Arterial spin labeling (ASL) is a noninvasive magnetic resonance imaging technique used to measure tissue perfusion without the need for contrast agents or radiation. It works by magnetically tagging arterial blood flowing into the region of interest and acquiring tagged and control images, which are subtracted to produce a perfusion image showing tagged blood flow. While ASL has better spatial and temporal resolution than PET, it has a low signal-to-noise ratio and sensitivity to long transit delays, limiting its clinical use compared to other perfusion imaging methods.

1. R A H M A N U D D I N
L E C T U R E R M E D I C A L
I M A G I N G
N W I H S
PWI: Arterial Spin Labeling
(ASL)

2. Introduction
 A noninvasive method
 To assess tissue perfusion
 No exogenous contrast agent injection or radiation
Venous
Angioma

3. Procedure
 Arterial flow toward ROI tagged by magnetic
inversion pulses
 Phase of protons changed
 After delay, allow tagged blood to enter in Slice of
interest (SOI), image is acquired
 Such image is called tagged image
 Second image, same SOI again acquired without in-
flowing tagged blood, image is called control image
 The tagged is subtracted from control
 Results in perfusion image representing tagged
blood that flowed into the image slice

4. ASL procedure

7. Procedure Cont…
 ASL involves T1-w imaging
 Has poor SNR ratio
 However, ASL has better spatial and temporal
resolution than PET
 Poor SNR and sensitivity to abnormally long transit
delay of tagged protons have limited its application
in clinical practice

8. Resolution of SPECT vs. ASL