SQUID-Detected Magnetic Resonance Imaging in Microtesla Magnetic Fields

McDermott, Robert; Kelso, Nathan; Sk, Lee; Mößle, M.; Mück, Michael; Myers, Whittier; Haken, Bernard ten; Seton, Hugh; Trabesinger, Andreas; Pines, Alexander; Clarke, John

doi:10.1023/b:jolt.0000029519.09286.c5

Cited by 74 publications

(54 citation statements)

References 15 publications

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“…This system consists of copper-wire coils wound on wooden or G-10 fiberglass supports that provide the necessary polarizing, precession, excitation, and gradient fields; references [4] and [28] describe this system in detail. We prepolarized the protons in a field of about 100 mT and then turned this field adiabatically so that the spins aligned along the B 0 field.…”

Section: Resultsmentioning

confidence: 99%

Correction of concomitant gradient artifacts in experimental microtesla MRI

Myers

Mößle

Clarke

2005

Journal of Magnetic Resonance

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Magnetic resonance imaging (MRI) suffers from artifacts caused by concomitant gradients when the product of the magnetic field gradient and the dimension of the sample becomes comparable to the static magnetic field. To investigate and correct for these artifacts at very low magnetic fields, we have acquired MR images of a 165-mm phantom in a 66-µT field using gradients up to 350 µT/m. We prepolarize the protons in a field of about 100 mT, apply a spin-echo pulse sequence, and detect the precessing spins using a superconducting gradiometer coupled to a superconducting quantum interference device (SQUID). Distortion and blurring are readily apparent at the edges of the images; by comparing the experimental images to computer simulations, we show that concomitant gradients cause these artifacts. We develop a non-perturbative, postacquisition phase correction algorithm that eliminates the effects of concomitant gradients in both the simulated and the experimental images. This algorithm assumes that the switching time of the phase-encoding gradient is long compared to the spin precession period. In a second technique, we demonstrate that raising the precession field during phase encoding can also eliminate blurring caused by concomitant phaseencoding gradients; this technique enables one to correct concomitant gradient artifacts even when the detector has a restricted bandwidth that sets an upper limit on the precession frequency. In particular, the combination of phase correction and precession field cycling should allow one to add MRI capabilities to existing 300-channel SQUID systems used to detect neuronal currents in the brain because frequency encoding could be performed within the 1-2 kHz bandwidth of the readout system.

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Section: Resultsmentioning

confidence: 99%

Correction of concomitant gradient artifacts in experimental microtesla MRI

Myers

Mößle

Clarke

2005

Journal of Magnetic Resonance

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“…Our SQUID-detected MRI system [17,18,19,20] is shown schematically in Fig referred to the lowest loop of the gradiometer. An array of Josephson tunnel junctions [18] protects the input circuit from the large transient currents that would otherwise be produced by the polarizing pulse.…”

Section: Experimental Configurationmentioning

confidence: 99%

“…An array of Josephson tunnel junctions [18] protects the input circuit from the large transient currents that would otherwise be produced by the polarizing pulse. The SQUID and gradiometer are immersed in liquid helium in a dewar [21] which generates negligible magnetic noise.…”

Section: Experimental Configurationmentioning

confidence: 99%

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SQUID-detected microtesla MRI in the presence of metal

Mößle

Han

Myers

et al. 2006

Journal of Magnetic Resonance

119

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In magnetic resonance imaging (MRI) performed at fields of 1 T and above, the presence of a metal insert can distort the image because of susceptibility differences within the sample and modification of the radiofrequency fields by screening currents.Furthermore, it is not feasible to perform nuclear magnetic resonance (NMR) spectroscopy or acquire a magnetic resonance image if the sample is enclosed in a metal container. Both problems can be overcome by substantially lowering the NMR frequency. Using a microtesla imaging system operating at 2.8 kHz, with a superconducting quantum interference device (SQUID) as the signal detector, we have obtained distortion-free images of a phantom containing a titanium bar and threedimensional images of an object enclosed in an aluminum can; in both cases high-field images are inaccessible.

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“…[The situation could be very different in low field MRI [McDermott et al, 2004;Matlachov et al, 2004;Matlachov et al, 2007] if the strength of the neuromagnetic field near a dendrite is more nearly comparable with that of the measuring field.] Neglecting a transverse component of B 0 , the evolution of M + is given by…”

Section: Theory Basic Equationsmentioning

confidence: 99%

Modeling direct effects of neural current on MRI

Heller

Barrowes

George

2007

Human Brain Mapping

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Abstract:We investigate the effect of the magnetic field generated by neural activity on the magnitude and phase of the MRI signal in terms of a phenomenological parameter with the dimensions of length; it involves the product of the strength and duration of these currents. We obtain an analytic approximation to the MRI signal when the neuromagnetically induced phase is small inside the MRI voxel. The phase shift is the average of the MRI phase over the voxel, and therefore first order in that phase; and the reduction in the signal magnitude is one half the square of the standard deviation of the MRI phase, which is second order. The analytic approximation is compared with numerical simulations. For weak currents the agreement is excellent, and the magnitude change is generally much smaller than the phase shift. Using MEG data as a weak constraint on the current strength we find that for a net dipole moment of 10 nAm, a typical value for an evoked response, the reduction in the magnitude of the MRI signal is two parts in 10 5 , and the maximum value of the overall phase shift is % 4 Á 10 À3 , obtained when the MRI voxel is displaced 2/3 the size of the neuronal activity. We also show signal changes over a large range of values of the net dipole moment. We compare these results with others in the literature.

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SQUID-Detected Magnetic Resonance Imaging in Microtesla Magnetic Fields

Cited by 74 publications

References 15 publications

Correction of concomitant gradient artifacts in experimental microtesla MRI

Correction of concomitant gradient artifacts in experimental microtesla MRI

SQUID-detected microtesla MRI in the presence of metal

Modeling direct effects of neural current on MRI

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