Assessment of temporal state-dependent interactions between auditory fMRI responses to desired and undesired acoustic sources.

A confounding factor in auditory functional magnetic resonance imaging (fMRI) experiments is the presence of the acoustic noise inherently associated with the echo planar imaging acquisition technique. Previous studies have demonstrated that this noise can induce unwanted neuronal responses that can mask stimulus-induced responses. Similarly, activation accumulated over multiple stimuli has been demonstrated to elevate the baseline, thus reducing the dynamic range available for subsequent responses. To best evaluate responses to auditory stimuli, it is necessary to account for the presence of all recent acoustic stimulation, beginning with an understanding of the attenuating effects brought about by interaction between and among induced unwanted neuronal responses, and responses to desired auditory stimuli. This study focuses on the characterization of the duration of this temporal memory and qualitative assessment of the associated response attenuation. Two experimental parameters--inter-stimulus interval (ISI) and repetition time (TR)--were varied during an fMRI experiment in which participants were asked to passively attend to an auditory stimulus. Results present evidence of a state-dependent interaction between induced responses. As expected, attenuating effects of these interactions become less significant as TR and ISI increase and in contrast to previous work, persist up to 18s after a stimulus presentation.

Acoustic echoplanar scanner noise and pure tone hearing thresholds: the effects of sequence repetition times and acoustic noise rates.

PURPOSE: Our goal was to determine the effects of acoustic echoplanar scanner noise on pure tone hearing thresholds in normal volunteers and to determine the influence of echoplanar sequence repetition time on threshold effects. METHOD: With use of a calibrated audiometer, pure tones ranging from 125 to 8,000 Hz were delivered monaurally to 10 normal-hearing volunteers in a quiet MR scanner suite and in the presence of acoustic scanner noise produced by three separate single shot blipped echoplanar pulse sequences varying only in repetition time (TR = 1,000, 2,000, or 3,000 ms), with all other parameters including the number of slices held constant. The magnitude of noise-induced threshold changes and the slopes of the threshold curves produced by each of the three echoplanar pulse sequences were then analyzed using multiple comparisons and a least significant difference method. The shapes of the threshold curves produced in each background state were best fit using a quadratic effect for frequency in a mixed effects linear model and compared using F test statistics. RESULTS: All of the volunteers demonstrated entirely normal hearing thresholds throughout the full range of tonal frequencies tested (< 25 dB) when no acoustic scanner noise was present in the scanner suite. Pure tone hearing thresholds significantly increased (p < 0.01) in the presence of acoustic scanner noise, with the magnitude of change inversely proportional to the repetition time and therefore the rate of periodic noise production by the echoplanar sequence used. The shape of the threshold curve in the presence of noise produced by the 1,000 ms TR sequence was not equivalent across the frequency spectrum tested but had a quadratic distribution with peak effects at 750-2,000 Hz. As the repetition time was increased and the periodic noise rate decreased, the magnitude of the noise-induced threshold changes significantly lessened (p < 0.01) and the quadratic distributions of the threshold curves changed significantly (p < 0.01), tending toward a more planar configuration. CONCLUSION: Background acoustic echoplanar scanner noise can significantly increase pure tone thresholds in the optimal frequency hearing range (125-8,000 Hz). However, the threshold effects are not equivalent across the frequency spectrum, and the magnitude of threshold changes is dependent on the rate at which periodic acoustic scanner noises are produced for a given sequence repetition time.

Functional MR of the primary auditory cortex: an analysis of pure tone activation and tone discrimination.

