Echo-Planar Imaging BOLD fMRI in Mice on a 9.4T Vertical Bore Microimager

Govind Nair, Timothy Q Duong

Center for Comparative NeuroImaging, PsychiatryUniversity of Massachusetts Medical School, Worcester, MA 01655

Grant supportsWhitaker Foundation, RG-02-0005

American Heart Association, SDG-0430020NIH, NEI R01 EY014211

NIH, NINDS R01 NS45879

Introduction• Longitudinal imaging of transgenic mice and mouse

disease models allows studies to be performed over their entire life span.

• Narrow-vertical bore magnets (microimagers) are well suited for imaging mice – low cost– availability at high fields– availability of high-performance gradients

• While anatomical imaging is readily available, fMRI in mice on microimagers remains a major challenge

Introduction• Mice fMRI on microimager had been reported using

conventional gradient-echo sequence (Arhens 2001; Huang 1996) and fast spin-echo with exogenous contrast agent sequence (Mueggler 2003)

• These sequences generally yield– Reduced temporal resolution– Reduced SNR per unit time– Reduced sensitivity to BOLD contrast– Increased physiological noises

• Echo-planar imaging overcomes these problems albeit– Increased susceptibility artifact– Harder to implement due to larger eddy current (small bore)– Poor shimming capability on small-bore magnets

Introduction

• Other challenges include– Limited spaces for physiological monitoring

– More difficult to use mechanical ventilation

– Increased susceptibility-induced signal loss due to small brain size and larger air-tissue interfaces

• The goal of this study was

– To develop a sensory-stimulation mouse model for fMRI studies

– Explore echo-planar imaging for fMRI on a 9.4 T microimager

Methods

• Mouse head immobilized with ear, tooth and shoulder bars

• Anesthetized with isoflurane

• Spontaneously breathing mice

• Monitored respiration via a transducer

• Maintained body temperature at 37 ± 0.5 C

Methods• Three sets of experiments were performed:

– Graded isoflurane (0.25, 0.50, 0.75, 1.0, 1.25%) were explored using 10% CO2 to determine the optimal BOLD CNR (n = 9)

– Hindpaw electrical stimulation (1-7 mA) on mice anesthetized under the optimal isoflurane level (n = 6)

– Stimulation were explored in details with 4 and 6 mA and under 0.75% and 1.0% isoflurane (n = 5)

• Relatively high currents were used because isoflurane is a potent anesthetic, relative to the widely used -chloralose

• Bench top observations were also observed in some of the hindpaw-stimulation animal and four additional animals

Imaging Parameters• 9.4 T / 89 mm vertical magnet, 100 G/cm gradient (45 mm ID)

• Surface coil (1.2-1.5 cm ID) – remote tuning and matching from top

• Shimming over an 8-mm thick slab; linewidth of 30-45 Hz

• Single-shot, spin-echo EPI – TR / TE = 2500 ms / 38 ms (TE ~ T2 at 9.4T)– FOV = 2 x 1 cm, matrix = 64x32 (312x312x600 m)– Nine 0.6-mm slices (0.15 mm gap)

• Paradigms– 2 mins baseline, 2 mins CO2 – 2 mins baseline, 1 mins stimulation, 2 mins baseline

• Anatomy obtained with similar parameters but at higher resolution

Data analysis

• Hypercapnia– BOLD percent changes were calculated from a whole-brain ROI

– BOLD contrast-to-noise ratio (CNR) was computed

• Hindpaw stimulation– Cross-correlation maps were calculated

– ROI’s of the hindpaw primary sensory cortex was drawn with reference to the average of all activation maps and anatomy

– Time courses of different conditions were obtained from the same ROI’s without using an activation-map mask

– Percent changes were computed

Hypercapnic Challenge

Isoflurane level(% in air)

Basal Respiratory rate (breaths per minute)

Percentage increase during 10% CO2

0.25 155 ± 19 33 ± 13

0.50 143 ± 24 32 ± 14

0.75 128 ± 15 26 ± 12

1.00 127 ± 10 16 ± 06

1.25 116 ± 18 19 ± 10

SE EPI and BOLD maps due to CO2 challenge

15%

0%

Single-shotNo average

Hypercapnic challenge

0.00 0.25 0.50 0.75 1.00 1.25 1.500

2

4

6

8

10

12

14

BOLD CNR

Percentage isoflurane in air

BO

LD

per

cent

cha

nges

3

4

5

6

7

8

9

Co

ntra

st-to

-no

ise

ratio

0 20 40 60 80 100-2

0

2

4

6

8

10

12

14isoflurane

BO

LD

per

cent

cha

nges

Time series (x 2.5 seconds)

1.00 % 0.75 % 0.50 % 0.25 %

Hypercapnic Stimulation

(N = 1) (N = 9)

Hindpaw stimulation (Group II, n = 4)

0 1 2 3 4 5 6 7 8 9-1

0

1

2

3

4

5

BO

LD

Per

cent

Cha

nges

Stimulation Current (mA)0 20 40 60 80 100

-4

-2

0

2

4

6

8

10

BO

LD

Per

cen

t C

han

ges

Time series (x 2.5 seconds)

1 mA 3 mA 5 mA 7 mA

Stimulation

0.9

CC

0.3

Hindpaw stimulation (Group III, n = 5)

0.75% isoflurane 1.00% isoflurane0

1

2

3

4

BO

LD

Pe

rce

nt

Ch

an

ge

s (

%)

4 mA 6 mA

Work in progress: GE EPI and segmented EPI

Gradient-echo EPI

Gradient-echo BOLD responses to 10% CO2

Multi-segment EPI (78x78x500 m2, no signal average)

Conclusions

• Implemented spin-echo EPI for fMRI study

• Developed a mouse model for sensory stimulation fMRI study– Optimized isoflurane concentration

– Stimulation currents

• These optimal parameters are in good agreement with an isoflurane-anesthetized sensory-stimulation model in rats where MABP, HR and RR and blood-gas measurements were carefully monitored.

• Improvement in spatial resolution and BOLD contrast are under investigation.

Current RR HR MABP

baseline 61 15 394 35 135 6

4 mA 62 18386 26

(387 32)136 6

(136 4)

6 mA 64 15405 37

(406 39)

139 11 (141 6) *

8 mA 64 18 415 38 *

(419 44) **

143 6 *

(148 8) **

* P = 0.01, ** P 0.008 (Liu, Schmidt et al., in press 2004)

RAT DATA: MABP traces and physiology

under 1.15-1.25% isoflurane (n = 6, SD)

4mA

6mA

8mA

20 mmHg

10 s