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184


SNR versus resolution in 3D 1H MRS of the human brain at high magnetic fields

Li BS; Regal J; Gonen O
It is commonly accepted that the signal-to-noise ratio (SNR = peak-signal/RMS-noise) per-unit-time of proton MR spectroscopy (1H-MRS) is linearly proportional to the voxel volume. Consequently, with a headcoil and 30-min acquisition, 1 cm3 is considered the SNR-limited spatial resolution barrier in the human brain. However, since local linewidths, Delta(upsilon*) = (piT2*)(-1), at high magnetic fields (B0), are dominated by regional inhomogeneities (DeltaB0), i.e., T2* << T2, reducing the voxel dimensions may increase T2*. This could compensate, in part, for signal loss with volume decrease. It is shown that for two cubic voxels of sides l1 and l2, l1 > l2, as the volume decreases by (l1/l2)3, their SNR ratio is reduced by only (l1/l2)2 due to a commensurate T2* increase of l1/l2. This is demonstrated in a phantom and the brains of volunteers, with 3D 1H-MRS in a headcoil at 4 T. It is shown that while the cubic voxels' dimensions were all halved, reducing their volume eightfold, their metabolites' SNR decreased only fourfold, due to their Delta(upsilon*s') twofold decrease. In other words, both spatial and spectral resolutions were doubled at a significantly, x2, smaller-than-expected SNR loss. This advantage was exploited to produce quality high spatial resolution, 0.75 x 0.75 x 0.75 cm3, metabolic maps in a 27-min acquisition
PMID: 11746567
ISSN: 0740-3194
CID: 27734

Quantifying radiation therapy-induced brain injury with whole-brain proton MR spectroscopy: initial observations

Movsas B; Li BS; Babb JS; Fowble BL; Nicolaou N; Gonen O
PURPOSE: To quantify the extent of neuronal cell loss imparted to the brain by means of radiation therapy through the decline of the amino acid derivative N-acetylaspartate (NAA) by using proton (hydrogen 1) magnetic resonance (MR) spectroscopy. MATERIALS AND METHODS: Proton MR spectroscopy in a clinical MR imager was used to ascertain the amount of whole-brain NAA before and immediately after whole-brain radiation therapy 3-4 weeks later. Eight patients (four women, four men; median age, 55 years; age range, 39-70 years) were studied. All subjects had lung cancer (non-small cell lung cancer [n = 5], small-cell lung cancer [n = 3]) and received either palliative or prophylactic whole-brain radiation therapy. Six of them also underwent a Mini-Mental Status Examination (MMSE) for correlation with the whole-brain NAA. Two-tailed Student t tests were used to evaluate the data. RESULTS: A significant (P = .042) average decline in whole-brain NAA of -0.91 mmol per person was observed in the cohort. No corresponding changes occurred in MMSE scores. There was no significant difference in whole-brain NAA decline between prophylactic and therapeutic whole-brain radiation therapy. CONCLUSION: Since whole-brain NAA loss was detected even when MMSE scores were unchanged, the former seems to be a more sensitive measure of radiation therapy injury than is the latter
PMID: 11687671
ISSN: 0033-8419
CID: 27735

Spatial resolution vs signal-to-noise-ratio (SNR) per-unit-time of 3D 1H MR spectroscopy in the human brain at 4 Tesla [Meeting Abstract]

Li, BS; Regal, J; Gonen, O
ISI:000172126600963
ISSN: 0033-8419
CID: 105110

Multivoxel 3D proton spectroscopy in the brain at 1.5 versus 3.0 T: signal-to-noise ratio and resolution comparison

