Patrick J. Ledden

Improved auditory cortex imaging using clustered volume acquisitions

The effects of the noise of echo‐planar functional magnetic resonance imaging on auditory cortex responses were compared for two methods of acquiring functional MR data. Responses observed with a distributed volume acquisition sequence were compared to those obtained with a clustered volume acquisition sequence. In the former case, slices from the volume were acquired at equal intervals within the repetition time, whereas the latter acquired all slices in rapid succession at the end of the imaging period. The clustered volume acquisition provides a period of quiet during which a stimulus may be presented uninterrupted and uncontaminated by the noise of echo‐planar imaging. Both sequences were implemented on a General Electric Signa imager retrofitted for echo‐planar imaging by Advanced NMR Systems, Inc. The sequences were used to acquire 60 images per slice of a fixed volume of cerebral cortex while subjects were presented an instrumental music stimulus in an On vs. Off paradigm. Data were acquired for both sequences using TR values of 2, 3, 4, 6 and 8 sec. The clustered volume acquisition sequence was found to yield greater measures of dynamic range (percent signal change, mean statistical power per unit imaging time) across the tested range of TR values. Observations of more consistent spatial extent of responses, greater mean signal changes, and higher and more consistent values of mean t‐statistic per unit imaging time demonstrate the efficacy of using a clustered volume acquisition for fMRI of auditory cortex. Hum. Brain Mapping 7:89–97, 1999.

Imaging Subcortical Auditory Activity in Humans

There is a lack of physiological data pertaining to how listening humans process auditory information. Functional magnetic resonance imaging (fMRI) has provided some data for the auditory cortex in awake humans, but there is still a paucity of comparable data for subcortical auditory areas where the early stages of processing take place, as amply demonstrated by single‐unit studies in animals. It is unclear why fMRI has been unsuccessful in imaging auditory brain‐stem activity, but one problem may be cardiac‐related, pulsatile brain‐stem motion. To examine this, a method eliminating such motion (using cardiac gating) was applied to map sound‐related activity in the auditory cortices and inferior colliculi in the brain stem. Activation in both the colliculi and cortex became more discernible when gating was used. In contrast with the cortex, the improvement in the colliculi resulted from a reduction in signal variability, rather than from an increase in percent signal change. This reduction is consistent with the hypothesis that motion or pulsatile flow is a major factor in brain‐stem imaging. The way now seems clear to studying activity throughout the human auditory pathway in listening humans. Hum. Brain Mapping 6:33–41, 1998.

Design of a SENSE‐optimized high‐sensitivity MRI receive coil for brain imaging

An 8‐channel receive‐only detector array was developed for SENSE MRI of human brain. The coil geometry was based on a gapped element design and used ultra‐high impedance preamplifiers for mutual decoupling of the elements. Computer simulations of the electric and magnetic fields showed that excellent signal‐to‐noise ratio (SNR) and SENSE performance could be achieved by placing the coil elements close to the head and maintaining a substantial gap between the elements. Measurements with a 1.5 T prototype coil showed a 2.7‐fold improvement of the SNR averaged over the brain compared to a conventional quadrature birdcage receive coil and an average geometrical noise amplification factor (g‐value) of 1.06 and 1.38 for rate‐2 and rate‐3 SENSE, respectively. Magn Reson Med 47:1218–1227, 2002. Published 2002 Wiley‐Liss, Inc.

