BRAINCAKEMedical Imaging
Revision notes for Medical Imaging in OCR Physics A (H556). Read the start of any lesson here, then sign in free for the whole lesson, the R.E.C.I.P.E. recall steps and the quiz.
- Production of X-raysX-rays are high-energy electromagnetic photons.
- X-ray TubesAn X-ray tube is an evacuated glass tube containing a cathode and an anode. • Cathode (heater filament): a current heats the filament and electrons…
- X-ray Attenuation MechanismsWhen a beam of X-rays passes through matter, its intensity falls.
- Photoelectric Effect, Compton Scattering and Pair ProductionThree of the attenuation mechanisms change or destroy the X-ray photon.
- Attenuation of X-raysAs a parallel X-ray beam passes through a material, its intensity decreases exponentially with thickness: I = I0 e−μx I0 is the incident intensity…
- Contrast MediaSoft tissues such as the stomach, intestines and blood vessels have attenuation coefficients very close to those of the tissue around them.
- CAT ScanningA CAT scan (computerised axial tomography, also called a CT scan) uses X-rays to build up images of thin slices through the body.
- Medical TracersA medical tracer is a compound labelled with a radioactive nuclide.
- Technetium-99m and Fluorine-18Two tracers are named in the OCR specification.
- The Gamma CameraA gamma camera detects the gamma photons emitted by a tracer inside the patient and forms an image of where the tracer is.
- Diagnosis with a Gamma CameraFor a gamma camera scan the patient is given a tracer, usually technetium-99m bound to a compound that the organ being studied takes up.
- PET ScanningPET (positron emission tomography) uses a positron-emitting tracer, usually fluorine-18 in FDG, to make 3D images of active tissue such as tumours…
- UltrasoundUltrasound is sound with a frequency above 20 kHz, above the range of human hearing.
- The Piezoelectric EffectUltrasound for medical imaging is produced and detected using the piezoelectric effect.
- Ultrasound TransducersAn ultrasound transducer uses a piezoelectric crystal to both transmit ultrasound and receive the echoes.
- A-scans and B-scansUltrasound scans use the time taken for echoes to return from boundaries inside the body.
- Acoustic ImpedanceHow much ultrasound is reflected at a boundary depends on a property of each medium called its acoustic impedance Z.
- Reflection at BoundariesWhen ultrasound travels from a medium of acoustic impedance Z1 into one of Z2, part of it is reflected at the boundary.
- Impedance Matching and Coupling GelAt an air-skin boundary about 99.9% of the ultrasound intensity is reflected, because the acoustic impedance of air is tiny compared with that of…
- Doppler ImagingDoppler imaging uses ultrasound to measure the speed of blood flow, for example to find narrowed arteries or check blood flow through the heart and…