Grid  Parameters that characterize the anti-scatter grid ✓ Grid Ratio The ratio of the height of the interspace material to its width—the septa dimensions do not affect the grid ratio metric ✓ Bucky Factor It describes the relative increase in x-ray intensity or equivalently, mAs, needed when a grid is used, compared to when a grid is not used. ✓ Grid Frequency The number of grid septa per centimeter ✓ Focal Length The interspace regions in the antiscatter grid should be aligned with the x-ray source, and this requires that the grid be focused. For systems where the source-to-detector distance can vary appreciably during the clinical examination (fluoroscopy), the use of lower grid ratio grids allows greater flexibility and will suffer less from off-focal grid cutoff, but will be slightly less effective in reducing the amount of detected scattered radiation ✓ Moving Grids A Bucky grid is a grid that moves with a reciprocating motion during the x-ray exposure, causing the grid bars to be blurred by this motion and not visible in the image.  Parameters that characterize the performance of the anti-scatter grid ✓ Primary Transmission Factor Tp is the fraction of primary photons that are transmitted through the grid ✓ Scatter Transmission Factor Ts is the fraction of scattered radiation that penetrates the grid ✓ Selectivity ✓ Contrast Degradation Factor (CDF) Reduction in contrast due to scattered radiation Contrast and Dose in Radiography The data shown in Fig. 6-20 were computed so that when the kVp was reduced, the mA was increased to ensure the same degree of film darkening. Dose, Contrast vr. kVp