Imaging Flashcards
what does MRI stand for?
magnetic resonance imaging
what does CT stand for?
computed tomography
what does PET stand for?
positron emission tomography
why is image contrast needed in clinical imaging?
to distinguish different tissues from each other, and pathological tissues from normal tissue
how can contrast medium be given?
via GI tract (orally/rectally), via vasculature (IV/IA), into joint (intra-articular), into spine
how do radiographs work?
exploit attenuation of X-rays in tissue to generate contrast between tissues of different densities
what is often used as a contrast medium in GI tract radiographs, what does this help to identify?
barium sulphate, mucosal abnormalities in GI tract
how does an ultrasound work?
transmits high frequency sound waves into tissue, assesses returning signal to generate images of tissue non-invasively
how is the transducer used in ultrasound?
handheld, placed over area of interest
what is gel used for in ultrasound?
used on skin to enhance transmission of signal
why is ultrasound used in children and pregnant individuals?
safe as doesn’t involve ionising radiation
what is ultrasound imaging particularly useful for?
assessing solid organs such as liver and kidneys, assessing female reproductive tract, fetal assessment, detecting fluid accumulation, guiding biopsies, assisting interventional procedures such as drainages
how does CT work?
cross-sectional imaging tool that generates 3D images using a rapidly moving gantry containing an X-ray source and detectors
what contrast is often used in CT?
iodine based contrast agents given intravenously (allows intra-abdominal organs to be assessed)
what is CT used for and why?
widely used to detect range of pathological conditions. rapid, relatively inexpensive, can be used to generate images of whole body and generate images at multiple points to evaluate dynamic changes following administration of contrast medium
small downside of CT?
involves low dose of ionising radiation
what principles is MRI based on?
principles of nuclear magnetic resonance (NMR) and localisation of NMR signal within body
what nucleus is routinely probed in clinical practice for MRI and why?
hydrogen nucleus (proton)- water is most abundant hydrogen containing molecule in body for detection
why is MRI a very versatile tool?
wide range of tissue contrast mechanisms
what is DCE MRI?
dynamic contrast-enhanced MRI
how doe DCE MRI generate imaging of tumours?
by quantitatively assessing perfusion and the leakiness of the vasculature that is characteristic of the neovasculature that forms within malignant tumours
what is DWI?
diffusion weighted imaging
what does DWI do?
detects the small changes in the molecular movement of water that occur in the presence of tumours
how does PET imaging work?
utilises positron emission from unstable nuclei- subsequently undergoes annihilation in tissue when it collides with an electron- resultant energy production can be used to localise positron with high sensitivity
what molecule is typically used in PET?
a derivative of glucose (2-fluoro-2deoxyglucose) labelled with the positron-producing nucleus fluorine-18 (18F-FDG)
how does 18F-FDG enter cells?
uptake by glucose transporters, phosphorylation by hexokinase means it accumulates within cell
why is 18F-FDG an excellent method to identify tumours?
more rapidly transported and metabolised in tumours than normal tissue
what is PET frequently combine with?
CT
roles of imaging in oncology?
detecting and staging cancers (CT + MRI), guiding biopsies to regions of interest (ultrasound), detecting changes in tumour size during and after treatment (CT), repeat imaging over time to assess patient progression, early cancer detection
what change in glucose metabolism in tumours is a key hallmark of cancer?
often undergoes anaerobic metabolism to lactate even in presence of oxygen despite this being energetically unfavourable
what oncometabolites may be elevated in cancer driving tumour formation?
2-hydroxyglutarate in isocitrate dehydrogenase (IDH) mutated tumours; fumarate in fumarate hydrase (FH) mutated tumours; succinate in succinate dehydrogenase (SDH) mutated tumours
what is a highly sensitive method to detect altered metabolism as part of tumour staging?
18F-FDG with PET-CT
how can MR spectroscopy be used to assess metabolic changes?
by detecting specific metabolites and how these change with treatment
what does hyperpolarised carbon-13 MRI do to the signal-to-noise of the detected nuclei?
increases it 4-5 times
what molecule is typically used in hyperpolarised carbon-13 MRI?
13C-labelled pyruvate injected intravenously
what happens to the 13C-labelled pyruvate injected in hyperpolarised carbon-13 MRI?
converted to 13C-lactate in tumour tissue, converted into 13C-bicarbonate in brain and heart, converted into 13C-alanine in muscle and liver, enter TCA cycle
what enzyme converts 13C-pyruvate to 13C-lactate?
LDH
what enzyme converts 13C-pyruvate to 13C-bicarbonate?
pyruvate dehydrogenase
what enzyme converts 13C-pyruvate to 13C-alanine?
alanine transaminase
what enzyme enters 13C-pyruvate into the TCA cycle?
pyruvate carboxylase
what is DMI?
deuterium metabolic imaging
how does DMI work?
patients drink glucose labelled with deuterium (nucleus detectable using MRI), this is subsequently taken up by tumours and converted into 2H-lactate or 2H-glutamate/2H-glutamine, as measures of non-oxidative and oxidative metabolism respectively
which imaging methods don’t use ionising radiatio?
MRI, ultrasound
what is gadolinium used as a contrast in MRI to measure?
tumour vascularity, whether it’s disordered, heterogenous, leaky
why does GD3+ have a large magnetic moment?
7 unpaired electrons in outer cell
how is toxicity of GD3+ reduced, where does GD3+ accumulate?
chelation, accumulates in skin and brain
how is increased neuronal activity detected in functional MRI?
increase in neuronal activity causes local vasodilatation and blood flow. detected with blood oxygen level dependent (BOLD) imaging
what is SPECT?
single photon emission computed tomography
