Arterial Spin Labelling Flashcards
What does BOLD reflect and what is it related to?
- Reflects changes in blood oxygenation levels
2. Related to changes in metabolic rate of oxygen consumption and removal of oxygen from haemoglobin
What is BOLD affected by?
Changes in blood volume and flow
What is the MR methods for perfusion/Arterial Spin Labelling?
- Use RF pulses to apply magnetic label to the water as a freely diffusible tracer
- Water is labelled by inversion of its magnetisation - different from the surrounding signal
- Two images: one with blood labelled and one without (control image)
- Substraction of lbaelled image from the control results in a perfusion-weighted image
- This can be converted to a quantitative perfusion map with knowledge of T1 (+ other parameters)
What is Arterial spin labelling?
magnetic resonance (MR) imaging technique that enables the measurement of brain perfusion noninvasively at the tissue level.
When is ASL feasible?
On low-field strength MR systems
What does ASL greatly benefit from?
The improved signal-to-noise ratio (SNR) of modern high-field-strength MR systems
What has led to a rapidly growing interest in ASL within the past few years?
- Increasing availability of 3-T imagers,
- Development of improved pulse sequences
- Multichannel receiver array coils
What did ASL paved the way for?
Widespread application in neurologic and psychiatric disorders
What is the basic concepts of ASL?
- ASL is based on the principle of magnetically labelling inflowing arterial blood protons prior to their entry into the tissue of interest - Viewed as tracer technique
- Water acts as the natural endogenous tracer to estimate tissue perfusion
- The label is created by applying radiofrequency pulses to invert the bulk magnetisation of the blood water protons
- Images are acquired after the labelling and inflow priod by using rapid acquistion technique such as echo-planar imaging, gradient- and spin-echo imaging (GRASE), or three-dimensional fast spin-echo imaging using a stack-of-spirals approach
- A pair of images is always acquired: a labelled image (blood water magnetisation is inverted) and a control image (blood water magnetisation is not inverted)
What is the signal difference between labeled and control images proportional to?
Amount of magnetisation inverted and delivered to the tissue
When is the signal difference proportional to cerebral blood flow (CBF)?
If all the labelled blood has arrived at the imaging voxel at the time of image acquistion
What are the current main implementations of ASL?
Pulsed and pseudocontinuous labelling
What is pulsed ASL?
- The arterial blood water is labelled by using a short adiabatic inversion pulse
- The labelling pulses are on the order of approximately 10 msec and designed to invert the blood water instantenously in a particular region - typically located inferior to the brain
- After labelling, a postlabel delay period is required (inflow time for PASL) during which time the inverted blood moves from labelling region into the brain - losing gradually its label through longitudinal T1 relaxation
- PASL is intrinsically a lower SNR technique
- The control acquistion for PASL consists of applying a radiofrequency pulse with equivalent power to the labelling pulse but which has a net zero effect on the blood water magnetisation in the labelling region
What is PCASL technique?
- A long labelling period (1-2 seconds) is made up of a train of very short pulses is designed to invert the inflowing blood magnetisation in an adiabatic or pseudo-steady state manner
- The blood is being continuously inverted as it flows through a ‘‘labelling plane’’ in the inferior-superior direction
- If the phase of every second pulse in the PCASl pulse train is shifted by 180 the flowing blood water is minimally perturbed and thus enables acquisition of non labelled control images
Why has PCASL recently been adopted as the labelling method of choice for clinical imaging?
- Ease of implementation
2. High SNR
What is continuous ASL?
