Fluoroscopy

fluoroscopy = continuous/near continuous low dose XR exposures (0.5-5mA)

  • major dose component, reduced by automatic addition of 0.2mm Cu filtration (but also increases tube loading)
  • region of interest fluoroscopy

fluorography = radioscopy = pulsed intense exposures (50-100mA); MTF limited at high lp/mm from optical coupling

ceiling-suspended or under table XRT/XII/C-arm or mobile C-arm systems

quantum sink = stage with worst SNR, corresponds to stage where XRs absorbed in detector

photofluorography = spot film photographic camera for recording static images onto large film (100-105mm)

cine-fluorography = cine camera recording onto roll of 35mm film

video camera converts image into electronic/video signal

  • video-fluoroscopy/screening fed to monitor for immediate display
  • digital fluorography fed to computer (DSI = digital spot imaging; DCI = digital cardiac imaging; DSA = digital subtraction angiography)

film-changer places cassette at XII input in radiography-fluoroscopy systems

X-Ray Image Intensifier (XII)

several thousand times more sensitive than standard screen-film

Design

may have antiscatter grid (reducing scatter to primary XR to 5-40%)

input window (thin C, Al or Ti)

input phosphor screen converts XR to light (CsI:Na)

thin transparent wafer (to prevent chemical interaction)

photocathode causes PE interactions with emission of electrons (Cs-Sb), curved to equalise path lengths of electrons

high electric field of 25-35kV in evacuated glass/stainless steel tube increases energy of electrons

electrostatic focussing lenses (G1, G2, G3 with positive potential) invert image and minimise distortion, direct electrons to anode

thin layer of Al to prevent light being reflected back to tube and removes spent electrons

output phosphor converts to green light with ~75% transmitted to window (P-20 phosphor = ZnCdS:Ag; thickness 5nm)

output window (lead glass or fibre-optic window)

glass envelope encased in lead-lined canister to protect from stray XRs

if XII used with cine or spot film camera, iris-type diaphragm introduced after collector lens to prevent overexposure of the film

second generation XII

CsI:Na as input phosphor (instead of P-20) with greater packing density, favourable K-edges for diagnostic XRs (higher QDE), lower conversion efficiency, grown in needle-like structure (with light preferentially emitted in direction of photocathode), superior spectral matching with photocathode, superior vacuum properties (minimising out-gassing at high T), fewer XR interacting with output phosphor (which would cause fog/veiling glare; due to higher QDE)

variable field size XIIs

up to 6 field sizes available with magnification controlled by altering voltage on G3 (increasing causes reduced effective input phosphor area), but decrease in minification gain and hence conversion factor, thus K must be increased if quantum mottle is to be maintained

the higher skin dose is partially controlled with proper collimation to match effective input phosphor area

Brightness Gain

conversion factor (Cd/m2/μGy/s) = ≈ 11.5-40

brightness gain = minification + flux gain; reduction with age due to radiation damage to output phosphor and loss of vacuum causing electron scattering

minification gain = reduction in size of image =

flux gain = increase in number of photons created between input and output phosphors (QDE of photocathode 5-10% with 25photons:1electron, acceleration causing 1electron:1000photons; 75% transmitted to window) ≈ 60

Image Quality

contrast ratio = ≈ 20-30:1; reduced by veiling glare

veiling glare = scattering at entrance window + scattering and lateral diffusion in input and output phosphors (dependent size of crystals and phosphor thickness) + electron scattering in body of XII + retrograde emission of light by output phosphor

XR dose required to satisfy quantum mottle limitations of detection on object contrast inverse of the contrast ratio squared (however patient scatter makes a more drastic contribution to noise)

MTF drops 5-15% at very low lp/mm due to veiling glare

crossover defocusing of electrons = space charge effects, higher at high XR intensities

resolution deteriorates at periphery of field of view

electron optics artefacts

  • spherical aberration (peripheral blurring)
  • astigmatism = perpendicular planes have different foci
  • coma = off-axis point sources not focussed together causing distortion
  • pincushion and barrel distortion from unequal focussing (but circularly symmetric) of electrons at periphery; barrel with reduced peripheral magnification, pincushion with more
  • S-distortion is circularly asymmetric focussing of electrons from stray magnetic fields
  • vignetting = non-uniform brightness of output image from unequal magnification thus unequal illumination (due to minification of brightness gain); with peripheral brightness reduced by 10-20%
  • chromatic aberration = electrons produced at photocathode have different kinetic energies and thus different focal points

