Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Scintillation camera planarSPECTPETDigital fluoroscopy, cardiac catheter labComputed radiography, digitizedchest filmsMammography (18 x 24 cm) or (24 x 30 cm)X-ray CTMRIUltrasound64 2 or 128 264 2 or 128 2128 2512 2 or 1024 2Typically 2,000 x 2,500Typically 1,800 x 2,300 to4,800 x 6,000512 264 2 to 1,024 2512 28 or 168 or 16168 to 1210-12CT, computed tomography; MRI, magnetic resonance imaging; PET,positron emission tomography; SPECT,single photon emission computed tomography.a digital image in four different pixel formats. The number in each pixel is convertedinto a visible light intensity when the image is displayed on a video monitor. Typicalimage formats used in radiology are listed in Table 4-7.Imaging modalities with higher spatial resolution require more pixels per imageso the image format does not degrade the resolution. In general, an image formatshould be selected so that the pixel size is on the order of the size of the smallest objectto be seen. In the case of a fluoroscope with a 23 cm field of view, the 512 2 pixel formatwould be adequate for detecting objects as small as about a half a millimeter insize. To detect objects half this size, a larger format, such as 1024 by 1024, should beselected. When it is necessary to depict the shape of an object, such as a microcalcificationin x-ray mammography, an image format much larger than that needed tomerely detect the object is required. Figure 4-9 shows the degradation of spatial resolutioncaused by using too small an image format. The penalty for using larger formatsis increased storage and processing requirements and slower transfer of images.The largest number that can be stored in a single pixel is determined by thenumber of bits or bytes used for each pixel. If 1 byte (8 bits) is used, the maximalnumber that can be stored in one pixel is 255 (2 8 - 1). If2 bytes (16 bits) are used,the maximal number that can be stored is 65,535 (2 16 - 1). The amount of contrastresolution provided by an imaging modality determines the number of bits requiredper pixel. Therefore, imaging modalities with higher contrast resolution requiremore bits per pixel. For example, the limited contrast resolution of ultrasound usuallyrequires only 6 or 7 bits, and so 8 bits are commonly used for each pixel. Onthe other hand, x-ray CT provides high contrast resolution and 12 bits are requiredto represent the full range of CT numbers. Figure 4-10 shows the degradation ofspatial resolution caused by using too few bits per pixel.Pixel size is determined by dividing the distance between two points in thesubject being imaged by the number of pixels between these two points in theimage. It is approximately equal to the field of view of the imaging device dividedby the number of pixels across the image. For example, if a fluoroscope has a 23-cm (9-inch) field of view and the images are acquired in a 512-by- 512 format,then the approximate size of a pixel is 23 cm/512 = 0.45 mm, for objects at theface of the image intensifier.