Variation in extracerebral layer (ECL) thickness within a subject and

between subjects Tae Sun Yoo

Department of Medical BiophysicsUniversity of Western Ontario

Supervisor: Dr. Keith St Lawrence and Ph.D. candidate Jonathan Elliott

Near Infrared Spectroscopy & Principles• Near infrared spectroscopy is a powerful optical technique used to measure cerebral blood flow (CBF) and cerebral blood oxygenation.

1) Bolus injection light-absorbing dye, indocyanine green (ICG)

2) Monitor passage of ICG through brain by NIRS

3) CBF determined by shape of time-concentration data

Blood Flow Technique:

Clinical Relevance of NIRSBrain injury is a leading cause of deaths and

disability in Canada.NIRS can be used to assess brain health at the

bedside of head-trauma patients

NIRS works well with infants, but not with adultsProblem is signal contamination from

extracerebral layerConsequence is reduced sensitivity to brain

leading to underestimation of CBF.

ApproachMRIcro was used to import MRI images

and Image J was used to obtain length of extracerebral layer (ECL) thickness values across the circumference of the head (from 0 to 360˚)

Hypothesis Variations in ECL thickness across the

subject will be sufficient to affect the NIRS brain attenuation signal to measure accurate cerebral blood flow (CBF).

MethodBrain MRI images were acquired from five

young healthy subjects & viewed by MRIcro.

Anterior Commissure (AC) was chosen as the reference location for all subjects

MRI images were exported in 2 transverse slices per subject.

MethodBrain MRI images were loaded onto “ImageJ”.

By using a line tool and fill function, outlines were drawn from 0 to 360˚ within the image.

Color picker function was used with pencil tool. land markers were made at two locations.

Total (ECL) thickness measurements were done by moving a line tool at 10˚increment with use of measurement function.

Scalp

Skull

Angle (θ) from 0° to 360° Record: Total ECL thickness made by Δ10° increment

0, 360˚

90˚

180˚

270˚

Angle (θ) Position

Note: MR images were calibrated by 1 x 1 mm ECL thickness

Region I: 320-40

˚

Region II: 50-130˚

Region III: 140-

220˚

Region IV: 230-

310˚

0

5

10

15

20

15.2 ± 4.58

15.7 ± 3.83

14.2 ± 2.12

14.9 ± 3.88

Mean ECL thickness of sub-jects (1-5) at [AC] based on

region

Mean

EC

L t

hic

kn

ess

(m

m)

Region I: 320-40

˚

Region II: 50-130˚

Region III: 140-

220˚

Region IV: 230-

310˚

0

5

10

15

20

10.9 ± 2.10

13.6 ± 2.3314.9 ± 2.18

13.2 ± 2.45

Mean ECL thickness of sub-jects (1-5) at [AC+20mm]

based on region

Mean

EC

L t

hic

kn

ess (

mm

)

Results

AC AC+20mm0

2

4

6

8

10

12

14

16

1815.1 ± 1.30

13.1±1.66

Mean ECL thickness from Sub 1-5 at AC vs AC+20mm

Mean

EC

L t

hic

kn

ess (

mm

)

n = 37

*p < 0.05

∆ in ECL thickness between AC & AC +20mm: ECL thickness was slightly thinner at AC above 20mm!

2 4 6 8 10 12 14 16 18 20 22 24 260%

20%

40%

60%

80%

100%

Effect of ECL thickness on% brain signal

ECL thickness (mm)

Bra

in c

on

trib

uti

on

to t

ota

l p

ath

len

gth

Mean, AC+20, Region I35%

As ECL thickness increased, % of brain signal is reduced

Subject 3, AC, Region I9%

Mean, AC+20, Region IBrain contribution: 35%

Subject 3, AC, Region IBrain contribution: 9%

0 10 20 30 40 50 60-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 10 20 30 40 50 60-0.02

0

0.02

0.04

0.06

0.08

0.1

0 10 20 30 40 50 60-2

0

2

4

6

8

10

12

14x 10

-3

Pure brain curve

Pure scalp curve

ICG

concen

tration (uM)

Time (s)

Time (s)

ICG

concen

tration (uM)

Time (s)

ICG

concen

tration (uM)

Discussion

Main limitations: There isn’t a decent imaging tool

which resolves different ECL layers separately.

Source of errors: . Small sample size (5 subjects) . Resolution of MR image quality (reduced with “zoom in” function)

Discussion

Future works on identifying ECL thickness variability:

By standardizing variation of ECL thickness, more precise and reliable CBF measurements can be made on adults.

In near future, individualized approach on making an adjustment on CBF measurements are possible by removing ECL (skull and scalp) contamination.

ConclusionMean ECL thickness across the circumference from

subject 1 to 5 at AC+20mm was thinner by 2mm than at AC.

Based on regions (I-IV), forehead region I at AC above 20mm was shown to be the thinnest mean ECL thickness.

Thus, thinner the ECL thickness, better the CBF measurements by increased in contribution of brain signal (more light propagates into brain and reduced ECL contamination).