MRI techniques that do not require contrast are advantageous for diagnosis and follow-ups of neurological conditions, especially those in which the use of contrast may be inadvisable, or simply not possible to administer. Arterial Spin Labelling (ASL) is a contrast-free MRI method to measure perfusion that has been used in brain imaging for some time. Canon Medical Systems (herein after Canon Medical) has been working together with clinical experts to enhance this technique further. Dr. Julien Savatovsky, Deputy head of Diagnostic neuroradiology at the Foundation Adolphe de Rothschild Hospital, France, explains what development and improvement of Canon’s new pseudo-Continuous ASL (pCASL) sequence brings.

“The best advantage of ASL is to allow perfusion assessment without requiring any contrast material at the opposite of DSC. So it can be beneficial in assessing patients with conditions, such as neurogenerative diseases or seizures, who cannot be injected all the time,” explained Dr. Savatovsky. “In some cases, you cannot perform DSC perfusion, because you cannot or would not want to inject the patient. ASL represents a real benefit for those patients”. He added “You can even repeat ASL several times within a day if needed, to compare the results with and without medication for example.”

“We usually found similar results between ASL and DSC. However, we have also found that there are few clinical applications in which ASL seems to be more sensitive than DSC,” he continued. “For example, in recent seizures, and recent migraines, you see a greater contrast between normal and abnormal site on the ASL perfusion compared to the DSC perfusion.”

ASL can be considered as having different modes of operation based on the way to “label” the blood with RF saturations. After having historically developed ASTAR, Canon Medical has focused on developing pseudo-Continuous ASL (pCASL) sequence together with Dr. Savatovsky. His team have ‘field-tested’ the new sequence and have found that it brings greater image quality than the previous solution.
“The image quality is greater than ASTAR because of a higher SNR and then, a higher resolution (Figure 1). You also have a higher coverage allowing more slices around the brain” he remarked. “And what is interesting is that it seems more reliable in terms of post label delay and sensitivity to slow flows, for example. Usually, you get ASL signal defects in some patients on posterior regions, especially the occipital lobes, but we don’t see that much on pCASL compared to other regular stored sequences. Canon Medical made the choice to implement their pCASL product with a spin echo readout. This makes the sequence less sensitive to susceptibility artifact compare to DSC. This is especially appreciated for patients that initially had blood products (e.g. post-operative tumors) or who are suffering from metastases with hemorrhagic transformation. To me, this choice was a key differentiator.”

Figure 1: Illustrative Vantage Galan 3T / XGO images with different ASL sequences on a patient suffering from visual acuity loss: previous ASTAR solution compare to the new pCASL solution, with the standard and a high-resolution protocols (Resolution=2×2×6 and 1.5×1.5×5 mm³ respectively). Higher SNR can be appreciated on pCASL sequences, with a better structure depiction on the high-resolution protocol. No hypoperfusion has been found on this patient.

Dr. Savatovsky and his team are researching the boundaries of the new sequence together with Canon Medical.

“We are working towards creating the most optimal protocol for clinical practice,” he explained. “Actually, the recommended voxel size is at two millimeters, with still a very high SNR, but we felt that we could even do higher resolution studies, that’s why we created a high-resolution protocol at 3T (Figure 1). We think that with a bit of development we could even reach the resolution that we have for DSC perfusion.”

The team have already incorporated pCASL and DSC perfusion into their brain perfusion protocols on both their Canon systems, 1.5 and 3T.

“We are already convinced that pCASL fits for a number of clinical indications, and we will continue using it for stroke imaging with normal diffusion, for example, or on any neurological deficits, seizures and recent headaches, as well as for our tumor protocol” (Figure 2). “Obviously, it’s not a ‘time-saver’, because both pCASL and DSC are required, but it helps in clinical confidence. As you have the same information in the two series, but importantly also because the pCASL can reveal additional information, especially in the periphery of the brain, or in areas where you have blood products that makes the DSC perfusion difficult to interpret.” //

Figure 2: Illustrative 1.5T images with 3D T1 FSE, DSC and pCASL sequence on a patient suffering from a brain tumor, after surgery (white arrows). A left anterior fronto-cingular hypoperfused area can be visualised, corresponding to the tumor resection. Around this cavity, no significant cerebral blood flow increase is detected, excluding the risk for neoangiogenesis. No additional perfusion abnormalities were found in the parenchyma.