Signal-to-noise ratio and parallel imaging performance of a 16-channel receive-only brain coil array at 3.0 Tesla†

The performance of a 16‐channel receive‐only RF coil for brain imaging at 3.0 Tesla was investigated using a custom‐built 16‐channel receiver. Both the image signal‐to‐noise ratio (SNR) and the noise amplification (g‐factor) in sensitivity‐encoding (SENSE) parallel imaging applications were quantitatively evaluated. Furthermore, the performance was compared with that of hypothetical coils with one, two, four, and eight elements (n) by combining channels in software during image reconstruction. As expected, both the g‐factor and SNR improved substantially with n. Compared to an equivalent (simulated) single‐element coil, the 16‐channel coil showed a 1.87‐fold average increase in brain SNR. This was mainly due to an increase in SNR in the peripheral brain (an up to threefold SNR increase), whereas the SNR increase in the center of the brain was 4%. The incremental SNR gains became relatively small at large n, with a 9% gain observed when n was increased from 8 to 16. Compared to the (larger) product birdcage head coil, SNR increased by close to a factor of 2 in the center, and by up to a factor of 6 in the periphery of the brain. For low SENSE acceleration (rate‐2), g‐factors leveled off for n > 4, and improved only slightly (1.4% averaged over brain) going from n = 8 to n = 16. Improvements in g for n > 8 were larger for higher acceleration rates, with the improvement for rate‐3 averaging 12.0%. Magn Reson Med 51:22–26, 2004. Published 2003 Wiley‐Liss, Inc.

Multislice Perfusion and Perfusion Territory Imaging in Humans With Separate Label and Image Coils

An arterial spin labeling technique using separate RF labeling and imaging coils was used to obtain multislice perfusion images of the human brain at 3 T. Continuous RF irradiation at a peak power of 0.3 W was applied to the carotid arteries to adiabatically invert spins. Labeling was achieved without producing magnetization transfer effects since the B1 field of the labeling coil did not extend into the imaging region or couple significant power into the imaging coil. Eliminating magnetization transfer allowed the acquisition of multislice perfusion images of arbitrary orientation. Combining surface coil labeling with a reduced RF duty cycle permitted significantly lower SAR than single coil approaches.

Quantitative assessment of auditory cortex responses induced by imager acoustic noise

A clustered volume acquisition functional MRI pulse sequence was modified to assess the response to the acoustic noise of echo‐planar imaging in the auditory cortex and to determine whether it is possible to obtain data which is relatively free of acoustic contamination. The spatial location and strength (percent signal change) of cortical responses to the imager noise were examined by introducing extra gradient readouts, without slice excitation, to provide acoustic stimulation immediately prior to acquisition of a cerebral volume. The duration of acoustic stimulation was controlled by varying the number of extra gradient readouts. Slice acquisitions were clustered at the end of the repetition time (TR) period to prevent a response from being induced by the volume acquisition itself (“Intra‐Acquisition Response”). The cerebral volumes were acquired using a long TR in order to limit the integration of the cortical response across volume acquisitions (“Inter‐Acquisition Response”). Cortical responses were observed to be largest and most significant on the medial two‐thirds of Heschls gyrus, the location of primary auditory cortex. Mean signal changes induced by the imager noise were observed to be as high as 0.95%. A 2 sec delay prior to onset of the BOLD response was empirically determined. These results demonstrate that clustered volume acquisitions may be utilized for up to 2 sec of volume acquisition without inducing an appreciable Intra‐Acquisition Response and can be used, with a sufficiently long TR, to provide data which are similarly free of any Inter‐Acquisition Response. Hum. Brain Mapping 7:79–88, 1999.

Whole-brain 3D perfusion MRI at 3.0 T using CASL with a separate labeling coil†

A variety of continuous and pulsed arterial spin labeling (ASL) perfusion MRI techniques have been demonstrated in recent years. One of the reasons these methods are still not routinely used is the limited extent of the imaging region. Of the ASL methods proposed to date, continuous ASL (CASL) with a separate labeling coil is particularly attractive for whole‐brain studies at high fields. This approach can provide an increased signal‐to‐noise ratio (SNR) in perfusion images because there are no magnetization transfer (MT) effects, and lessen concerns regarding RF power deposition at high field because it uses a local labeling coil. In this work, we demonstrate CASL whole‐brain quantitative perfusion imaging at 3.0 T using a combination of strategies: 3D volume acquisition, background tissue signal suppression, and a separate labeling coil. The results show that this approach can be used to acquire perfusion images in all brain regions with good sensitivity. Further, it is shown that the method can be performed safely on humans without exceeding the current RF power deposition limits. The current method can be extended to higher fields, and further improved by the use of multiple receiver coils and parallel imaging techniques to reduce scan time or provide increased resolution. Magn Reson Med 52:131–140, 2004. Published 2004 Wiley‐Liss, Inc.