Optical Coupling

fibre-optic coupling (compared to tandem lens systems) has reduced veiling glare (thus improved contrast), better light transmission, smaller size, minimal degradation of low spatial frequency MTF (15%2%), better spatial uniformity by avoiding spherical aberration effects

tandem lens system

collector lens has large diameter and f-number of 0.75, with output phosphor at focus to produce parallel beam of light

iris diaphragm increases effective fcoll and improves resolution and contrast; size of aperture altered to adjust speed

half/partially silvered mirror at 45° splits the light beam, with most reflected to strike camera lens and ~15% transmitted to video camera; may be automatically removed during screening to improve quality of fluoroscopic images

camera lenses have diameter ~1/2 of collector lens, with film placed at its focus; ; d1 = output phosphor size; d2 = film size

accuracy of lens placement is critical

digital imaging not as good as cne film resolution, but no longer requires beam splitting hence reduced dose

Closed Circuit Television/Video System

camera, control unit (amplifies video = voltage vs time signal) and monitor connected by cables

pixels arranged in scan lines; Australasian broadcast TV has 625lines/frame, frame rate 25Hz and frame period 40ms

Pick-Up Tube (PUT)/Photoconductive/Vidicon Video Camera

design

glass window (to maintain vacuum)

signal plate (thin transparent electrically conducting graphite with ~40V)

target (thin film photoconductive material when absorbs light emits electrons that escape matrix and swept out by anode potential leaving the element positively charged [hence increased conductivity; latent image]; globules of target suspended in mica matrix)

  • Hivicon/Vidicon Sb2S3 (antimony trisulphide; Saticon/Primicon SeAsTe (selenium arsenic tellurium), Plumbicon PbO, Chalnicon/Pasecon CdSe, Amorphous Silicon a-Si

anode (fine wire mesh at ~900V allowing most of electron beam to pass through)

vacuum tube (~15 x 3cm) surrounded by a focussing coil for steering the electron beam and more recently deflectrons (deflecting electrodes, with improved spatial resolution, uniformity, ruggedness, low weight, image rotation capabilities and ability to implement simple image distortion corrections)

cathode filament boils off electrons via thermionic emission

focussed readout electron beam accelerates to cathode while being steered vertically or horizontally by deflectrons and decelerates past anode before hitting target with very little energy and restores lost surface electron density of target to equilibrium value; thus a video signal induced at the signal plate

scanning

raster = pattern that camera target is read, from top left to bottom right in scan lines

frame = one scan of the entire target

retrace losses = time for electron beam to be switched to left for new scan line ≈7.2ms/8% of video signal (during which monitor electron beam is suppressed/blanked), sync pulse given to indicate start of video line

progressive scanning = each line scanned sequentially with field = frame rate (25Hz)

slow progressive scanning for fluorography

interlaced scanning = even lines read then odd lines in 2 separate fields to compete a frame (50Hz)

pulsed progressive readout = radiation pulse applied while electron beam blanked then 1st frame readout and 2nd frame of electron beam scrub frame to reduce residual signal

aspect ratio = height:width = 4:3; for high resolution video systems = 1:1

Semiconductor Video Camera = Charged Coupled Devices (CCDs)

advantages: compact, excellent temporal stability, high dynamic range (>4x), improved spatial resolution and negligible lag, not subject to drift associated with vacuum tubes, automatic self-calibration (corrects for pixel inhomogeneities and vignetting), homogenous spatial resolution, no time jitter

disadvantages: expensive and unable to operate in continuous mode compared to PUTs

light-sensitive semiconductor chip containing thousands of electronic sensors (photodiodes)

signal is read from each sensor element electronically with electronic shift registers; in the interline transfer technique signals transferred to vertical shift registers then horizontal shift registers to be read out one horizontal line at a time

large diameter a-Se video cameras can detect XRs directly dispensing need for XII (see flat panel receptors below)

Camera Control Unit

oscillator driver circuits to regulate deflectrons, focusing coils and synchronises video signal betw camera and monitor

AGC = automatic gain control = main video amplifier adjusts gain so video signal range 0 to 1V is fully utilised; also increases quantum mottle

Video signal

signal bandwidth/bandpass = frequency range that electronic components are able to transmit

max freq = 312.5lp/scan line x 625 scan lines x 25 frames/s = 4.88MHz + retrace losses = 5.25Mhz + aspect ratio issue

