Digital Fluoroscopy

Abstract

Fluoroscopy is an imaging modality that shows anatomical structures and the motion of organs and the movement of contrast media in blood vessels and organs with the goal of obtaining functional information. It has evolved from conventional fluoroscopy recording images on film to current digital fluoroscopy (DF) in which digital dynamic images are obtained and stored in a computer. There are two modes of DF, one that is based on using an image intensifier coupled to a digital imaging chain and one that is based on the use of digital flat-panel detectors (FPDs). The former is being replaced by the latter, which is capable of producing dynamic images that can be displayed and viewed in real time. For this reason, these detectors are sometimes referred to as dynamic FPDs. Two types of dynamic FPDs are currently available for digital fluoroscopy, namely, the cesium iodide amorphous-silicon (CsI a-Si) TFT indirect digital detector and the a-selenium TFT direct digital detector. An important characteristic of these detectors is that they must have high frame rates and fast data transfer rates. Frame rates of 15–30 frames per second (fps) or greater are possible at readout speeds of 30–50 ms. Additionally, dynamic FPDs can operate in at least two readout modes: the fps in continuous X-ray mode and the fps in the pulsed X-ray mode. The overall goal of the latter is to reduce the dose to the patient during the examination.

FPD DF systems offer two approaches to image magnification: (a) electronic magnification (zoom) and (b) binning. With the former approach, there is no increase in spatial resolution, and both original and magnified images have the same signal-to-noise ratio (SNR). Binning has the disadvantage of less spatial resolution because the effective area of each image pixel is four times larger, and it has the advantage of lower data rates and less image mottle than ungrouped pixels. Advantages of FPD DF include distortion-free images, improved contrast resolution, high detective quantum efficiency (DQE), and uniform image quality over the whole displayed rectangular image. The chapter concludes with a brief description of several image post-processing operations specifically for digital fluoroscopy, such as grayscale image manipulation, temporal frame averaging, last image hold, and edge enhancement and a brief overview of digital subtraction angiography (DSA).