CAT scans, gamma cameras and PET scans

why are CAT scans better than x-rays?

• CAT scans create 3D images but x-rays only do 2D ones, so CAT scans give a better idea of size, shape and position of tumour so easier to treat

• CAT scans can also see cross-sections of tumour

• CAT scans can differentiate between materials of similar densities and attenuation coefficicents but x-rays can’t

why are CAT scans not better than X-rays?

• CAT scans are longer so patient receives more ionising radiation and will be more uncomfortable

• CAT could damage their cells more because they receive several years of background radiation

• patient has to stay still for a long period of time in CAT scans

what does CAT stand for?

• computerised axial tomography

• it records a large number of 2D x-ray images from different angles and assembles them into a 3D image using software or something

• the computer controls the scanning process and analysis of electrical signals

how does a CAT scan work?

• patient lies on back in a horizontal table that can slide in/out of gantry

• gantry has x-ray tube on one side and an array of electronic x-ray detectors on opposite side (both rotate)

• the x-ray tube produces a fan-shaped beam of x-rays 1-10mm thick

• the detectors record the intensity of the transmitted x-rays and send electrical signals to a computer

• every time 360 rotation, a two-d image or slice is acquired and the table moves 1cm through the ring, then the beam irradiates the next slice of the patients body

• radiographer can view each 2D slice and make a 3D image

what is the purpose of a gamma camera?

• detects gamma radiation (high energy photons) emitted by tracers and turns it into an image

• shows the function and processes of the body instead of just anatomy

• can scan brain/lungs/liver etc

• can also kill tumour cells when it is emitted from multiple sources and concentrated on a target

• minimises damage to healthy cells and max damage to tumour

what factors are taken into account when choosing a tracer?

• whether the tracer is going to travel through the digestive system or circulate in blood

• can be drunk/eaten/breathed in

• if a certain tissue needs to be targeted, radioisotopes can be combined with other elements to achieve specificity

• should have a short half life to minimise the amount of radiation that the patient receives

• must be gamma emitter because gamma is weakly ionising and can travel through bones to the detectors

facts about fluorine-18 or something

• is used in PET scans

• has a short half life and has to be produced on site in hospital using particle accelerator

• 110 min half life

• beta plus decay

• decays into oxygen-18 nucleus, positron, neutrino and gamma photon

• can be made by high-speed protons colliding with oxygen-18 nuclei

who or what is technetium-99m?

• the m means metastable apparently (it stays in a high energy state for a long period)

• has a 6 hour half life

• instead of storing technetium, hospitals store molybdenum-99 because it decays into technetium

• beta minus decay

• its photons have 140keV

• can be combined with Na and O₂ to make NaTcO₄ which can target braincells

how does the gamma camera turn gamma photons into images?

• photons travel towards collinator (honeycomb of long thin lead tubes)

• collinator only lets gamma photons through along the axis of the tubes

• the photons then reach the scintillator crystal (NaI)

• when one gamma photon strikes the scintillator thousands of visible light photons are produced

• 10% of photon energy converted to light energy

• visible light photons travel into photomultiplier tubes, in hexagonal pattern

• for each light photon that hits the cathode, electron is released through photoelectric effect

• electron is accelerated repeatedly towards anodes, releasing 2-4 electrons each time

• repeats until electrical pulse formed, representing 1 pixel

• software is used to process electrical signals and determine where the tracer decayed or something which produces a high quality image showing concentrations

what happens in a PET scan?

• tracer injected into patient

• the patient lies on a table inside a ring of gamma detectors

• the tracer decays and emits beta plus positrons

• inside the patient, each positron collides with and annihilates a nearby electron

• this causes two gamma photons to be emitted in opposite directions to conserve momentum

• detectors on opposite sides of the ring detect the gamma photons

• the computer identifies the precise point of the origin of decay by calculating the time difference between the detections of the 2 gamma photons

• repeated many times to make 2D slices

• software is used to construct a 3D image of the tracer’s location

• tracer density can be determined from rate of gamma photons emitted in each region

what tracers are used in PET scans?

• fluorodeoxyglucose

  • (it’s like glucose but it has a fluorine-18 atom in it somewhere instead of oxygen)

  • accumulates in tissues with a high rate of respiration because the body treats it like glucose

• carbon monoxide

  • made from carbon-11

  • emits positron and has half life of 20 minutes

  • good at clinging to haemoglobin in RBCs so can be transported through the blood

what are the advantages of PET scans?

• non-invasive

• diagnoses cancer

• observes organ function

• produces 3D image of processes in body

• assess effects of new meds

what are the disadvantages of PET scans?

• very expensive because producing radioactive tracers on site is hard

• only found in larger hospitals

• only used for patients with complex health problems