Automatic Brightness/Dose Control (ABC)

high subject contrast can lead to video signal saturation (reaching max value)

photocell/light detector between XII and camera to measure average brightness of output phosphor; or summed video signal over central circular region

feedback to XR generator to vary mA, pulse length then/or kVp, sluggish in old generators

can also feedback to adjust video voltage

built-in limitations for skin K <50/100mGy/min

proposed better model is to optimise SNR2/dose

Video Monitor

electron gun with surrounding control grid (regulates number of electrons in beam and hence pixel brightness, with negative charge = dark current)

focussing and deflecting coils (around the neck of the tube)

phosphor with surrounding anode (15kV, hence electrons energetic and produce many visible photons, with secondary electrons that are produced in the phosphor removed by the anode)

vacuum envelope

Image Quality

resolution

modern CsI:Na XII have resolution ~4-6lp/mm

horizontal resolution determined by number of scan lines and bandwidth (adjusted to give same resolution as vertical), finite size of readout beam and lateral diffusion of charge in the target

vertical resolution less than theoretical due to aliasing (lp betw scan lines) hence 2n video lines needed for n line pairs

  • Kell factor = ; actual requirement of 1.25 x 2n lines (Kell factor = 0.8)
  • resolution ~92% less due to retrace requirements

standard television systems limit overall resolution with CsI XIIs

contrast

log (I/I0) = γlog (L/L0); I = signal current; L = relative light exposure; γ = gamma

Vidicon tubes limited by large dark current (high I0), newer PUTs have γ close to unity

monitor enhances contrast by up to 2x

lag/temporal resolution

image persistence causes blurring with movement, but averages statistical fluctuations and minimises quantum mottle

reduced with pulse progressive readout

noise

shot noise = variations in electron beam current (from thermal emission) distributed in Poisson distribution and dependent on signal current

amplification process adds noise

SNR (signal to noise ratio) in dB = 20 (log10SNR)

DSA requirements

cameras must have low noise, low lag, excellent spatial resolution (1024 x 1024 matrices), good contrast, SNR 500-1000, γ close to unity, minimal lag (hence read in pulsed progressive mode)

Image Recorders

Spot Film Cameras

uses lens/mirror combination onto roll of 70/105mm or cut 100mm film at 1-12frames/s; resolution limited by XII

advantages (over conventional film): reduced exposure, short exposure times, ease of operation (no cassettes), continuous monitoring (via video chain), less film usage (hence lower cost)

disadvantages: limited FOV, small film size, inferior resolution

photofluorography = mass miniature radiography using 70/90/105mm roll or 100mm cut film not recommended due to high dose compared with conventional radiography

Cine Fluorography

recording of images onto 35mm (18 x 24mm) movie/cine film

camera lens

aperture (defines size of image)

shutter rotates in front of aperture to cut of light during film advance processes from afterglow (phosphor decay constant of 0.4ms; CsI:Na <1μms)

pressure plate holds film against aperture

electric motor with supply and take-up reels advances film

XR pulses (to reduce dose) and shutter are synchronised by signal from drive motor at 25-60/2

generator must be 3-phase 12-pulse or constant potential with high current and short pulse widths (fast switching from secondary triode/tetrode or grid control)

tube must have high thermal capacity and instantaneous ratings (metal/ceramic or liquid metal bearings)

ABC may be able to be performed within the one cine pulse; preset kVp (at optimum contrast) and mA (max) with varying pulse width (can be altered quickly)

short pulse widths reduce motion unsharpness (from contrast or patient movement)

Flat Panel Receptors

scintillator (CsI)

TFT = thin film transistor array (with a photodiode for each pixel detector element, convert light > electron)

can adjust pixel size by combining adjacent pixels for fluoroscopy

cone-beam CT – rotation of receptor and XRT with reconstruction to produce CT images

Fluoroscopy Safety

general

QA and QC programmes to ensure equipment in good working order, optimal XII system

ensure correct pt, pt prepared, test is required with no other alternatives, not pregnant

XRT

increased filtration, ROI filters

highest kVp, lowest mAs

pulsed fluoroscopy

minimise fluorography or boost mode

geometry and beam

best geometry (large SID, far XRT-pt)

move field around by changing beam angle or rotate XRT/XII 180° if procedure becomes prolonged

minimise attenuation between pt and XII

remove grid (if pt small) or when using air-gap

XII

largest FOV, most collimation, minimised use of magnification

viewing

use LIH, image browsing, key image save

ensure ideal viewing conditions (dimmed lights)

ensure DICOM monitor standard

increase XII gain and signal amplification for tolerable level of noise

reduce not-essential conversations and distractions

ensure good communication with staff and patient so they know what to do and when

5 min cumulative time and DAP meter with warning levels

reducing dose to staff

remove staff not required from room, esp during fluorography

ensure adequate shielding (leaded apron 0.25-0.5mm Pb, thyroid shields, glasses, gloves, needle holders, drapes, mobile/fixed shields)

maximize distance from patient (inverse square law)

under-couch XRT and XRT on side away from staff