In this presentation, arterial spin labeling MRI is explained as an alternative for acquiring perfusion and 4D angiography images. Moreover, the main advantages and disadvantages will be discussed as well as applications shown.
-to understand the technical background of arterial spin labeling (ASL)
-to know what information can be obtained by ASL and the differences in information content as compared to dynamic susceptibility contrast perfusion imaging
-to be able to name the main advantages and disadvantages of using ASL as opposed to contrast enhanced MRI techniques
Arterial spin labeling MRI exploits labeled blood as a completely non-invasive tracer to measure perfusion or to acquire dynamic (4D) MR angiography. Most frequently, the blood is labeled by employing a fast (+- 15 msec) slab-selective inversion pulse below the imaging slices (pulsed ASL) or by creating a labeling plane below the imaging slices (continuous or pseudo-continuous ASL) that is switched-on for approximately 1.5-2.5 second and that inverts all blood flowing through this plane. In both approaches the blood is labeled by inverting the magnetization and after a suitable delay-time (the post-label delay (PLD) time) images are acquired of the region of interest. This image (the labeled image) will reflect both static tissue signal and the inflow of labeled blood. By acquiring a second image that is acquired in a similar manner as the labeled image, but without inverting the inflowing blood (control image) and subtracting these two images, only the signal of the inflowing blood will be depicted.
Based upon the choice of the PLD and the readout module, one will either perform angiography (short PLD in combination with a high spatial resolution readout) or perfusion imaging (long (+_2sec) PLD and low (typical 3x3x3 mm) spatial resolution readout). Multiple repeats are performed to either fill the complete, high spatial resolution k-space (angiography) or to average the signal (perfusion imaging; often a segmented 3D readout acquisition is used with only a few averages).
Since labeling is performed by inverting the longitudinal magnetization, we will loose our tracer by longitudinal relaxation and it will be governed by the T1 of blood (approx. 1.65 sec at 3Tesla). This implies that the delay between labeling and readout cannot be too long, because otherwise already too much of our label will have decayed and the SNR would be too low. This is the main compromise that one has to solve when using ASL in the radiological clinic.
Because ASL is a subtraction technique, fluctuations of static tissue will lead to artefacts and lower effective SNR. Therefore, a pre-saturation is performed, frequently in combination with several global (or almost global) inversion pulses to suppress the background signal.
The consensus paper on ASL imaging (see references: Alsop et al, MRM 2015) describes what settings to use for ASL perfusion imaging. In summary, the recommendation is to use pseudo-continuous ASL (PCASL) with 1800 ms labeling duration, a 1.8-2.0 sec PLD, background suppression, and a segmented 3D readout, such as 3D-GRASE.
Quantification of PCASL data is based on the principle that almost all labeled spins will extravasate from the vascular compartment and accumulate in the tissue compartment and will remain there until readout. Because for PCASL we know how long we have labeled the blood, quantification is relatively simple, since the amount of label detected in a voxel is dependent on the cerebral blood flow, the label-duration and the relaxation rate of the label. Complicating factor is that the label will have resided both in blood as well as in tissue, which have different T1s. In the consensus paper, it is recommended to neglect this fact, and to just calculate with the T1 of blood. Moreover, quantification errors will occur, when the PLD was chosen shorter than the time it takes for the blood to travel from the labeling plane to the tissue compartment, because then not all labeled spins will have reached their final destination, resulting in an underestimation of the CBF.
Compared to dynamic susceptibility contrast (DSC) MRI, PCASL provides only information on the CBF, whereas DSC also provides information on cerebral blood volume (CBV), mean transit time (MTT), and timing parameters. By acquiring ASL-images with different PLDs, the arterial transit times can also be determined by ASL, although this is not yet frequently done.
Several new approaches have been proposed to enhance the information that can be obtained from ASL. For example, it is possible to restrict the labeling to a single artery to selectively display the vasculature or tissue fed by this particular artery. This is especially useful in the workup towards an intervention, to fully understand the vascular layout of the patient.
Moreover, new labeling approaches have been proposed that do not label based upon location, but based on the velocity or acceleration of the blood. These methods create also label within the imaging region and suffer therefore less from transit time effects.
Finally, time-encoded PCASL dynamically encodes the label and thereby allows in a highly time-efficient manner to acquire dynamic ASL images. This can be combined with selective ASL techniques or by a multi-contrast readout module to gain even more information about the label traversing the vascular tree.
Some suggested literature: