Medical Imaging Systems - Part 4: Fluoroscopy PDF

Medical Imaging Systems Part 4: Fluoroscopy Fluoroscopy Fluoroscopy is an imaging procedure that allows real-time x-ray viewing of the patient with high temporal resolution. Real Time imaging is usually considered to be 30 frames per second, which is the standard television fram...

Medical Imaging Systems Part 4: Fluoroscopy Fluoroscopy Fluoroscopy is an imaging procedure that allows real-time x-ray viewing of the patient with high temporal resolution. Real Time imaging is usually considered to be 30 frames per second, which is the standard television frame rate. Unrecorded fluoroscopy sequences are used for advancing catheters during angiographic procedures (positioning), and once the catheter is in place, a sequence of images is recorded using high frame rate pulsed radiography as radiographic contrast media are injected in the vessels or body cavity of interest. For recording images of the heart, cine cameras offer up to 120- frame-per second acquisition rates using 35-mm cine film. Fluoroscopic Imaging Chain Components:  Fluoroscopy Image Intensifier: ❑ A 10-minute “on time” fluoroscopic procedure, if conducted at 30 FPS, produces a total of 18,000 individual images! Due to the extremely large number of images required to depict motion and to achieve positioning, fluoroscopic systems must produce each usable image with about one thousandth of the x-ray dose of radiography, for radiation dose reasons. ❑ In practice, standard fluoroscopy uses about 1 to 5 µR incident upon the image intensifier per image, whereas a 400-speed screen-film system requires an exposure of about 600 µR to achieve an optical density of 1.0. ❑ IIs and fluoroscopic flat panel detectors operate in a mode that is several thousand times more sensitive than a standard radiographic detector and, in principle, can produce images using several thousand times less radiation. ❑ standard fluoroscopy uses about 9 to 17 nGy (~1 to 2 R) incident upon the detector per image, whereas a computed radiography (CR) system requires an exposure of about 5 to 9 Gy (~0.6 to 1 mR) to the detector. Image Intensifiers  Input Screen: All modern Image Intensifiers use cesium iodide (CsI) for the input phosphor. CsI is not commonly used in screen-film cassettes because it is hygroscopic and would degrade if exposed to air. CsI has the unique property of forming in long, needle-like crystals. Figure 9-3 A detailed view of the input window of an II is shown. Incident x-rays pass through the antiscatter grid (not shown), and must also pass through the vacuum support window (,1 mm Al) and the support for the input phosphor (,0.5 mm Al). X-rays are absorbed by the CsI input phosphor, producing green light that is channeled within the long CsI needles to the antimony trisulfide photocathode, which then ejects electrons into the vacuum space of the II. Scanning electron micrographs illustrate the crystalline needle–like structure of CsI. X-ray optical photon electrons  The Output Phosphor: ◼ The output phosphor is made of zinc cadmium sulfide doped with silver (ZnCdS:Ag), which has a green (~530 nm) emission spectrum. Figure 9-5 The output window of the II is shown. The electron strikes the very thin (0.2 mm or 200 nm) anode and phosphor, and the impact of this high-velocity charged particle causes a burst of green light in the phosphor, which is very thin (1 to 2 mm). The light exits the back side of the phosphor into the thick glass window. Light photons that hit the side of the window are scavenged by the black light absorber, but a high fraction of photons that reach the distal surface of the window will be focused using an optical lens and captured by a digital camera. Light propagates by diffusion (multiple scatter interactions) in a turbid phosphor medium, but in the clear glass window, light travels in straight lines until surfaces are reached, and then standard optics principles (the physics of lenses) are used for focusing. The spatial pattern of electrons released at the input screen must be maintained at the output phosphor. To preserve a resolution of 5 line pairs/mm at the input plane, the output phosphor must deliver resolution in excess of 70 line pairs/mm. This is why a very fine grain phosphor is required at the output phosphor. Video Cameras: Figure 9-6 A diagram illustrating the closed circuit analog TV system used in older fluoroscopy systems is illustrated. At the TV camera (A), an electron beam is swept in raster fashion on the TV target (e.g. SbSO3). The TV target is a photoconductor, whose electrical resistance is modulated by varying levels of light intensity. In areas of more light, more of the electrons in the electron beam pass across the TV target