This poster was originally presented at the RANZCR Annual Scientific Meeting 2011, October 6-9, in Melbourne/AU.
Congress:
RANZCR ASM 2011
Type:
Educational Exhibit
Keywords:
SPECT, PET, MR-Diffusion/Perfusion, MR physics, Neuroradiology brain, Imaging sequences
Authors:
M. McGuiness, A. Lane, A. Coulthard; Brisbane/AU
DOI:
10.1594/ranzcr2011/R-0032
Imaging Findings OR Procedure Details
Arterial Spin Labeling (ASL)
- An MRI technique,
which utilizes radiofrequency labeled arterial blood water protons as an endogenous tracer.
- The major benefit is the avoidance of ionizing radiation and intravenous contrast agents.
- Since its development in 1992 improvements in labeling approaches and image processing have broadened potential clinical applications.
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Involves a preparation component where blood is ‘labeled’ in the feeding vasculature in different magnetic states to create control and labeled images.
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Following a ‘post-labeling delay’ to allow the blood to reach the parenchyma the acquisition component acquires the data.
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This is done utilizing a rapid acquisition technique (such as spin echo) as the labeled tracer relaxes to equilibrium magnetization rapidly.
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As there is only a 1-2% signal difference between control and labeled images,
multiple images are acquired and signal intensity averaged.
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The control image is subtracted from the labeled image to create a quantitative perfusion map expressed in units of mL/100g/min.
This measurement is that of cerebral blood flow,
other perfusion studies may assess parameters such as cerebral blood volume and mean transit time.
Fig. 0: Illustration of the concept of ASL, comparing 150-PET (Fig 1A) and ASL (Fig 1B) approaches. In ASL, endogenous arterial blood water is magnetically labeled instead of exogenously administered tracer, and the magnetic label decays with T1 instead of radioactive decay.
References: Detre JA. (2008) Arterial Spin Labeled Perfusion MRI. Clinical Neurology. med.usa.siemens.com
Key parameters required for ASL quantification are the arterial transit time,
the labeling efficiency,
and T1 values in blood and tissue. ASL methods particularly benefit from high magnetic field strengths due to both lengthened T1,
which allows more label to accumulate,
and higher sensitivity.
The most common ASL approaches use either pulsed labeling (PASL) with an instantaneous spatially selective saturation or inversion pulse,
or continuous labeling (CASL),
most typically by flow driven adiabatic fast passage.
CASL
- Tagging based on location and velocity
- Uses long and continuous RF pulses (2-4s) in combination with a slice-selective gradient to induce a flow-driven adiabatic inversion of the arterial magnetization in a narrow plane of spins,
usually just below the imaging plane
- Allows brain magnetization to reach a steady-state that maximizes the signal difference between labeled and unlabeled conditions
- The continuously inverted spins provide a theoretically higher SNR than that obtained with PASL
PASL
- Tagging based on location
- Uses short RF pulses (5-20ms) to saturate or invert a thick slab (10-15cm) of blood volume,
known as a tagging region,
proximal to the imaging region
- In some PASL methods,
a spatially selective inversion pulse is administered to the tissue rather than to arterial blood.
Fig. 0: Schematic diagram illustrating arterial spin labeling strategies. Continuous ASL (CASL, left) labels arterial spins as they flow through a labeling plane. Pulsed ASL (PASL, middle and right) labels arterial spins using a spatially selective labeling pulse. Applications to single-slice imaging are illustrated for simplicity. Somewhat more complex schemes are used for multislice imaging.
References: Detre JA. (2008) Arterial Spin Labeled Perfusion MRI. Clinical Neurology. med.usa.siemens.com
Advantages of ASL
- No ionizing radiation
- No exogenous contrast – negates need for intravenous access and concerns regarding contrast reactions and risk of gadolinium induced nephrogenic systemic fibrosis
- Can be acquired as part of a multimodal MRI examination
- Repeatability and reproducibility – less than 10% variation in repeat studies
- Potential to selectively label vessels in cerebral vasculature to map perfusion territories and avoid more invasive angiography
- Absolute quantification of cerebral blood flow allows diagnosis of conditions which cause global hyper- or hypoperfusion such as hypercapnia
Disadvantages of ASL
- Inherently low signal to noise ratio leading to poor spatial resolution
- Tracer decays rapidly with T1 of blood (approximately 1200ms).
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- Rostral images may have lower signal due to image acquisition from inferior to superior
- Gadolinium further shortens the T1 of blood,
thus ASL images should be acquired prior to contrast administration
- Sensitive to motion artifact
- Relies on assumption about T1 of blood and tissue (affected by factors such as anaemia),
labeling efficiency and arterial transit time
- At present CBF is the only haemodynamic parameter measured,
however there are some promising techniques in development
Clinical Applications
The potential clinical applications of ASL are vast.
Some of the more promising indications will be discussed.
Cerebrovascular disease (CVD)
- In acute stroke imaging ASL may underestimate cerebral blood flow
- The signal is likely to be reduced due to the effect of stenosis and collateral flow on arterial transit time
- Applying a longer delay may give a more accurate assessment of blood flow at the expense of tracer decay and signal loss
- While ASL is good at detecting at-risk tissue,
it can not alone reliably differentiate between ischaemia and infarction
- Use in combination with DWI to assess ischaemic penumbra,
and ease of reproducibility make ASL suitable for monitoring reperfusion post-intervention
- In chronic CVD challenge studies can assess cerebrovascular reserve with repeatability and absolute measurement capability
- There is also the potential of performing selective arterial labeling to assess perfusion of a single arterial territory
Alzheimer’s disease (AD)
- ASL is able to quantify the loss of function in areas involved in AD which,
coupled with other MRI sequences,
can anatomically define areas with associated tissue loss
- ASL may be utilised in early detection of AD
Brain tumours
- Studies have shown correlation between tumour grade and ASL signal
- Images may be co-registered with anatomical scans at diagnosis
- Reproducibility makes ASL suitable for tumour surveillance
- ASL is able to differentiate between radiation necrosis,
tumour recurrence and infection
- In future ASL may be used to assess response to anti-angiogenic therapy,
which has shown promise as a tumour therapy
Seizure disorders
- Epileptogenic foci generally demonstrate hypoperfusion in the ictal phase and hyperperfusion in the interictal phase
- Ease of repetition,
lack of exogenous contrast and short tracer half-life make ASL suitable for interictal studies
- Compared to SPECT it has the benefit of structural localization sequences in the same study