Fluoroscopy Robert Metzger, Ph.D.

Real-Time Imaging Fluoroscopy is an imaging procedure that allows real-time x-ray viewing of the patient with high temporal resolution Use TV technology, which provides 30 frames per second imaging Allows acquisition of a real-time digital sequence of images (digital video), that can be played back as a movie loop Cine cameras offer up to 120 frame per second acquisition rates using 35-mm cine film. Digital cine also available

Fluoroscope Imaging Chain c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 232.

The Image Intensifier There are 4 principal components of an II: (a) a vacuum bottle to keep the air out (b) an input layer that converts the x-ray signal to electrons (c) electronic lenses that focus the electrons, and (d) an output phosphor that converts the accelerated electrons into visible light c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

The Image Intensifier X-RAYS CCD TV CAMERA MIRROR ADC LENS OUTPUT PHOSPHOR APERTURE FOCUSING ELECTRODES DISPLAY ELECTRONS PHOTO-CATHODE LAYER INPUT PHOSPHOR ...CsI X-RAYS

The Input Screen The input screen of the II consists of 4 different layers: (a) vacuum window, a 1 mm aluminum window that is part of the vacuum bottle keeps the air out of the II, and its curvature is designed to withstand the force of the air pressing against it a vacuum is necessary in all devices in which electrons are accelerated across open space c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

The Input Screen The input screen of the II consists of 4 different layers: (b) support layer, which is strong enough to support the input phosphor and photocathode layers, but thin enough to allow most x-rays to pass through it 0.5 mm of aluminum, is the first component in the electronic lens system, and its curvature is designed for accurate electronic focusing c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

The Input Screen The input screen of the II consists of 4 different layers: (c) input phosphor, whose function is to absorb the x-rays and convert their energy into visible light cesium iodide (CsI) is used long, needle-like crystals which function as light pipes, channeling the visible light toward the photochathode with minimal lateral spreading 400 mm tall, 5 mm in diameter c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

The Input Screen The input screen of the II consists of 4 different layers: (d) photocathode is a thin layer of antimony and alkali metals that emits electrons when struck by visible light 10 to 20% conversion efficiency 23 to 35 cm diameter input image (FOV) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 233.

Input Phosphor Energy Conversion Aluminum Support Photocathode CsI Needles Figure courtesy from Jonathan Tucker, Brooke Army Medical Center, SA, TX

Input Phosphor Energy Conversion 60 keV X-Ray Aluminum Support Photocathode Figure courtesy from Jonathan Tucker, Brooke Army Medical Center, SA, TX CsI Needles

Input Phosphor Energy Conversion Aluminum Support 3,000 light photons  = ~ 420 nm Photocathode Figure courtesy from Jonathan Tucker, Brooke Army Medical Center, SA, TX CsI Needles

Input Phosphor Energy Conversion Aluminum Support Photocathode ~ 400 electrons To Anode CsI Needles Figure courtesy from Jonathan Tucker, Brooke Army Medical Center, SA, TX

Electron Optics Electrons are accelerated by an electric field Energy of each electron is substantially increased and this gives rise to electron gain Focusing is achieved using an electronic lens, which requires the input screen to be a curved surface, and this results in unavoidable pincushion distortion of the image c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

Electron Optics The G1, G2, G3 electrodes along with the input screen and the anode near the output phosphor comprise the five-component electronic lens system of the II The electrons under the influence of the 25K to 35K V electric field, are accelerated and arrive at the anode with high velocity and considerable kinetic energy After penetrating the very thin anode, the energetic electrons strike the output phosphor c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

The Output Phosphor The output phosphor is made of zinc cadmium sulfide Anode is very thin coating of aluminum on the vacuum side of the output phosphor, which is electrically conductive to carry away the electrons once they deposit their energy in the phosphor Each electron causes the emission of approximately 1000 light photons from the output phosphor 2.5 cm diameter output phosphor c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

The Output Phosphor The reduction in image diameter leads to amplification (analogy: magnifying glass and sunlight) Minification gain of an II is simply the ratio of the area of the input phosphor to that of the output phosphor, e.g., 9’’ input phosphor, 1’ output phosphor, area is square of the diameter ratio, minification gain is 81 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

The Output Phosphor The output phosphor is coated right onto the output window Some fraction of the light emitted by the output phosphor is reflected at the glass window Light bouncing around the output window is called veiling glare, and can reduce image contrast c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 235.

