Neuroimaging at 1.5 T and 3.0 T: comparison of oxygenation-sensitive magnetic resonance imaging
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Magnetic Resonance in Medicine 45:595– 604 (2001)
Neuroimaging at 1.5 T and 3.0 T: Comparison of Oxygenation-Sensitive Magnetic Resonance Imaging
Gunnar Kruger,* Andreas Kastrup, and Gary H. Glover ¨
Noise properties, the signal-to-noise ratio (SNR), contrast-tonoise ratio (CNR), and signal responses were compared during functional activation of the human brain at 1.5 and 3.0 T. At the higher field spiral gradient-echo (GRE) brain images revealed an average gain in SNR of 1.7 in fully relaxed and 2.2 in images with a repetition time (TR) of 1.5 sec. The tempered gain at longer TRs reflects the fact that the physiological noise depends on the signal strength and becomes a larger fraction of the total noise at 3.0 T. Activation of the primary motor and visual cortex resulted in a 36% and 44% increase of “activated pixels” at 3.0 T, which reflects a greater sensitivity for the detection of activated gray matter at the higher field. The gain in the CNR exhibited a dependency on the underlying tissue, i.e., an increase of 1.8 in regions of particular high activation-induced signal changes (presumably venous vessels) and of 2.2 in the average activated areas. These results demonstrate that 3.0 T provides a clear advantage over 1.5 T for neuroimaging of homogeneous brain tissue, although stronger physiological noise contributions, more complicated signal features in the proximity of strong susceptibility gradients, and changes in the intrinsic relaxation times may mediate the enhancement. Magn Reson Med 45:595– 604, 2001. © 2001 Wiley-Liss, Inc. Key words: neuroimaging; spiral scan; magnetic field strength; CNR; SNR; physiological noise
suggesting that high field fMRI methods may be able to resolve oxygenation changes in small vessels and capillaries, which are spatially localized near the origin of the neuronal activity. In the present study, various BOLD-relevant properties were compared at 1.5 T and 3.0 T. In order to establish identical BOLD-sensitivities, we investigated the T * relax2 ation times for gray matter at each field strength and scaled the corresponding echo time (TE), and the excitation angle at 3.0 T. We compared intrinsic noise contributions and the SNR in gradient-echo (GRE) images and examined activation-induced BOLD responses during visual and motor activation at both fields in terms of spatial extent, the mean z -score, and the CNR of “activated voxels.” T * -maps 2 from various brain sections were calculated to investigate spatial aspects and the field dependency on signal distortions in the proximity of large susceptibility gradients. Imaging parameters such as temporal and spatial resolution and sampling time ( T s ) were identical for both scanners to keep SNR properties and total scan times unchanged. THEORY Signal-to-Noise The SNR in high-field MR-images has been shown to be proportional to B 0 (2) and is given by: SNR V Ts f T1 , T2 , T* , 2 [1]
MRI modalities are often limited by the signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR). Both SNR and CNR have been shown to increase with magnetic field strength B 0 (1,2). Consequently, the “optimal field strength” and the field dependency in blood oxygenation level dependent (BOLD) MRI have been the subject of various investigations (1,3– 6). For many MRI applications a magnetic field strength of 1.5 Tesla (T) seems to represent a good compromise. Functional MRI (fMRI), however, is particularly dependent on good SNR and CNR properties, since typically observed BOLD signal changes at 1.5 T are on the order of a few percent and often exceed the intrinsic noise only slightly. Several biophysical models of activation-induced changes of the oxygenation-sensitive MRI signals have proposed that the changes in the relaxation rate R * and subsequently the BOLD effect are propor2 tional to B 0 for large vessels and proportional to B 0 2 for small vessels and capillaries (7,8). Thus, higher fields may provide an important improvement in fMRI. Indeed, recent investigations have demonstrated a superlinear increase in the BOLD CNR with the field strength (1,4,5),
where is the operating frequency, V is the voxel size, T s is the sampling time, f 1 is a function that depends on the k -space trajectory in the given pulse-sequence, and is a function of various tissue parameters ( T 1 , T 2 , T * ). 2 Thus, Eq. [1] suggests that a higher field strength directly improves the SNR for a given pulse sequence. However, with higher fields T 2 and T * decrease, while T 1 increases. 2 Since T 1 increases with B 0 ( 30% for 3.0 T vs. 1.5 T) the linear gain from with the field in GRE acquisitions with the Ernst angle ( E ) is reduced by partial saturation effects unless TR T 1 . A recent investigation on the SNR demonstrates that the signal strength S increases with the square of the B 0 -field and that the noise is proportional to B 0 (2). The total noise N in an MR image consists of at least three different noise sources (2,9):
T 2
0 2
Lucas MRS Center, Department of Radiology, Stanford University, Palo Alto, CA. Grant sponsor: Deutsche Forschungsgemeinschaft; Grant numbers: Kr 1896/ 1-1; Ka 1419/1-1; Grant sponsor: NIH NCRR; Grant number: RR09784. *Correspondence to: Gunnar Kruger, Lucas MRS Center, Department of ¨ Radiology, Stanford University, Palo Alto, CA 94305. E-mail: gkruege@s-word.stanford.edu Received 24 June 2000; revised 1 September 2000; accepted 16 October 2000. © 2001 Wiley-Liss, Inc.
S
2
P
2
,
[2]
where T is the thermal noise, S is systematic noise, P physiological noise, and 0 is the sum of T and S . The physiological noise includes contributions from fluctua-
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