and reach the signal plate, producing a higher video signal in those lighter regions. The video signal (B) is a voltage versus time waveform which is communicated electronically by the cable connecting the video camera to the video monitor. Synchronization pulses are used to synchronize the raster scan pattern between the TV camera target and the video monitor. Horizontal sync pulses (shown) cause the electron beam in the monitor to laterally retrace and prepare for the next scan line. A vertical sync pulse (not shown) has different electrical characteristics and causes the electron beam in the video monitor to reset at the top of the screen. Inside the video monitor (C), the electron beam is scanned in raster fashion, and the beam current is modulated by the video signal. Higher beam current at a given location results in more light produced at that location by the monitor phosphor (D). The raster scan on the monitor is done in synchrony with the scan of the TV target. Characteristics of Image Intensifier (II) Performance: ◼ The function of the x-ray II is to convert an x-ray into minified light image. ◼ There are several characteristics that describe how well the II performs this function. 1. Conversion Factor: - Is the ratio of the output luminance measured in candela per meter squared (Cd m-2) divided by the input exposure rate measured in mR/sec, resulting in the peculiar units of Cd sec m-2 mR-1. The conversion degrades with time, and this ultimately can lead to the need for II replacement. 2. Brightness Gain: - Is the product of the electronic and minification gains of the II. - The electronic gain of an II is roughly about 50, and the minification gain changes depending on the size of the input phospore and the magnification mode. - As the effective diameter of the input phosphor decreases, the brightness gain decreases. Question: The brightness gain ranges from about 2,500 to 7,000, why?  Quantum Detection Efficiency: Figure 9-7 This figure compares the quantum detection performance as a function of x-ray beam energy (kV of a polyenergetic spectrum hardened by a 20-cm-thick patient). In an II system, an x-ray has to penetrate about 1.5 mm of aluminum before reaching the actual phosphor. A considerable fraction of incident photons will be attenuated, reducing QDE, especially at lower kV values. The flat panel detector system also uses a CsI phosphor, but only a carbon fiber cover is in the x- ray beam path, which acts as physical protection and as a light shield to the detector. Quantum detection efficiency (QDE): the fraction or percentage of the total radiation impinging on an image receptor that is actually detected by the receptor. Automatic Exposure Rate Control (AERC) Automatic Brightness Control (ABC): ◼ The purpose of ABC is to keep the brightness (SNR) of the image constant at the monitor. It does this by regulating the x-ray exposure rate incident on the patient. ◼ Fluoroscopic systems sense the light output from the II using a photodiode or the video signal itself. This control signal is used in a feedback circuit to the x-ray generator to regulate the x-ray exposure rate. ◼ For pulsed-fluoroscopy systems, the ABC circuitry may regulate the pulse width (the time) or pulse height (the mA) of the x-ray pulses. ◼ In practice, the mA and kV are increased together, but the curve that describes this can be designed to aggressively preserve subject contrast, or alternately to favor a lower dose examination. ◼ Some fluoroscopy systems have “low dose” or “high contrast” selections on the console, which select the different mA/kV curves. Automatic Brightness Control (ABC): Figure 9-8 AERC circuitry adjusts the x-ray output in response to the thickness of the patient. X-ray output can be increased for thick regions by increasing the mA, kV, or both. Increasing kV increases the dose rate but contrast is reduced, whereas increasing mA raises the dose rate but preserves contrast. Different AERC curves can be used on some systems (five are shown, typically fewer are available), which allow the user to select the low-dose or high-contrast AERC logic, depending on the clinical application. Automatic Brightness Control (ABC): Continuous Fluoroscopy Continuous fluoroscopy produces a continuous x-ray beam typically using 0.5 to 6 mA (depending on patient thickness and system gain). A camera displays the image at 30 FPS, so that each fluoroscopic frame is displayed for 33 ms (1/30 s). Variable Frame Rate Pulsed Fluoroscopy The x-ray generator produces a series of short x-ray pulses. The pulsed fluoroscopy system can generate 30 pulses/s, but each pulse can be very short in time. With pulsed fluoroscopy, the exposure time ranges from about 3 to 10 ms, which reduces blurring