Image Intensifier Performance Conversion Factor Light out of image intensifier (cd/m2) Conversion Factor = Exposure rate into image intensifier (mR/sec) Defined as a measure of the gain of an image intensifier ratio of light output to exposure rate input 100 to 200 for new image intensifier Degrades over time, ultimately can lead to II replacement

Image Intensifier Performance Brightness Gain BG = minification gain x electronic gain (flux gain) Minification gain = increase in image brightness that results from reduction in image size from the input phosphor to output phosphor size (di/do)2, di is input diameter which varies, do is output diameter typically 2.5 cm For 30 cm (12”) II, minification gain = 144

Image Intensifier Performance Brightness Gain BG = minification gain x electronic gain (flux gain) Electronic gain or flux gain is typically 50 The brightness gain therefore ranges from about 2,500 – 7,000 As the effective diameter of the input phosphor decreases (magnification increases), the brightness gain decreases

Field of View/Magnification FOV specifies the size of the input phosphor of the image intensifier Different sizes: 23 cm (9”), 30 cm (12”), 35 cm (14”), 40 cm (16”) FOV Magnification is accomplished electronically using electronic focusing that projects part of the input layer onto the output phosphor Since brightness gain decreases in mag. mode, the x-ray exposure rate is boosted. (12/9)2 = 1.8, (12/7)2 = 2.9 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 237.

ALL OF INPUT SURFACE USED TO GATHER X-RAYS NON-MAGNIFY MODE OF I.I. OUTPUT IMAGE ALL OF INPUT SURFACE USED TO GATHER X-RAYS

MAGNIFIED MODE OF IMAGE INTENSIFIER OUTPUT IMAGE LESS IMAGED ANATOMY IS EXPANDED OVER THE SAME OUTPUT SURFACE AND LOOKS MAGNIFIED ONLY A PORTION OF INPUT SURFACE USED TO GATHER X-RAYS

LARGE NON-MAG FoV e.g., 12 INCH SMALL, MAG FoV e.g., 6 INCH Magnification

Pincushion Distortion

Optical Coupling Parallel rays of light enter the optical chamber, are focused by lenses, and strike the video camera where an electronic image in produced A partially silvered mirror is used to shunt the light emitted by the image intensifier to an accessory port c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 239.

Video Camera c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 240.

Video Camera Analog video systems typically have 30 frames/sec operation, but they work in an interlaced fashion to reduce flicker, the perception of the image flashing on and off The human eye-brain system can detect temporal fluctuations slower than about 47 images/sec, and therefore at 30 frames/sec flicker would be perceptible With interlaced systems, each frame is composed of two fields and each field is refreshed at a rate of 60 times per second, which is fast enough to avoid perception of flicker

Lag Lag means that each new TV image actually contains residual image information from the last several frames Lag is good and bad Lag acts to smooth the quantum noise in the image, but can also cause motion blurring

Lag Effect of camera lag. Angiogram of a rapidly moving coronary artery shows a trailing "ghost" due to excessive camera lag (the direction of travel is from right to left).

Video Resolution Spatial resolution of a video in the vertical direction (top to bottom) of the TV image is governed by the number of scan lines By convention, 525 lines are used in N. America for TV 490 lines usable In the early days of TV, a man named Kell determined that about 70% of theoretical video resolution is appreciated visually, and this psychophysical effect is now called the Kell factor 490 x 0.7 = 343 lines or 172 line pairs useful for resolution For 9” field, resolution = 172 lp/229 mm = 0.75 lp/mm 17 cm or 7” field, resolution is 1.0 lp/mm 12 cm or 5” field, resolution is 1.4 lp/mm