Scalable multichannel MRI data acquisition system.

A scalable multichannel digital MRI receiver system was designed to achieve high bandwidth echo‐planar imaging (EPI) acquisitions for applications such as BOLD‐fMRI. The modular system design allows for easy extension to an arbitrary number of channels. A 16‐channel receiver was developed and integrated with a General Electric (GE) Signa 3T VH/3 clinical scanner. Receiver performance was evaluated on phantoms and human volunteers using a custom‐built 16‐element receive‐only brain surface coil array. At an output bandwidth of 1 MHz, a 100% acquisition duty cycle was achieved. Overall system noise figure and dynamic range were better than 0.85 dB and 84 dB, respectively. During repetitive EPI scanning on phantoms, the relative temporal standard deviation of the image intensity time‐course was below 0.2%. As compared to the product birdcage head coil, 16‐channel reception with the custom array yielded a nearly 6‐fold SNR gain in the cerebral cortex and a 1.8‐fold SNR gain in the center of the brain. The excellent system stability combined with the increased sensitivity and SENSE capabilities of 16‐channel coils are expected to significantly benefit and enhance fMRI applications. Magn Reson Med 51:165–171, 2004. Published 2003 Wiley‐Liss, Inc.

Measurement of human myocardial perfusion by double-gated flow alternating inversion recovery EPI.

This paper presents a flow‐sensitive alternating inversion recovery (FAIR) method for measuring human myocardial perfusion at 1.5 T. Slice‐selective/non‐selective IR images were collected using a double‐gated IR echoplanar imaging sequence. Myocardial perfusion was calculated after T1 fitting and extrapolation of the mean signal difference SISel ‐ SINSel. The accuracy of the method was tested in a porcine model using graded intravenous adenosine dose challenge. Comparison with radiolabeled microsphere measurements showed a good correlation (r = 0.84; mean error = 20%, n = 6) over the range of flows tested (0.9–7 ml/g/min). Applied in humans, this method allowed for the measurement of resting myocardial flow (1.04 ± 0.37 ml/g/min, n = 11). The noise in our human measurements (SEflow = 0.2 ml/g/min) appears to come primarily from residual respiratory motion. Although the current signal‐to‐noise ratio limits our ability to measure small fluctuations in resting flow accurately, the results indicate that this noninvasive method has great promise for the quantitative assessment of myocardial flow reserve in humans. Magn Reson Med 41:510–519, 1999.

Effects of inductive coupling on parallel MR image reconstructions.

Theoretical arguments and experimental results are presented that characterize the impact of inductive coupling on the performance of parallel MRI reconstructions. A simple model of MR signal and noise reception suggests that the intrinsic amount of spatial information available from a given coil array is unchanged in the presence of inductive coupling, as long as the sample remains the dominant source of noise for the coupled array. Any loss of distinctness in the measured coil sensitivities is compensated by information stored in the measured noise correlations. Adjustments to the theory are described to account for preamplifier noise contributions. Results are presented from an experimental system in which preamplifier input impedances are systematically adjusted in order to vary the level of coupling between array elements. Parallel image reconstructions using an array with four different levels of coupling and an acceleration factor up to six show average SNR changes of −7.6% to +7.5%. The modest changes in overall SNR are accompanied by similarly small changes in g‐factor. These initial results suggest that moderate amounts of inductive coupling should not have a prohibitive effect on the use of a given coil array for parallel MRI. Magn Reson Med 52:628–639, 2004.