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Imaging

X-ray

  • Stream of photons emitted from a radiation source
  • Photons are called x-rays if they are produced by electron interactions
  • Photons strike tissue with varying properties and impart energy to tissue
  • Photons emerge from patient with varying amounts of energy and strike image intensifier, producing image

 

CT

  • Uses x-rays to build cross sectional images which are reconstructed off the same principles as x-rays by photons passing through tissue and imparting different amounts of energy
  • CT ‘tube’ has x-ray emitter and detector on opposite sites, which rotate around patient constantly
  • Images are not produced by the detector (cf. II) but reconstructed by computer algorithms
  • Each image has thousands of pixels and voxels (volume element) which is determined by the attenuation coefficient measured by the detector

 

Hounsfield units – universal dimensionless unit used in CT to objective measure and compare images. Obtained from linear transformation of measured attenuation coefficients.

  • Water is assigned 0 HU, and air – 1000 HU. Bone results at 1000 HU or more.

MRI

  • Protons from hydrogen atoms in tissue are usually spinning in random fashion
  • MRI machine produces magnetic field which ‘lines up’ the protons in parallel to the magnetic field
  • Radiofrequency ‘coil’ transmits radiofrequency pulses over area of interest – when RF pulse stops, protons release their energy (in either T1 relaxation or T2 decay)
  • Receiver coil detects changes in electromagnetic energy and differences in time of energy changes (relaxation time) is interpreted in different signal intensity
  • T1 relaxation is the release of energy as protons realign in the longitudinal plane, which occurs rapidly in fat which is bright (as is haemorrhage).
  • T2 decay is the dephasing of nuclei, which occurs slowly in water and this is detected as bright on T2.
  • Diffusion weighted imaging detects diffusion of water molecules against magnetic gradient – more tightly packed and denser cells (malignant) will have less water diffusion (restricted diffusion).

 

MRI kidneys/adrenal – interpretation

Look first at T1 fat sat, T1 in and out of phase, and T2. Then contrast enhanced Gad images.

  • T1 fat sat – will show macroscopic fat (similar to CT)
  • T1 out of phase is the one with black crayon borders around organs – loss of water signal – microscopic/intracellular fat may be evident on out of phase images as heterogeneity, different to macroscopic fat
  • T1 in phase (normal T1) – blood, proteinaceous material and melanin is bright on T1
  • T2 – water is white – confirms cystic vs solid
  • Contrast enhanced T1 – determines enhancement

 

Nuclear medicine

  • Radionuclides are atoms with excess nuclear energy – unstable – which is emitted as gamma radiation or new particles
  • Radiopharmaceuticals have two parts – the radionuclide which has specific properties for imaging (99Tc, 68Ga, 18F), and a compound with desired physiological properties for what is being investigated (PSMA, FDG, MAG3, diphosphonate)
  • The ideal radionuclide or isotope for imaging studies emits only gamma rays with useful levels for detection – short range positrons – which are detected by gamma cameras.
  • These are then interpreted in conjunction with other imaging (single photon emission SPECT) or formal CT or PET.

 

PET

  • Positron emitting isotope undergoes beta decay, with protons converted to a positron, neutron and neutrino
  • Positron travels a short distance and annihilates with an electron – annihilation reaction results in formation of two high energy photons which are detected by PET scanner (recorded by scintillator crystals)

 

  • FDG is a glucose analogue which is metabolised faster by tumour cells – it is accumulated in cells and trapped, and then as it undergoes beta decay emits positrons which collide with electrons and undergo annihilation reaction, creating two photons.
  • SUVmax – maximal Standardised Uptake Value. May be significant variability between machines, users, and patients. Can be used to analyse treatment response but difficult to validate between centres and different patients.

 

Radioactivity

  • Radioactivity or radioactive decay is the spontaneous breakdown of unstable nuclides with release of subatomic particles and/or gamma radiation
  • Beta decay – emission of a beta particle (either positron or electron)
  • Alpha decay – emission of an alpha particle with two neutrons and two protons – these are relatively heavy particles which travel only short distances – causing significant ionising radiation and biological effect but only over short distance.

 

Ultrasound

  • Electrical pulses in the transducer cause the mechanical oscillations of crystals, converting the electrical energy into mechanical sound waves.
  • Ultrasound waves go from transducer and through tissues, and are reflected back to the transducer as reflected echoes in different proportions based on the density of the tissue
  • If there are no differences in the tissue density then no echoes are produced – i.e. urine in the bladder is anechoic.
  • If the tissue is very dense then the sound waves may be completely reflected resulting in total acoustic shadowing i.e. stones, bone or gas