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