Gonen O; Gruber S; Li BS; Mlynarik V; Moser E
BACKGROUND AND PURPOSE: The new 3.0-T imagers theoretically yield double the signal-to-noise ratio (SNR) and spectral resolution of 1.5-T instruments. To assess the possible improvements for multivoxel 3D proton MR spectroscopy (1H-MRS) in the human brain, we compared the SNR and spectral resolution performance with both field strengths. METHODS: Three-dimensional 1H-MRS was performed in four 21-29-year-old subjects at 1.5 and 3.0 T. In each, a volume of interest of 9 x 9 x 3 cm was obtained within a field of view of 16 x 16 x 3 cm that was partitioned into four (0.75-cm-thick) 16 x 16-voxel sections, yielding 324 (0.75-cm3) signal voxels per examination. RESULTS: In an acquisition protocol of approximately 27 min, average voxel SNRs increased 23-46% at 3.0 versus 1.5 T in the same brain regions of the same subjects. SNRs for N-acetylaspartate, creatine, and choline, respectively, were as follows: 15.3 +/- 4, 8.2 +/- 2.2, and 8.0 +/- 2.0 at 1.5 T and 22.4 +/- 7.0, 10.1 +/- 3.5, and 10.1 +/- 3.6 at 3.0 T. Spectral resolution (metabolite linewidths) were 3.5 +/- 0.5 Hz at 1.5 T versus 6.1 +/- 1.5 Hz at 3.0 T in approximately 900 voxels. Spectral baselines were noticeably flatter at 3.0 T. CONCLUSION: Expected gains in SNR and spectral resolution were not fully realized in a realistic experiment because of intrinsic and controllable factors. However, the 23-46% improvements obtained enable more reliable peak-area estimation and an 1H-MRS acquisition approximately 50% shorter at 3.0 versus 1.5 T
PMID: 11673168
ISSN: 0195-6108
CID: 27736

Investigation of global absolute N-acetyl aspartate levels in Alzheimer's disease [Meeting Abstract]

McGowan, JC; Clark, CC; Ge, Y; Udupa, J; Grossman, RI; Gonen, O
ISI:000167154800051
ISSN: 0197-4580
CID: 105111

New magnetic resonance spectroscopy strategies

Chapter by: Gonen O; Grossman RI
in: Magnetic resonance spectroscopy in multiple sclerosis by Filippi M; Arnold DL; Comi G [Eds]
Milano ; New York : Springer, 2001
pp. 97-112
ISBN: 8847001234
CID: 3795

The accuracy of whole brain N-acetylaspartate quantification

Gonen O; Grossman RI
A non-localizing pulse sequence to quantify the total amount of N-acetylaspartate (NAA) in the whole brain (WBNAA) was introduced recently [Magn. Reson. Med. 40, 684-689 (1998)]. However, it is known that regional magnetic field inhomogeneities, deltaB0s, arising from susceptibility differences at tissue interfaces, shift and broaden local resonances to outside the integration window, leading to an underestimation of the true amount of NAA in the entire brain. To quantify the upper limit of this loss, the whole-head proton MR spectrum (1H-MRS) of the water was integrated over the same frequency width as the NAA. The ratio of this area/total-water-line was 75 +/- 5% in 5 volunteers. The procedure was repeated with the brain-only water peak, obtained by summing signals only from voxels within that organ from a three-dimensional chemical-shift-imaging (3D CSI) set. It indicated that <10% of the water signal loss occurred in the brain. Therefore, by analogy, WBNAA accounts for >90% of that metabolite
PMID: 11167045
ISSN: 0730-725x
CID: 24401

Quantification of the variability of 3D proton magnetic resonance spectroscopy (1H-MRS) in healthy human brains [Meeting Abstract]

Li, BS; Babb, JS; Soher, BJ; Maudsley, AA; Gonen, O
ISI:000090071300612
ISSN: 0033-8419
CID: 105112

Head-to-head performance comparison of 3D multivoxel proton MR spectroscopy: 1.5 vs 3 Tesla in the human brain [Meeting Abstract]

Gruber, S; Li, BS; Soher, BJ; Mlynarik, V; Moser, EV; Gonen, O
ISI:000090071300613
ISSN: 0033-8419
CID: 105113

Assessing the damage to the brain from cancer radiation-therapy with whole brain N-acetylaspartate (WBNAA) obtained with 1H-MRS [Meeting Abstract]

Movsas, B; Li, BS; Babb, JS; Fowble, BL; Nicolaou, N; Gonen, O
ISI:000090071300801
ISSN: 0033-8419
CID: 105114