from patient motion (e.g., pulsatile vessels and cardiac motion) in the image. Therefore, in fluoroscopic procedures where object motion is high (e.g., positioning catheters in highly pulsatile vessels), pulsed fluoroscopy offers better image quality at the same average dose rates. Variable Frame Rate Pulsed Fluoroscopy Field of View/Magnification Modes - Magnification is produced by pushing a button that changes the voltage applied to electrodes in the II, and this results in in different electron focusing. - As the magnification factor increases, a smaller area on the input of the II is visualized. When the magnification mode engaged, the collimator also adjusts to narrow the x-ray beam to the smaller field of view Figure 9-10 No-magnification (left) and magnification (right) modes are illustrated. When the magnification mode is selected, the electrode voltages in the II are changed such that the electrons are focused to a smaller FOV. The motorized collimator blades also constrict the x-ray field to the smaller region. The AERC senses a loss of light immediately after switching to the magnification mode, and increases the exposure rate. Field of View/Magnification Modes (cont.) - The increase in the exposure rate is equal to the ratio of FOV areas. Example: A 30 cm (12-inch) II, which has 23-cm (9-inch) and 18-cm (7-inch) magnification modes. Switching from the 12-inch to the 9- inch mode will increase the x-ray exposure rate by a factor of (12/9)2 = 1.8. Question: what is the increase of the x-ray exposure rate by switching from the 12-inch to the 7-inch mode? The automatic brightness control circuitry compensates for the dimmer image by boosting the x-ray exposure rate. Therefore, as a matter of radiation safety, the fluoroscopist should use the largest field of view (the least magnification) that will facilitate the task at hand. Frame Averaging: Figure 9-11 A. This image shows the trade-off between temporal blurring and noise. Frame A integrates five frames, and there is minimal blurring of the moving central object, but the image is overall noisier. Frame B shows 10-frame blurring, with a considerably broader central object but lower noise in the static areas of the image. Frame C (20- frame averaging) shows dramatic blurring of the central, moving object but enjoys a very high SNR in the static regions of the image. D. This figure shows the influence of different recursive filters for temporal averaging. A parameter (α) setting of 1.0 results in no temporal filtering, and a noisy trace at this one (arbitrary) pixel is seen. As the value of α is lowered, the amount of temporal lag is increased and the trace becomes smoother. However, the rounded leading edges (at t = 1 and t = 40) show the increased lag that occurs in the signal. Spatial Resolution (MTF) in Fluoroscopy: ◼ The limiting spatial resolution of an imaging system is where the MTF approaches zero. ◼ The limiting resolution of modern IIs ranges between 4 and 5 cycles/mm in 23-cm mode ◼ Higher magnification modes (smaller fields of view) are capable of better resolution. Example: a 14-cm mode may have a limiting resolution up to 7 cycles/mm, why: Spatial Resolution (MTF) in Fluoroscopy: Contrast Resolution RADIATION DOSE: Entrance Skin Exposure (ESE) Figure 9-16 This figure shows the procedure for measuring the entrance dose rates on a fluoroscopy system. The exposure meter is positioned under the polymethyl methacrylate (PMMA) using blocks, and different thicknesses of PMMA are used to assess the dose rate over a range of magnification factors and operating modes (different fluoro schemes, cine, DSA, etc.) of the system. To determine the maximum tabletop exposure rate, a sheet of lead is positioned in the back of the PMMA—this drives the system to maximum output and allows the measurement of maximum exposure rates. RADIATION DOSE: Entrance Skin Exposure (ESE) ◼ The maximal legal entrance exposure rate for normal fluoroscopy to the patient is 10 R/min. For specially activated fluoroscopy, the maximum exposure rate is 20 R/min. ◼ Many methods can be employed during fluoroscopic procedures to reduce the patient dose. Dose to Personnel: ◼ As a rule of thumb, standing 1 m from the patient, the fluoroscopist receives from scattered radiation (on the outside of his or here apron) approximately 1/1,000 of the exposure incident upon the patient.  Radiation shields thyroid shield lead apron The basic tenets of radiation safety, time, distance, and shielding, lead eyeglasses Dose errors in Fluoroscopy can be very bad… Summary of operational factors that affect image quality and radiation dose to the patient and staff Summary of operational factors that affect image quality and radiation dose to the patient and staff