Video Resolution The horizontal resolution is determined by how fast the video electronics can respond to changes in light intensity This is influenced by the camera, the cable, the monitor but the horizontal resolution is governed by the bandwidth of the system The time necessary to scan each video line (525 lines at 30 frame/sec) is 63 msec 11 msec required for horizontal retrace, 52 msec available To achieve 172 cycles in 52 msec, the bandwidth required is 172 cycles/52 x 10-6 sec = 3.3 x 106 cycles/sec = 3.3 MHz Higher bandwidths are required for high-line video systems

TV LINES ARE COMPOSED OF DOTS TELEVISION IMAGE HORIZONTAL DIRECTION RASTER LINE VERTICAL DIRECTION TV LINES ARE COMPOSED OF DOTS

INTERLACED SCANS

INTERLACED SCANS

TYPICAL MEASURED RESOLUTION [ 1023 LINE T.V. ] FoV T.V. I.I. [CINE] 9 INCH 1.8-2.2 LP/mm 2.7-3.2 LP/mm 6 INCH 2.5-2.8 LP/mm 3.7-4.5 LP/mm 4.5 INCH 3.2-3.7 LP/mm 5.0-6.0 LP/mm

Summary Fluoroscopy is a live imaging procedure Image Intensifier main component and consists of the input phosphor, electronic lens system and output phosphor Input phosphor – Cesium Iodide, converts x-rays to light Photocathode – converts light into electrons Output phosphor – Zinc cadmium sulphide, converts electrons into light Artifacts – pincushion distortion, veiling glare, lag Brightness gain = minification gain x electronic (flux) gain Several magnification modes available, typically exposure rate increases with magnification Video camera produces the electronic image which we see on the TV monitor Use interlaced scanning to avoid flicker Horizontal (determined by bandwidth) and vertical (determined by the number of scan lines) video resolution

Flat Panel Digital Fluoroscopy Flat panel devices are thin film transistor (TFT) arrays that are rectangular in format and are used as x-ray detectors CsI, a scintillator is used to convert the incident x-ray beam into light TFT systems have a photodiode at each detector element which converts light energy to an electronic signal Flat panel detectors would replace the image intensifier, video camera, and other peripheral devices c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 242.

DIAGRAM OF GE FLAT PANEL IMAGE DETECTORS FROM GE

FLAT PANEL-LIGHT SENSOR scan line FET Pitch Very High Fill Factor Sensitive Area Pitch x Pitch Fill Factor= data line FROM GE Pitch

Peripheral Equipment Photo-spot camera used to generate images on photographic film, 100-mm cut film or 105-mm roll film full resolution of the II system, hardly seen nowadays Digital photo-spot high resolution, slow-scan TV cameras in which the TV signal is digitized and stored in computer memory Or CCD cameras with 10242 or 20482 pixel formats near-instantaneous viewing of the image on a video monitor allows the fluoroscopist to put together a number of images to demonstrate the anatomy important to the diagnosis digital images can be printed on a laser imager

Peripheral Equipment Spot-film devices attaches to the front of the II, and produces conventional radiographic screen-film images better resolution than images produced by II Cine-radiography cameras attaches to a port and can record a very rapid sequence of images on 35-mm film used in cardiac studies, 30 frames/sec to 120 frames/sec or higher uses very short radiographic pulses digital cine are typically CCD-based cameras that produce a rapid sequence of digital images instead of film sequence

Fluoroscopy Modes of Operation Continuous fluoroscopy continuously on x-ray beam, 0.5 – 4 mA or higher display at 30 frames/sec, 33 msec/frame acquisition time blurring present due to patient motion, acceptable 10 R/min is the maximum legal limit High dose rate fluoroscopy specially activated fluoroscopy 20 R/min is the maximum legal limit audible signal required to sound used for obese patients

Fluoroscopy Modes of Operation Pulsed fluoro: series of short x-ray pulses, 30 pulses at ~10 msec per pulse exposure time is shorter, reduces blurring from patient motion Can be used where object motion is high, e.g., positioning catheters in highly pulsatile vessels 15 frames/sec, 7.5 frames/sec also available Variable frame pulsed fluoroscopy is instrumental in reducing dose Ex., initially guiding the catheter up from the femoral artery to the aortic arch does not require high temporal resolution and 7.5 frames/sec could potentially be used instead of 30 frames/sec 7.5 frames/sec instead of 30 frames/sec, dose savings of (7.5/30) 25%

Frame Averaging Fluoroscopy systems provide excellent temporal resolution However, fluoroscopy images are relatively noisy, and in some applications it is beneficial to compromise temporal resolution for lower noise images This can be achieved by averaging a series of images or frames Real-time averaging in the computer memory for display Can cause noticeable image lag but noise in image is reduced as well Could also reduce dose in some circumstances c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 245.

