Fluoroscopy & Digital Photospot
Fluoroscopy provides a dynamic view of the anatomy, and to keep total patient dose low, the image intensifier is operated at a very high gain at low air kerma entrance rates in order to achieve the brightness necessary for the TV camera to produce a reasonably bright image on the in-room TV monitor. A real-time sequence of images at 30 frames/second presented to the viewer is perceived with much less mottle than a static last frame hold image because of the internal lag characteristics of the human eye-brain response, which is on the order of 200 ms. The consequence is that a noisy image sequence viewed in real-time will appear as an average of approximately 3-5 frames of image information content, thus presenting with less quantum mottle than the last frame hold image. Digital photospot images are acquired through the image intensifier / TV system using a much higher entrance air kerma (and of course much higher patient dose) but much better image quality in terms of resolution (typically a higher resolution TV camera is used in conjunction with a larger digital image matrix with correspondingly smaller pixel dimension) and lower quantum mottle are achieved, as shown below. Exposure differences are on the order of 50 – 100 times greater for the digital photospot compared to the single frame fluoroscopy image. This must be considered, however, in the context of a much larger number of images acquired during fluoroscopy compared to a relatively few digital photospot images acquired for the purposes of documentation of the anatomical findings with much better detail and image quality.
|Figure A. One frame of a fluoroscopic image of a pelvis phantom||Figure B.One digital photospot image of a pelvis phantom.|
Figure A is a single frame of a fluoroscopy run of a pelvic phantom with a nominal image intensifier diameter of 38 cm. The techniques used to acquire this fluoroscopy run were 75 kV and 2.4 mA, which are typical of fluoroscopy. The radiation air kerma rate at the entrance of the phantom (i.e., entrance skin air kerma rate) was 35 mGy/minute (i.e., ~4 R/minute). Fluoroscopy is performed by generating 30 frames every second, so that in one minute there will be a total of 60 seconds x 30 frames/second image, or 1800 images. Figure A shows just one of these 1800 images generated every minute and the patient entrance dose is thus ~(35 mGy entrance air kerma)/(1800 acquired images), or ~0.019mGy entrance air kerma per frame.
Figure B shows an example of a digital photospot image (radiograph) that is obtained by increasing the x-ray tube current to a high value, about hundred or so times higher than the low tube used during fluoroscopy (i.e.. 2.4 mA). In this example, the x-ray tube voltage decreased to 65 kV, and the digital photospot image was acquired using an x-ray beam intensity of 9 mAs. The skin entrance air kerma associated with Figure B was ~1.4 mGy, or a factor of 74 (i.e., 1.4/0.019) higher than the single fluoroscopy frame depicted in Figure A.
The number of photons used to generate a radiographic image determines the amount of mottle (AKA noise) in the image. Figure A makes use of very few photons and has a much higher level of mottle than figure B. Fluoroscopy images are generally very low quality, and are used to identify the location of a catheter rather than for diagnostic interpretation. Digital photospot images are acquired using radiation intensities that use ~100 or so times more photons, and are considered to be diagnostic.
|Figure C. One frame of a fluoroscopic run of a skull phantom (AP projection)||Figure D. One digital photospot image of the same skull phantom shown in Figure C.|
Figure C shows one single frame of a fluoroscopy run of a skull phantom, with a nominal image intensifier diameter of 25 cm. The techniques used (selected by the II system) to acquire this image were a x-ray tube voltage of 74 kV and tube current of 2.2 mA. The choice of x-ray techniques for the skull phantom are slightly lower than those used to image the pelvis phantom (Figure A) because this projection of the skull phantom is less attenuating the pelvis AP projection. The radiation air kerma at the entrance of the phantom was 26 mGy/minute (~2.9 R/minute), so the single frame shown in Figure C required an entrance skin air kerma of 0.014 mGy.
Figure D shows the corresponding digital photospot image (radiograph) taken at the completion of the fluoroscopy run. To generate the radiograph shown in Figure D, the imaging chain used an x-ray tube voltage of 68 kV and a total tube current exposure time product of 6 mAs (high current/short exposure time). The entrance skin air kerma required to generate the image in Figure D was 0.94 mGy, or a factor of 67 times higher (i.e., 0.94/0.014) than the single fluoroscopy frame depicted in Figure C. As expected, the image quality (i.e., mottle) of the image in Figure D is low, and the image is acceptable for diagnostic purposes. By contrast, the mottle in the image in Figure C is very high, and the single frame shown in Figure C would not be deemed to be of diagnostic quality for most clinical applications.In comparing a single frame from a fluoroscopy run (Figure A and Figure C) with a digital photospot (Figure B and Figure D), it is important to note that the imaging chain is identical except for the amount of radiation used to acquire the image. The amount of mottle (noise) in an image is inversely proportional to the square root of the number of photons used to acquire the image. In other words, a single frames from a fluoroscopy series that uses ~100 times less radiation than a digital photospot image will have approximately ten times more mottle. This increased mottle will limit the ability to detect low contrast lesions; a fluoroscopy frame is expected to have about a tenfold reduction in lesion detection (contrast or lesion effective thickness) than the corresponding digital photospot image.