- Apply an inversion pulse which inverts blood as it flows through the artery
- White: fully relaxed, M0
- Apply a pulse with a frequency offset that causes an inversion of the magnetisation as the blood flows through
- As the blood passes the imaging slice plane it becomes inverted so becomes negative –M0
- Inversion pulse is a flow induced inversion – the blood is flowing through a gradient while it has RF pulse applied [selective to the blood]
- As it moves through the gradient – it moves through a different field and that causes the magnetisation to tip
- Blood moves up through the arteries and the arterioles into imaging slice of interest – it will get to the capillary bed of that slice – water within the blood will then exchange across the BBB into the parenchymal tissue of that slice
- As the blood water goes into the tissue – the negative signal tend to reduce the overall signal Availible within tissue
- If more blood is supplied to a certain region – get more negative magnetisation delivered to that region so you get a greater drop in signal
- Can apply the pulse over a certain period of seconds – signal reduction to build up over that time
- The control will have full signal as it didn’t have any negative flow delivered to it
What is the main advantage of PCASL?
- Higher SNR
- this can be understood by considering the degree of labeling of the arterial blood as it arrives at the tissue and how this changes during the inflow time and/or postlabel delay
What happens during PASL?
A slab of arterial blood is inverted at inflow time of 0 and subsequently undergoes T1 relaxation during the inflow time
What does during PCASL?
The arterial blood is continuously inverted as it passes through the labelling plane which means that all blood has the same amount of T1 decay when it arrives at the tissue irrespective of when it was labelled
What is an important aspect of all ASL techniques?
The introduction of the postlabel delay (or inflow time) between the end of the labelling pulse and time of image acquisition
What happens if the postlabel delay is longer than the longest transmit time between the tagging plane and the imaging volume?
The ASL signal becomes insensitive to variations in the arterial arrival time as long as the blood and the tissue T1 values are similar (true for gray matter)
What does the post-label delay enable?
CBF quantification and minimises the appearance of intravascular signal in the ASL images, yet at the expense of SNR due to T1 relaxation of the bolus
What doers the choice of postlabel delay depend on?
- Subjects’ age with older subjects showing longer arterial arrival times
What is the recommended postlabel delay for paediatric and adult clinical population?
1500 msec and 200 msec respectively
What is the result of perfusion-weighted image and what is done to compensate for it?
The image is quite noisy because it is looking at quite a small difference between two images because the blood volume is relatively low compared to the volume of the whole tissue
- Do averaging to make sure we have high SNR
What are the clinical applications of ASL?
cerebrovascular stroke vascular malformation encephalitis migraine-associated hyperperfusion 3 dementia and cognitive disorders
What do most clinically available ASL sequences use?
Only a single delay between labelling and image acquisition based on original values calculated from healthy young adults
What happens in the case of proximal vessel occlusion/
There is a delayed arrival of blood in the parenchyma, which may falsely suggest a reduced relative CBF as estimated with ASL and increased ASL signal in the feeding arterial vessels known as arterial transit artefacts
What is CASL post-labelling delay (PLD)?
- waits different amount of time after you have turned off the inversion pulse
- if there is a short delay then a lot of the labelled blood can still be in the large artery – region of high intensity which corresponds to labelled H20/blood that is still in the arteries and the arterioles – that is not reflecting the tissue perfusion – haven’t allowed long enough for the labelled blood to go up and exchange into the brain tissue
What occurs during longer and longer post-labelling delay?
More signal is shifted into the extravascular space
What is one of the most attractive features of ASL?
It’s ability to quantify perfusion, an important indicator of tissue health as well as neuronal activity
What are the major assumptions of quantification of CBF?
- The entire labelled bolus is delivered to the target tissue
- There is no outflow of labelled blood water. Brcause the tissue water pool is much larger than blood water pool, and water exchange between blood and tissue is rapid, this is generally a valid assumption
- The relaxation of the labelled spins are governed by blood T1
What are the two compartments of ASL CBG quantification model?
- The blood [intravascular compartment]
2. Tissue [Extravascular compartment]
Where is it assumed that there is a free diffusable tracer for ASL signal model?
When it gets to the capillaries it exchanges freely between the blood and the tissue
What does the T1 recovery determine?
How the brain magnetisation changes
What does the factor λ scale?
The signal intensity of tissue to that of blood In principle, λ should be an image because tissue water density differs in different tissue types, but often a brain average value is used
What are the problems with ASL?