Last Frame Hold Last-frame hold when the fluoroscopist takes his or her foot off the fluoroscopy pedal, rather than seeing a blank monitor, last-frame-hold enables the last live image to be shown continuously useful at training institutions no unnecessary radiation used on patient

Road-Mapping Road Mapping software-enhanced variant of the last-frame-hold feature side-by-side video monitors, one shows captured image, the other live image In angiography, subtracted image can be overlayed over live image to give the angiographer a vascular “road map” right on the fluoroscopy image is useful for advancing catheters through tortuous vessels

Automatic Brightness Control The purpose of the automatic brightness control (ABC) is to keep the brightness of the image constant at monitor It does this by regulating the x-ray exposure rate (control kVp, mA or both) Automatic brightness control triggers with changing patient size and field modes

Automatic Brightness Control The top curve increases mA more rapidly than kV as a function of patient thickness, and preserves subject contrast at the expense of higher dose The bottom curve increases kV more rapidly than mA with increasing patient thickness, and results in lower dose, but lower contrast as well c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 247.

Image Quality Spatial Resolution A 2D image really has 3 dimensions: height, width, and gray scale Height and width are spatial and have units such as millimeters The classic notion of spatial resolution is the ability of an image system to distinctly depict two objects as they become smaller and closer together The closer together they are, with the image still showing them as separate objects, the better the spatial resolution At some point, the two objects become so close that they appear as one, and at this point, spatial resolution is lost

Image Quality Spatial Resolution The spatial domain simply refers to the two spatial dimensions of an image, width (x-dimension) and length (y-dimension) Another useful way to express the resolution of an imaging system is to make use of the spatial frequency domain F (line pairs/mm or cycles/mm) =1/2, where  is the size of the object (mm) Smaller objects (small ) correspond to higher spatial frequencies and larger objects (large ) correspond to lower spatial frequencies So, objects that are 0.36 mm correspond to 1.4 lp/mm 0.19 mm corresponds to 2.7 lp/mm 1 mm correspond to 0.5 lp/mm

Image Quality: Spatial Resolution Spatial frequency is just another way of thinking of object size A device used to measure the spatial resolution is the bar pattern c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 249.

Image Quality Spatial Resolution The modulation transfer function, MTF of an image system is a very complete description of the resolution properties of an imaging system The MTF illustrates the fraction (or percentage) of an object’s contrast that is recorded by the imaging system, as a function of the size (i.e., spatial frequency) of the object The limiting spatial resolution is the size of the smallest object that an imaging system can resolve The limiting resolution of modern image intensifiers is between 4 and 5 cycles/mm c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 248.

Image Quality Contrast Resolution The ability to detect a low-contrast object on an image is highly related to how much noise (quantum noise and otherwise) there is in the image The ability to visualize low-contrast objects is the essence of contrast resolution. Better contrast resolution implies that more subtle objects can be routinely seen on the image The contrast resolution of fluoroscopy is low by comparison to radiography, because the low exposure levels produce images with relatively low signal-to-noise ratio (SNR)

Image Quality Contrast Resolution Contrast resolution is increased when higher exposure rates are used, but the disadvantage is more radiation dose to the patient Fluoroscopic systems with different dose settings allow the user flexibility from patient to patient to adjust the compromise between contrast resolution and patient exposure

Noise and Contrast Comparison of x-ray noise amplitudes in coronary angiograms acquired at fluoroscopic (2 µR per frame) (a) and angiographic (16 µR per frame) (b) exposure levels.

