Perfusion MRI in GSK Study

Units

Perfusion is measured in mL/100g of tissue/min, or units of CBF

Normal human gray matter is perfused at a rate of 50-60mL/100g/min

Measurable ParametersCBF – Cerebral Blood Flow – The amount

of blood moving through a given amount of tissue per unit time (can be absolute: aCBF, or relative: rCBF)

CBV – Cerebral Blood Volume – The amount of blood in a given amount of tissue at any time

MTT – Mean Transit Time – The average amount of time it takes any water molecule or particle of contrast agen to pass through the voxel vasculature

MRI Methods

Bolus Tracking - use of a contrast agent such as gadolinium as a tracer

Arterial Spin Labeling (ASL) - labels the water molecules of blood magnetically to use as a tracer

Arterial Spin Labeling Method

Tracer is a magnetic label applied to the water molecules in the blood

Magnetic label is produced by saturating or inverting the longitudinal component of the MR signal

The tagged water will pass into brain tissue and alter it’s longitudinal magnetization

A control image where no tagging is done is collected after each tagged image

Calculating CBF

M = Mcontrol – Mtag

A difference image is created from the control and tag image that is proportional to CBF.

Surround Subtraction

Perfusion images are calculated from a combination of the control and tagged images

Odd indexed images are taggedEven indexed images are controlsPerfusion weighted time series

= {img[1] – (img[0]+img[2])/2}, {(img[1]+img[3])/2-img[2]}, …

Problems with Pulsed ASL

Slice profile effect alters the quality of the subtraction

Measurements are sensitive to the transit time of blood water between labeling and imaging sites

Solution

Add one or more spatial saturation pulse during the inversion time to saturate either the imaging slice or the tagging region. This is referred to as quantitative imaging of perfusion using a single subtraction (QUIPSS and QUIPSSII)

QUIPSS RF Pulse Sequence1. Slice selective in-plane presaturation

applied to imaging slice2. Inversion tag is applied3. Second saturation pulse is applied to

tagging region after a delay, TI1 (done only in QUIPSSII)

4. After additional delay, at time TI2, an image is acquired using single shot EPI after the inversion tag

GSK Study Basic Info

3 Study Days: A – Placebo, B – Ketamine, C – Ketamine&Lamotragine

2 Perfusion runs ~ 5minutes longOne before the bolus and one after

the bolusOne Proton Density scan acquired

GSK Study Run Breakdown

1. Localizer2. 3D T13. T1 flash4. Fixation5. VOD6. VOD7. VOD

8. Fingertap9. Proton Density10.Perfusion11.Fixation (bolus delivered)12.VOD13.VOD14.VOD15.Perfusion

Output Images in Analysis

dM – difference image (relative CBF)

BOLDCBF (absolute CBF is calculated if

Proton Density image was specified)