Quantitative Clinical uses of MRI Flashcards
is conventional MRI qualitative or quantitative?
qualitative
overview quantitative MRI
an advanced way of imaging- ensures the same patient in different scanners gives the same results- the numbers mean something (can compare between patients or same patient over time). Image intensity is physically meaningful
however, takes longer scan time as repeat scans are required
outline an example of how MRI biomarkers have been used
one from:
- studying MS , known to effect WM initially. Quantitative research looking at distribution of WM found there’s a large variation in disease progression
- used to select treatment after acute stroke. A key treatment is the injection of thrombolytics into blood to dissolve clot, it has been shown to be effective in early stroke in penumbra, however if area of tissue is permanently damaged it will worsen the damage. Using diffusion and perfusion scan to compare areas infarcted and penumbra areas (which can be recovered if treated quickly)
- looking at treatment response of radiotherapy to cancer- scanning to measure perfusion in brain to see if angiogenic activity is still in tumor
overview qualitative MRI
different scanners produce different images of the same anatomy, only looking at image contrast, not intensity
the majority of clinical MRIs are qualitative
compare the use of MRI biomarkers to biofluids (such-as blood or urine)
it gives more primary details of organ (detecting disease/treatment effects in situ) and is not confounded by changes outside of target organ
compare the use of MRI biomarkers with biopsy
non-invasive (doesn’t cause damage.disturb the tissue state and is repeatable)
no sampling error (e.g. in heterogenous disease- like different areas of tumor)
overview the unique features of MRI biomarkers
comprehensive multi-scale assessment (anatomy/structure/function) and enables the study of organ interactions (e.g. multiorgan post-hospitalisation covid-19 study)
overview the history of MRI biomarker development
1972- first biomarker proposed- faster T1 relaxation signal seen in malignant tissues, however qualitative MRI still favoured clinically
early 2000s- renewed interest in quantitative biomarkers when looking at tumor phenotypes quantitative scan numbers were better at decoding tumors than radiologists looking at qualitative scans
2010s- emergence of quantitative radiology
2020s- imaging biomarkers combined with AI- e.g. UoS findings able to detect heart disease
what are MR contrast agents
paramagnetic molecules (such as gadopentetate dimeglumine) which alters the magnetic environment in its locale, therefore alters signal
how are T1,T2 and T2* signals effected by presence of gadolinium-based contrast agents?
shortens them
T1 weighted images exhibit increased signal
T2* weighted images exhibit decreased signal
why can gadolinium not be used alone?
it is toxic so needs to be combined with something to make it inert but retain its magnetic responses
what 2 pathologies is gadolinium useful for imaging?
stroke and tumour
what is MR perfusion imaging (DSC-MRI) (stands for)
dynamic susceptibility contrast MRI
what is perfusion?
local blood flow through a region of brain tissue
what are the 3 measurable parameters (quantitative biomarkers) of interest?
- CBF- cerebral blood flow
- CBV- cerebral blood volume
- MTT- mean transit time
overview methodology for DSC MRI perfusion imaging
- acquire baseline pre-contrast images
- inject contrast agent (bolus)
- observe signal change during first passage of bolus through brain
- T2* weighted sequence is sensitive to intravascular
contrast agent (T1 more sensitive extravascularly) - requires high temporal resolution- multi-slice imaging
- can watch contrast bolus agent in passage films
draw perfusion measurement curve and indicate what different aspects mean
see notes
outline jet-colour maps of perfusion
often used to indicate contrast agent uptake, however, can fol the eye, providing artificial transitions making it look like there are clear boundaries where there aren’t
use of perfusion imaging to look at stroke…
whether/what treatment is required and can often see hyperaemia (increased blood flow compensation for previous flow deficit)
use of perfusion imaging to look at tumour…
aggressive tumours tend to have a rim of vasculature and necrotic core, using contrast uptake perfusion imaging can establish how aggressive tumour is (vasculature) and then can be used to check the efficacy on treatment of tumour (post-steroid)
arterial spin labelling (ASL)
measures perfusion to invert magnetisation using exogenous contrast agent
blood magnetically labelled in neck as it flows to the brain-difference between labelled and non-labelled images is proportional to cerebral blood flow
the signal change is tiny, so not always clearly seen, especially in WM
what does DCE-MRI permeability imaging stand for
dynamic contrast enhanced MRI
overview the BBB
normal brain- endothelial cells form tight junctions creating the BBB- allows permeability of essential molecules such as glucose but not toxins
severely impedes drug delivery
breakdown of BBB can occur in various disease states e.g. tumours
what is the purpose of permeability imaging?
it attempts to quantity the permeability of the blood vessel endothelial wall
in normal subjects, vessel integrity is maintained and no leakage occurs, however, in disease imperfections in vessel wall are seen and can be measured
what are the measurable parameters of interest in permeability imaging?
