Biomedical subjects

T Loenneker

Publications and source records attributed to T Loenneker.

5 recordsLinked to original sources

PubMed2001

Silent BOLD imaging.

Pulsed magnetic field gradients in magnetic resonance imaging produce high levels of acoustic noise. In functional magnetic resonance imaging, acoustic scanner noise causes unwanted masking effects. Recently, we proposed a method to perform magnetic resonance imaging experiments undisturbed by acoustic scanner noise by utilizing the property of standard gradient coils to poorly submit acoustic noise in the low frequency range. The silent gradient scheme is now incorporated into a standard T(2)*-weighted sequence. Additionally, simultaneous multi-slice excitation (SIMEX) pulses were implemented to improve the intrinsic low volume coverage of the silent sequence. The proposed silent SIMEX technique was tested and compared with a standard noisy technique using auditory and visual stimulation paradigms. The scanner noise during the silent experiments could be reduced below the range of the ambient noise of the magnet room. This feasibility study shows a trend of decreased activated areas in the noisy experiment for both, the visual and auditory paradigm.

Brain↗

PubMed2000

Effect of pentobarbital on visual processing in man.

To investigate the effect of sedative agents on visual processing in humans, we analysed the BOLD contrast signal response to a visual stimulation paradigm in 15 healthy, adult volunteers using functional magnetic resonance imaging. The subjects were tested during alert state and under sedation following intravenous administration of pentobarbital. The injection of pentobarbital not only significantly reduced the response signal strength but the reduction in BOLD contrast signal was related to the ratio of amount of sedative administered and the subject's body weight. The three subjects with the highest relative sedative dosage even displayed an inverted (negative) BOLD contrast signal. A significant reduction in the number of positively correlating pixels was found 15 min after administration of pentobarbital. All measured parameters returned to near pre-sedative levels by the end of the experimental session. The relative dosage dependence of the strength of the BOLD signal the negative BOLD signal in the three subjects with the highest relative sedative dosage indicates that pentobarbital had a more pronounced effect on cerebral blood flow than on cerebral oxidative metabolism.

Adult↗

PubMed1999

"Silent" MRI with soft gradient pulses.

A method to reduce the acoustic noise generated by gradient systems in magnetic resonance imaging (MRI) is proposed based on the linear response theory. Since the acoustic frequency response function of typical gradient coils is low in the range below 200 Hz, the noise level can be significantly reduced by using gradient pulse sequences whose spectra are limited to this frequency range. Such "soft," i.e., band-limited, pulse shapes can be designed using sinusoidal ramps individually adjusted to available delays. "Silent" versions of three basic MRI sequences [gradient-echo (GE), spin-echo (SE), and rapid acquisition with relaxation enhancement (RARE)] were programmed on 2 and 3 T whole-body scanners. High-quality images could be acquired at noise levels as low as 40 dBA (GE and SE) and 60 dBA (RARE).

Brain↗

PubMed1999

Visual processing in infants and children studied using functional MRI.

We studied the development of visual processing in 58 children, ranging from 1 d to 12 y of age (median age 29 mo), using functional magnetic resonance imaging. All but nine children had either been sedated using chloral hydrate (n = 12) or pentobarbital (n = 28). Nine children were studied under a full halothane/ N2O:O2 anesthesia. In the first postnatal month, 30% of the neonates showed a positive blood oxygenation level-dependent (BOLD) contrast signal, whereas, for infants between the ages of 1 mo and 1 y, 27% did so. Thirty-one percent of children between 1 and 6 y of age and 71% of children aged 6 y and above showed a positive BOLD contrast signal change to our visual stimulation paradigm. Besides the usual positive BOLD contrast signal change, we also noted that a large portion of the children measured displayed a negative BOLD contrast signal change. This negative BOLD contrast signal change was observed in 30% of children up to 1 mo of age, in 27% between 1 mo and 1 y of age, in 47% between 1 and 6 y of age, and in 14% of children 6 y and older. In the children in which we observed a negative correlating BOLD contrast signal change, the locus was more anterior and more lateral than the positive BOLD contrast signal, placing it in the secondary visual cortical area. The results indicate that when using functional magnetic resonance imaging on children, the primary visual cortical area does not respond functionally in the same manner as that of the adult until 1.5 y of age. This supports earlier clinical and electrophysiologic findings that different cortical mechanisms seem to contribute to visual perception at different times postnatally.

Adult↗

PubMed1996

Multislice interleaved excitation cycles (MUSIC): an efficient gradient-echo technique for functional MRI.

A method providing improved slice efficiency for gradient-echo imaging requiring long echo times, such as in functional neuromagnetic resonance imaging, is presented. To enhance the volume coverage while maintaining the short imaging time of a conventional single-slice gradient-echo technique, an interleaved multislice excitation is performed during the echo time. This technique allows detection of susceptibility changes, e.g., the acquisition of stimulated human cortical activation maps, on clinical MR instruments at multiple planes within total imaging times of a few seconds. The efficiency of the technique is demonstrated in the detection of temporary changes in T2* in functional MRI experiments of the human visual cortex at magnetic field strengths of 2 Tesla and 3 Tesla. Fourteen 128 x 128 slices can be acquired in 13 s to cover a large volume-of-interest in the same time that would be required for single-slice acquisition using the conventional technique.

Adult↗