PURPOSE: To use functional MR imaging to measure the effect of frequency (pitch), intensity (loudness), and complexity of auditory stimuli on activation in the primary and secondary auditory cortexes. METHODS: Multiplanar echo-planar images were acquired in healthy subjects with normal hearing to whom auditory stimuli were presented intermittently. Functional images were processed from the echo-planar images with conventional postprocessing methods. The stimuli included pure tones with a single frequency and intensity, pure tones with the frequency stepped between 1,000, 2,000, 3,000, or 4,000 Hz, and spoken text. The pixels activated by each task in the transverse temporal gyrus (TTG) and the auditory association areas were tabulated. RESULTS: The pure tone task activated the TTG. The 1,000-Hz tone activated significantly more pixels in the TTG than did the 4,000-Hz tone. The 4,000-Hz tone activated pixels primarily in the medial TTG, whereas the 1,000-Hz tone activated more pixels in the lateral TTG. Higher intensity tones activated significantly more pixels than did lower intensity tones at the same frequency. The stepped tones activated more pixels than the pure tones, but the difference was not significant. The text task produced significantly more activation than did the pure tones in the TTG and in the auditory association areas. The more complex tasks (stepped tones and listening to text) tended to activate more pixels in the left hemisphere than in the right, whereas the simpler tasks activated similar numbers of pixels in each hemisphere. CONCLUSION: Auditory stimuli activate the TTG and the association areas. Activation in the primary auditory cortex depends on frequency, intensity, and complexity of the auditory stimulus. Activation of the auditory association areas requires more complex auditory stimuli, such as the stepped tone task or text reading.

Magnetization transfer or spin-lock? An investigation of off-resonance saturation pulse imaging with varying frequency offsets.

PURPOSE: To characterize near-resonance saturation pulse MR imaging on a 1.5-T scanner in order to gain insight into underlying mechanisms that alter tissue contrast and to optimize the technique for neuroimaging. METHODS: Off-resonance saturation pulses were applied to T1-weighted, spin-density-weighted, and T2-weighted sequences at frequency offsets ranging from 50 Hz to 20,000 Hz down field from water resonance. Suppression ratios were determined at each offset for phantom materials (MnCl2 solution, gadopentetate dimeglumine, corn oil, water, and agar), normal brain structures, and a variety of brain lesions. RESULTS: Signal suppression of MnCl2 on T1-weighted images occurred at offsets of less than 2000 Hz even though no macromolecules were present in the solution. Only those phantom materials and tissues with short or intermediate T1 relaxation times and relatively large T1/T2 ratios were sensitive to changing frequency offsets. Suppression of brain increased from approximately 20% at 2000 Hz offset to approximately 45% when the offset was reduced to 300 Hz. In human subjects, the net effect of reducing the frequency offset was to increase T2 contrast on T1-weighted, spin-density-weighted, and T2-weighted images. Distilled water and contrast material did not suppress except at very low offsets ( < 300 Hz). A frequency offset of 300 Hz was optimal for maximizing conspicuity between most contrast-enhancing lesions and adjacent brain while preserving anatomic detail. CONCLUSION: Suppression of MnCl2 indicates that magnetization transfer is not the sole mechanism of contrast in near-resonance saturation MR imaging. Spin-lock excitation can reasonably explain the behavior of the phantom solutions and the increase in T2 contrast of tissues achieved as the frequency offset is decreased from 2000 Hz to 300 Hz. Below 300 Hz, saturation is presumably caused by spin-tip effects. With our pulse design, an offset of 300 Hz is optimal for many routine clinical imaging examinations.

Topical fumagillin in the treatment of microsporidial keratoconjunctivitis in AIDS.

OBJECTIVE: To report microsporidial keratoconjunctivitis in a patient infected with HIV who was treated with topical fumagillin. CASE SUMMARY: A 37-year-old white man who was experiencing redness, pain, irritation, decreased vision, and a foreign body sensation occurring in both eyes was referred to the ophthalmology clinic. The patient had a medical history significant for AIDS, Pneumocystis carinii pneumonia, and Cytomegalovirus retinitis. Conjunctival smears were taken and stained positive for the presence of Microsporidia. The patient was diagnosed with bilateral microsporidial keratoconjunctivitis and fumagillin therapy was initiated. After 5 days of therapy, the patient reported significant improvements characterized by a decrease in blurred vision with only slight blurring in the left eye, decrease in headache, and decreased foreign body sensation. The patient continued topical fumagillin therapy for more than 14 months, with only slight blurring in the left eye and no apparent ocular toxicity as a result of fumagillin therapy. DISCUSSION: Although rare in occurrence, increasing numbers of documented microsporidial infections are being reported in the medical literature, particularly in individuals who are seropositive for HIV. Clinicians need to be cognizant of microsporidial keratoconjunctivitis as another opportunistic infection in this patient population. CONCLUSIONS: Although a curative agent has yet to be discovered, fumagillin represents a safe, effective, low-cost, topical agent for the treatment of microsporidial keratoconjunctivitis.