PS/Vt (Ktrans) - permeability surface areas product per unit volume of tissue
Ve- extravascular extracellular space volume fraction
overview methodology of permeability imaging
- acquire baseline pre-contrast images
- inject bolus contrast agent
- observe signal up to 30 mins after injection
- convert signal change into contrast agent concentration
- T10 weighted sequence is sensitive to leakage of contrast agent in to the extravascular extracellular space (EES)
what does a single post-contrast time provide?
information on amount of leakage at a specific time
discuss contrast agent uptake curves, can use a diagram
using curves of time vs signal enhancement can detect between benign and malignant tissue.
Malignant tumor tissue: large and fast uptake w/o washout
benign/necrotic tumour tissue: large but more progressive uptake
normal tissue: minimal contrast uptake
outline signal enhancement vs contrast agent concentration
signal enhancement is not linearly related to contrast agent concentration. It exhibits high-dependency on pre-contrast tissue T1 (T10)
T10 must be measured to enable contrast agent concentration to be calculated from signal enhancement measurements
visually, contrast agent concentration and signal enhancement look very similar
what can model BBB permeability?
pharmacokinetic model can be created to describe interaction between blood plasma and extravascular extracellular space (measures leakage)
an equation for this provides reasonable model of the contrast agent uptake observes in human tissues
discuss vasculature in regards to tumours
without vasculature, tumours are limited in growth ~1-2mm diameter, in order to grow, cancer cell send signalling molecules that generate angiogenesis
new vessels are often imperfect without tight endothelial junctions (can be up to 2000nm pore as opposed to ~4nm in normal endothelial cell). Higher grade, more aggressive tend to be more vascular (and leaky) hence can be observed using permeability imaging
name 3 intracranial tumour types
- Glioblastoma Mutliform (GBM)
-Meningioma
- Low Grade Oligodendroglioma (LGO)
what seems to be a good predicter of steroid response (tumour treatment)
regional CBF
what are the 5 key imaging techniques used in clinical practice?
- volumetry (VMB)
- fMRI
- DTI
- perfusion
-spectroscopy
outline volumetry use in clinical practice
- used to look at grey matter volume
- good for longitudinal study to monitor atrophy over time
- particularly used in neurodegenerative disease- AD/PD
- fairly simple to do
outline fMRI in clinical use
- relies on BOLD contrast
- due to large masses of data provided by EPl it’s only started being used in clinical practice more recently
- useful in understanding which pparts of cortex are recruited for a specific function e.g. to see which hemisphere is dominant in language, as language centres are in the temporal lobe, which can be near regions causing seizures, this ensures surgery won’t remove language centres.
- also used in tumour planning/pre-surgery in order to understand which parts of cortex are being recruited as tumour may change this- ensures preservation of function
- paradigms for visual/motor/language systems are well-established
- the downside is that fMRI is very motion sensitive so if you are looking at a patient who is unable to stay still in machine it may be unviable
overview DTI (diffusion tensor imaging) in clinical practice
- one of the most useful methods clinically
-it measures Brownian motion of water, which produces maps of trace diffusion and diffusion coefficient can be calculated - can identify disease e.g. stroke leads to vasogenic oedema with larger extracellular space allowing greater fluid diffusion
- whereas tumours tend to show as hypercellular tissues where cells are closely packed, hence reducing diffusion
- DTI allows you to be able to differentiate between more diseases based on the flow of water e.g. a cerebral abscess may be unclear in a standard T1 scan as it could look like a number of things e.g. stroke, however, with Diffusion imaging, reduced diffusion in the tissue indicates tightly packed cells so this could be a cerebral abscess (filled with dead bacterial and WBCs, hence reduced diffusion)
- diffusion along WM tracts is anisotropic, seen in DTI wherby it follows the tracts, clinically this can be used to determine the edges of tumour and gives a better idea of the extent of tumour- useful for tumour planning
- normal DWI uses 3 directions, DTI uses around 30
- also motion sensitive so may be difficult in certain patients
Overview Neurite Orientation Dispersion and Density Imaging (NODDI)
an advance version of DTI which can model extra-neurite, intra-neurite and CSF compartments within each voxel, hence we can measure different types of tissue
