MR perfusion without contrast (Arterial Spin Labeling, ASL)

Teaching points by Dr MGK Murthy

1.  So far, in the absence of  better techniques for studying direct neural activity, Science has relied on Vascular supply to regions as  an Index , using  (a) Gadolinium  MRI   (b) Radioactively labeled water (15 O water )imaged with  with PET  Scanner  (c)  BOLD techniques of MRI (using  Oxygen uptake for e.g.: as in functional  MRI) 

2. However a new technique  now using , Magnetically labeled water molecules(Arterial Spin Labeled, ASL) on MRI scanners  to study Brain perfusion ( CBF ,Cerebral Blood Flow)  is becoming increasingly popular  due to

 

(a) No injections or chemicals 

(b) No Radiation

(c) Changes in ASL  on activation  of  brain areas more than  possible with BOLD techniques 

Disadvantage:  Takes longer for acquisition/ Less slices/ Thicker slices /Inherently Noisier (as compared to BOLD)

 TECHNIQUE:

(a) Arterial blood water labeled just below the slice of interest  by applying inversion RF pulse of 180  deg  ---------------Inversion of net magnetization of Blood Water (Water molecules labeled magnetically)-------------------After a period of time (Called Transit Time) ---------- paramagnetic tracer will flow in to slice or Region of interest --------------Exchanges with Tissue water ------------Results in Decrease in  Total Tissue Magnetization --------------MR signal and image intensity will decrease -----Image taken at this stage called TAG image 

(b) Repeat the above acquisition (a) without  Inversion pulse of 180 deg --------------referred to as Control image 

(c) Subtraction of control image (b) from  magnetically labeled water  TAG image (a) === Reflects the amount of arterial blood delivered to each voxel in the Slice (ROI) with  in  the transit time (that is CBF)

(d) Currently four types of ASL exist  (i)  pulsed ASL (PASL) (5-20ms pulsed RFs) , (ii) continuous ASL (CASL)(1-2 second RF ) (iii) pseudo-continuous ASL (PCASL) and  (iv) velocity-selective ASL (VS-ASL)

(e) While PASL, CASL, and pCASL invert the inflowing blood at a specific location, VS-ASL saturates the blood that is moving at a faster velocity than the specified cutoff value to achieve perfusion contrast

Disadv of VA-ASDL is low SNR  and  the need to calculate the optimum velocity for labeling cutoff 

(f) Two problems exist in general for ASL Technique  (i) subtle Movement artifacts  adversely affect the image because of the subtraction nature of the same. 

(ii) they are also  highly sensitive  for susceptibility artefacts  arising from   surgical hardware/ hemorrhage/  air in sinuses  etc  

(g) Two critical steps recommended include (i) Labeling plane should be placed perpendicular to  Feeding arteries in general  and (ii)  avoid potential artefact  sources like the  sinuses/ surgical hardware  etc  

4. Usefulness of ASL technique is quite self explanatory  in  Various  day to day   clinical scenarios including in children/ Pregnant women / renal compromise  patients etc .

Other uses include 

(i)  CVA ( in Acute  --- for penumbra blood flow   and   in Chronic stroke  for study of Revascularizing regions)

 (ii) Dementia ( for e.g. Alzheimer's suggests reduced CBF in precuneus/ post cingulate/ post parietal locations)

(iii) Neuro-oncology (for studying  recurrences / primary tumours ) 

 (iv) Epilepsy (increased CBF in Ictal phase and Decreased CBF in interictal  phase) 

5. Psychiatry diseases    profile for utility of ASL technique merits special consideration

(i) Chronic and treatment-resistant depression = Altered  perfusion in  pathologically relevant subgenual anterior cingulate cortex compared to healthy controls

(ii) Adolescent depression shows decreased CBF in executive/ affective / motor regions with late life depression  shows increased CBF in white matter)

(iii) Schizophrenia = decreased CBF in prefrontal cortex

(iv) Border line personality disorders= Decreased CBF  in medial orbito frontal / Increased CBF in lateral orbito frontal regions 

Can be  also used to evaluate  treatment response in all conditions 

6. Territorial ASL =study  of one  vessel territory like ACA/MCA etc (using Pseudo continuous  ASL technique called pCASL)

7.Available in most current scanners across all the vendors 

8. Takes  Approximately  5 minutes of extratime to imaging