• Can only acquire a single slice
- When we apply RF pulse to do the inversion – we are applying the RF pulse at a certain off-set frequency away from imaging slice – cause inversion of blood as it flows through the plane
• Also get a magnetisation transfer effect
- Reduction of signal in the brain tissue
- Trying to do experiment
- Confounding effect – reduction in signal intensity via magnetisation transfer
- By applying RF pulse of resonance, affect macromolecular spins within slice of interest which also causes signal loss – if we don’t account for it – misinterpret magnetisation transfer effect as perfusion = drop in signal
- Magnetisation transfer effect is entirely symmetrical in frequency
What are the limiting factors for ASL?
- Low intrinsic SNR of method
- typically looking for a difference of < 5% - Spin label lifetime limited by T1
- difficult to measure perfusion in regions with extended transit time - For CASL, long pulses needed to cause inversion
- in practice, train of shorter RF pulses must be used to keep within SAR limit –> pCASL
What are recent ASL improvement
- 3T scanners increase SNR and lengthen the blood T1
- Background suppression eliminates noise from static tissue
- 3D acquisition techniques increase SNR
What are advantages of perfusion fMRI using ASL?
- Can image in regions of high static susceptibility
- Can quantify resting and activated CBF
- Provides signal change in absolute units (ml/100g/min)
- Reduced intersubject variability
What are ASL advantages?
- Non-invasive repeated measurements
- Quantitative CBF in standard units ml/100g/min
- for fMRI:
- can image in regions of high static susceptibility
- can assess baseline CBF
What are ASL disadvantages?
Short lifetime of tracer (no recirculation problems but fast decay of tracer)
- low SNR
What may ASL complement ?
structural information of standard MR imaging in neurocognitive decline and may allow early diagnosis of dementia
What may ASL depicit?
mismatch perfusion in acute stroke and help identify tissue at risk in chronic cerebrovascular disease.
depict arteriovenous shunting in arteriovenous malformation and fistulas
What may ASL be used to localise?
the epileptogenic focus in seizure disorders
What can ASL replace in neoplasms?
replace standard gadolinium-enhanced dynamic susceptibility contrast-enhanced imaging, notably in patients with allergies or renal insufficiency or in children.
What may ASL help detect?
subtle functional changes in psychiatric disorders such as posttraumatic stress disorder or mild traumatic brain injury, where standard structural MR imaging typically provides no detectable anomaly.
What may emerging new applications of ASL include?
superselective ASL to map vascular territories and cerebrovascular reserve imaging
What does ASL MR perfusion not require?
Intravenous administration of contrast
What does ASL exploit?
The ability of MRI to magnetically label arterial blood flow below the imaging slab
What is the parameter most commonly derived from ASL?
Cerebral blood flow (CBF)
What is ASL?
Non-invasive and non-ionising MRI technique that measures tissue perfusion (blood flow), by using magnetically-labelled arterial blood sister protons as an endogenous tracer
What are the number of techniques described to achieve ASL perfusion?
Based on the magnetic labelling process
- Pulsed (PASL)
- Continuous (CASL)
- Pseudocontinuous (PCASL)
- Velocity- selective ASL
What is the main idea in ASL?
Obtain a labelled image or tagged image and a control image in which the static tissue signals are identical but the magnetisation of the inflowing blood is different
How are the water molecules in the arterial blood magnetically labelled?
Using a radio frequency pulse that saturated water protons
What does substraction between labelled and controlled image eliminate ?
The static signals and the remaining signals are linear measures to the perfusion, which is proportionate to the cerebellar blood flow (CBF)
Why is ASL signal-to-noise ratio very low?
The signals from the tagged blood is only 0.5-1.5% of the entire tissue signals
Why is EPI used for ASL acquisition?
Because of its high signal to noise ratio
Why is ASL data acquired before gadolinium administration?
Gadolinium will cause T1 shortening leading to a decrease in the measurable signals in both the labelled and controlled images
