A2 PHYSICS: G485 - PIGUY MEDICAL IMAGING >>REVISION>> X - R AY S • created when high energy electrons from thermionic emission strike metal target & rapidly decelerate • Also when electrons make transitions between lower atomic energy levels I = I0 " differing attenuation coefficients • barium increases μ of soft tissue, allowing distinction of it e.g. intestines -μx Intensity is constant in collimated beam INTERACTION photoelectric effect • intensity is power per cross-sectional area • x-ray image occurs when tissues have Key I - intensity μ - attenuation coefficient x - distance INTENSITY e e compton effect e e pair production n ee+ • image intensifier contains a scintillator sodium iodide crystal, for every ɣ photon absorbed it emits thousands of visible light photons, so less exposure needed & response time is quicker IMAGE INTENSIFIER plastic coating double sided film metal backing intensifying screens DIAGNOSIS METHODS I COMPUTER IMAGING CAT x-ray slice PET ɣ detector 3D • medical tracers! 1. radioactive tracer e.g. technetium-99 is bonded to biologically active molecule e+ e- guy 2. injected and absorbed by organ detector 3. ɣ camera detects & builds 2d image 1. lead tubes absorb all photons except those travelling along the axis of tubes 2. sodium iodide crystal as scintillator absorbs a ɣ photon and emits thousands of visible light photons 3. photomultiplier tubes detect visible light, produces an electrical signal 4. image quality improved by narrower lead tubes or longer scanning time GAMMA CAMERA ɣ photolead multiplier dynodes NaI e- photocathode to computer • gamma camera ɣ detector DIAGNOSIS METHODS II MRI SCANNERS • non-invasive techniques (no surgery) x-rays CAT +ve -ve low exposure to x-ray; quick ionising radiation; can’t see tissues with similar attenuations differentiates between x-ray exposure v. high; soft tissues; scans takes ionising radiation seconds; high res. 3d PET high res. 3d; shows function of organs; ionising radiation; MRI no ionising radiation; high res. 3d slow (~45 minutes) no metal implants; can’t see functions ultrasound no ionising radiation; shows movement (real time images) lower res 2d; does not reflect off bone 1. nuclei with uneven no. of nucleons possess spin, act like tiny magnets B S N 2. they experience a torque and precess in a magnetic field 3. this occurs at the Larmor frequency 4.25x107 x B 4. by resonance, it absorbs energy if radio waves of Larmor frequency is received 5. so nuclei flip from low to high energy state & drop back down after time B B 6. relaxation time indicates tissue 7. gradient of magnetic field at all points varies Larmor frequency indicates position U LT R A S O U N D U LT R A S O U N D • ultrasound (>20kHz) is produced by the piezoelectric effect 1. alternating pd applied to piezoelectric crystal (quartz) causes it to oscillate. 2. alternating pd is at resonant frequency to maximise amplitude 3. epoxy resin backing prevents backwards propagation of ultrasound & rapidly damps oscillations after alternating pd stops Z= c 2 I r (Z Z ) __ = _______ 2 1 I0 (Z2 + Z1)2 ! A-scans are voltage peaks, where reflected ultrasound creates a p.d. by the piezoelectric effect ! B-scan is multiple Ascans for 2d image • ultrasound scanning 1. transducer is used to send pulses of ultrasound into the patient. Key f - frequency c - speed of ultrasound v - speed of blood towards source I - intensity z - acoustic impedance - density of tissue DOPPLER 2. pulses are reflected at tissue boundaries 3. intensity of the reflected pulse depends upon the acoustic impedance 4. delay time of the retuning pulse is used to determine the depth f ___ f’ = __ 2v 1- c Change of reflected ultrasound frequency indicated speed of the blood flow.