Digital Fluoroscopy.pdf

Radiographic Instrumentation II RXD23002 Digital Fluoroscopy By : Yussri Jemenin Digital Fluoroscopy Flat panel replace Image Intensifier ❖ Flat panel devices are thin film transistor (...

Radiographic Instrumentation II RXD23002 Digital Fluoroscopy By : Yussri Jemenin Digital Fluoroscopy Flat panel replace Image Intensifier ❖ 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. Flat Panel Digital Fluoroscopy FROM GE  FLAT PANEL-LIGHT SENSOR scan line FET Very Pitch High Fill Factor Fill Factor= Sensitive Area Pitch x Pitch FROM GE Pitch data line  Fluoroscopy analog & digital Analogue Screen/film Computed CR Plates radiography X-Ray Image CCD Capture A-Si Indirect image detection (amorphous (phosphor) silicon) Digital A-Se Integration (amorphous Digital selenium) detector systems Direct Direct image photon Si wafer detection counting (no phosphor) Adapted from 6 KCARE UK Type of detector  Type of detector 1) Indirect detection systems the X-rays are absorbed in a phosphor layer and produce light photons which convert into electric charges in a photo detector and then to an electric signal Imaging Plates based on photostimulable phosphors CsI(Tl) – aSi Flat Panels Phosphor screen +OF taper+ CCD array (slot scanning) 2) Direct detection systems the X-ray photons directly convert into charges (electron-hole pairs) and thus to an electric signal in a photoconductor aSe Flat Panels 3) Direct photon counting systems single photons are counted, i.e. the number of photons directly represent the intensity level in a pixel Sectra Microdose Mammography (MDM) based on Si detectors Xcounter based on gas avalanche photodiodes  Indirect image detection CCD systems CCD systems use a scintillator like gadolinium disulphide to convert x-rays to visible light Light is collected by optics to demagnify the 35x45cm2 film to 2-4 cm2 CCD essentially visible light is converted into charge that is amplified and readout A negative is the thickness of the detector system because of the optical system  a-Si Indirect Detectors CsI-scintillator with a-Si switching diodes or TFT-read out 100 m pixel size X-rays giving ~5 lp/mm scintillator Used by GE and others Pixel matrix amplifier, multiplexer & contacts line driver photo diode ADC switch  Indirect detection Also called FPD (flat panel detectors) The idea is that charge proportional to the x-rays received is stored on a capacitor The charges are conducted out by transistors one row at a time and subsequently amplified, multiplexed, and digitized The readout is very fast Direct Detection : a-Se  Direct image detection Indirect or direct conversion thin-film transistor (TFT) arrays Indirect conversion uses a scintillator layer (like CsI:Tl) to convert x-rays to visible light and amorphous silicon photodiodes to convert visible light into charge Direct conversion uses an x-ray photoconductor layer (usually amorphous selenium) to convert x-rays to charge An applied electric field directs the charges to the charge collection electrodes  Direct image detection ❖ 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%  Fluoroscopy Modes of Operation ❖ 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  Fluoroscopy Modes of Operation ❖ 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  Fluoroscopy Modes of Operation ❖ 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  Digital Image Quality Gray Levels at a constant 512 x 512 Gray Levels at a constant 512 x 512 matrix size. matrix size. 8 Grey Levels (3 bits) 4 Grey Levels (2 bits)  Digital Image Quality Effect of Matrix Size. Effect of Matrix Size. 128 x 128 matrix 64 x 64 matrix  Digital Image Quality Effect of Matrix Size. Effect of Matrix Size. 512 x 512 matrix 256 x 256 matrix