Digital fluoroscopy (DF)

a fluoroscopy unit from which the signal is digitized, and where the digital images are generated in a computer and displayed on a cathode ray tube (CRT) screen. A digital fluoroscopy unit has many resemblances with a common analogue TV-based fluoroscopy unit. However, there are certain properties which are required for operation in digital mode.

The optical system behind the output screen of the image intensifier is for a digital system equipped with an adjustable iris diaphragm. The reason for this is that in a digital system, the X-ray tube current can sometimes be much higher (often 100 times higher) than that used for normal fluoroscopy. The high-mA operation mode is used with digital fluorography or digital subtraction angiography (DSA). As for normal cinefluorography, the light from the output screen must be limited with a diaphragm in order not to saturate the television camera tube.

The TV camera tube must be chosen so that the lag (also called inertia) is as small as possible, as the purpose of the system is to record image sequences where the image content rapidly changes, e.g. contrast passages through vessels or display of moving contrast-filled vessels (as in the heart). Furthermore, the electronic noise level in the TV chain must be extremely low. When using high-mA techniques for subtraction, the electronic noise could contribute significantly to the noise level in the subtraction images. In such images, the noise is a problem, because it is added quadratically when images are subtracted (see digital subtraction angiography (DSA)). One of the TV camera tube combinations fulfilling the requirement for low electronic noise are called plumbicon tubes because they have lead oxide as input screen material. They have a particularly low lag and very low dark current. Furthermore, the video output is directly proportional to the light input. The plumbicon has, however, due to the low inertia, a clearly visible quantum noise in the fluoroscopic image. In digital systems are therefore often use another type of TV camera tube called a saticon. In this tube the input screen material is SeAsTe, which has the same low electronic noise level as the plumbicon but a slightly higher inertia. The higher inertia results in a markedly lower quantum noise level.

For common fluoroscopy, the readout of the image on the TV camera input screen is often done in interlaced mode (see interlaced horizontal scanning. This means that, in order to form the resulting 625 (or 525) line image, the TV camera tube is scanned twice, every other line being scanned each time. The interlaced mode has been introduced because the demands of noninterlaced mode on electronics and bandwidth of the system would lead to undesirable consequences for cost and it enables the possibility to transmit television images wirelessly. The readout of the "first" image in interlaced mode will to some extent reduce the residual signal left for scanning in the "second" image as some charge from the second lines is picked up by the first scan, thus resulting in a variation of brightness between the first and second image. The eye, however, merges the two neighbouring lines together and the impression will be a nearly flicker-free image.

Digital fluoroscopy systems often operate in pulsed mode, that is, the X-rays come in short pulses. This necessitates other methods for reading out the input screen of the TV camera tube. First, the tube must have the property of "storing" the image in the input screen. In progressive scan mode (see noninterlaced readout) the image is read out line by line in 40 milliseconds (33 ms for 60 Hz systems). Since, however, a higher repetition rate is needed for display (50 or 60 frames per second), the progressively scanned image cannot be used for display. Therefore, the image is converted to an interlaced display mode and is displayed as two interlaced images using 20 (or 16.7) milliseconds per field.

For further details on requirements on digital images, see digital radiography (DR).

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