Shizhe Li1, Li An1, Christopher Johnson1, Maria Ferraris-Araneta1, Milalynn Victorino1, Jyoti Tomar1, and Jun Shen1
1National Institutes of Health, Bethesda, MD, United States
Synopsis
This study demonstrates the feasibility of detecting carbonic
anhydrase activity in the frontal lobe of the human brain. Upon saturation of
carbon dioxide, a very large magnetization transfer effect catalyzed by
carbonic anhydrase was measured in the frontal lobe of healthy human subjects
using 13C MRS with oral administration of [U-13C6]glucose. The results showed that it is feasible to examine
carbonic anhydrase activity using magnetization transfer 13C MRS in the frontal cortex—where structural lesions,
disturbed function, and morphology are strongly associated with many
psychiatric symptoms.
INTRODUCTION
Carbonic anhydrase (CA) is an enzyme that
catalyzes the reversible hydration of carbon dioxide and dehydration of
bicarbonate. It plays a central role in life including in the CNS. CA
abnormalities have been implicated in many brain disorders including several
psychiatric disorders.1-3 Both carbonic anhydrase inhibitors and
activators may have significant therapeutic effects.2-6
Using the technique of 13C magnetization transfer (MT) MRS, CA
activity was previously measured in in the occipital lobe of the human brain.7
Here we demonstrate that CA activity can be measured in the frontal lobe of the
human brain using 13C MT MRS, demonstrating the feasibility of
characterizing CA in psychiatric disorders. The MT effects catalyzed by CA in
frontal and occipital lobes are also compared.METHODS
Hardware: 13C MRS was performed
on a Siemens 7T scanner using a home-built coil assembly. The proton coil was a
shielded quadrature half-volume coil (two overlapping octagon loops, the nominal
size of each loop was 12.7 x 12.7 cm2). The 13C coil was a 7-cm
surface coil. For each subject, the angle of the headrest was adjusted so that
the skin of the forehead is parallel to the B0 field. As a result, the
axial 13C B1 field is perpendicular to the static
magnetic field (Fig. 1).
Glucose administration:
Healthy volunteers were instructed to fast overnight before the scan. Solution
of 20% w/w 99% enriched [U-13C6]glucose was orally
administrated (n = 3) at the dosage of 0.75 g [U-13C6]glucose
per kg of body weight.8
13C
MRS: A rectangular
voxel of 3.5 cm (SI) x 4 cm (AP) x 5 cm (LR) in the frontal lobe was shimmed by adjusting first- and second-order shims. The typical half-height water linewidth
from the shimmed voxel was 17~20 Hz. The pulse sequence consisted of the following interleaved
acquisition scheme: {control irradiation – bicarbonate excitation –
acquisition} – {carbon dioxide saturation – bicarbonate excitation –
acquisition}. TR = 30 s, SW = 8 kHz, data point = 2048, with NA = 8 per
spectrum. The 13C excitation hard pulse (duration = 0.25 ms) was
placed at the bicarbonate frequency (160.7 ppm). No proton decoupling was
applied. During the 30 s delay,
a 50 ms pulse block was repeatedly executed. In each pulse block, a 1.0 ms
proton hard pulse (120o) was applied first at the water frequency to
generate heteronuclear NOE, then a 48.0 ms continuous wave pulse (γB1 = 50 Hz) was applied at 228.0 ppm for
control irradiation or at 125.0 ppm to saturate carbon dioxide. The average RF power for the entire sequence was
3 W.RESULTS
Fig. 2 shows time-course spectra acquired from a subject after
oral administration of [U-13C6]glucose without saturation
of carbon dioxide. Glutamate C1 and C5; glutamine C1 and C5; aspartate C1 and C4;
as well as bicarbonate C1 were detected in the carboxylic/amide region. Fig. 3A
shows the MT effect catalyzed by CA during the 90-120 min interval after oral administration.
Spectrum (a) is the sum of the last three control spectra (NA = 24), (b) is the
sum of the last three spectra with saturation of carbon dioxide (NA = 24), and (c)
is the difference spectrum. The saturation transfer ratio (the ratio of the bicarbonate peak area with carbon dioxide
saturation to that with control irradiation) was 0.33 ± 0.06 (n = 3) in the
frontal lobe. For the comparison purpose, the corresponding spectra from the
occipital lobe of a different subject are shown in Fig. 3B7. The saturation
transfer ratio calculated from the occipital lobe spectra was 0.34 ± 0.03 (n = 3).
An unpaired t-test was performed to compare the saturation transfer ratios from the two regions. There was no statistical difference in the carbonic anhydrase
activities between the occipital (n = 3) and frontal lobes (n = 3) of healthy
subjects.DISCUSSION
This
study demonstrates that it is practically feasible to measure CA activity in
vivo in the frontal lobe of the human brain and to characterize this important
enzyme in many neuropsychiatric disorders. To our best knowledge, this is the
first report of detection of carbonic anhydrase activity in the frontal lobe of
human brain in vivo.
Frontal
lobe 13C MRS has been challenging due to technical hurdles. The
frontal sinus, nasal cavity, and sphenoid sinus lay below the frontal cortex.
The sinuses reduce the total volume of brain tissue near the 13C
coil. They also create strong B0 field inhomogeneity in the frontal
lobe. As the result, the linewidth of the frontal lobe spectra (Fig. 3A)
was generally greater than that from the occipital lobe (Fig. 3B). Despite the above difficulties our results
showed that high quality 13C MT data can be readily acquired. The saturation
transfer ratio of the frontal lobe was found to be essentially the same as that
of the occipital lobe. The dramatic signal change in the bicarbonate signal
intensity due to MT effect makes this technique highly viable for a broad range
of clinical applications involving carbonic anhydrase.Acknowledgements
The authors gratefully acknowledge the support
of the Intramural Research Program of the National Institute of Mental Health,
National Institutes of Health.References
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