Noise and Contrast 16 µR per frame. Note improved resolution and contrast due to the higher exposure.

Digital Image Quality Effect of Matrix Size. 512 x 512 matrix

Digital Image Quality Effect of Matrix Size. 256 x 256 matrix

Digital Image Quality Effect of Matrix Size. 128 x 128 matrix

Digital Image Quality Effect of Matrix Size. 64 x 64 matrix

Digital Image Quality Gray Levels at a constant 512 x 512 matrix size. 256 Grey Levels (8 bit)

Digital Image Quality Gray Levels at a constant 512 x 512 matrix size. 4 Grey Levels (2 bits)

Digital Image Quality Gray Levels at a constant 512 x 512 matrix size. 8 Grey Levels (3 bits)

Image Quality Temporal Resolution Fluoroscopy has excellent temporal resolution, that is over time Blurring in the time domain is typically called image lag Lag implies that a fraction of the image data from one frame carries over into the next frame Video cameras such as the vidicon demonstrate a fair amount of lag

Image Quality Temporal Resolution Lag in general is undesirable, beneficial for DSA Frame averaging improves contrast resolution at the expense of temporal resolution With DSA and digital cine, cameras with low-lag performance (plumbicons or CCD cameras) are used to maintain temporal resolution

Fluoroscopy Suites Gastrointestinal Suites R and F room, large table that can be rotated from horizontal to vertical to put the patient in a head-down or head-up position II above or under the table, spot film device usually there Remote Fluoroscopy Rooms Designed for remote operation by the radiologist Tube above table, II under table Reduce dose to the physician and no lead apron needed c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 250.

Fluoroscopy Suites Peripheral Angiography Suites Table floats, allows patient to be moved from side to side and head to toe C-arm or U-arm configuration 30 to 40 cm image intensifier used Power injectors are normally ceiling- or table-mounted Cardiology Catheterization Suite Similar to angiography suite, 23 cm II used to permit more tilt in cranial caudal direction Cine cameras used, biplane rooms common c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 250.

Fluoroscopy Suites Biplane Angiographic Systems Two complete x-ray tube/II systems used, PA and Lateral Simultaneous acquisition of 2 views allows a reduction of the volume of contrast media injected in patient Portable Fluoroscopy- C Arms C-Arm devices with an x-ray tube placed opposite from the II 18-cm (7-inch) and 23-cm (9-inch) and several other field sizes available Operating rooms and ICUs c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 250.

Radiation Dose Patient Dose The maximum exposure rate permitted in the US is governed by the Code of Federal Regulations (CFR), and is overseen by the Center for Devices and Radiological Health (CDRH), a branch of the Food and Drug Administration (FDA) The maximum legal entrance exposure rate for normal fluoroscopy to the patient is 10 R/min For specially activated fluoroscopy, the maximum exposure rate allowable is 20 R/min

Radiation Dose Patient Dose Typical entrance exposure rates for fluoroscopic imaging are About 1 to 2 R/min for thin (10-cm) body parts 3 to 5 R/min for the average patient 8 to 10 R/min for the heavy patient Maximum dose at 120 kVp for most vendors c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 252.

Dose to Personnel Rule of Thumb: standing 1 m from the patient, the fluoroscopist receives from scattered radiation (on the outside of apron) approximately 1/1,000 of the exposure incident upon the patient The scatter field incident upon the radiologist while performing a fluoroscopic procedure is shown A radiologist of average height, 178 cm (5’10”) is shown overlaid on the graph and key anatomic levels are indicated c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 253.

Dose to Personnel The dose rate as a function of height above the floor in the room is shown for 6 different distances D, representing the distance between the edge of the patient and the radiologist 80 kVp beam and 20 cm patient thickness assumed for calculation Roentgen-area product (RAP) or dose-area product (DAP) meters can be used to provide real-time estimate of the amount of radiation the patient has received c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 253.

Additional Reading Additional topics on digital fluoroscopy and digital subtraction angiography can be found at the RSNA Education Portal. http://www.rsna.org/education/archive/aapm/toc.html#fluoroscopy