Biomedical subjects

Richard A E Edden

Publications and source records attributed to Richard A E Edden.

4 recordsLinked to original sources

PubMed2006

Optimized detection of lactate at high fields using inner volume saturation.

In localized proton MR spectroscopy ((1)H-MRS) in vivo, the detection of lactate (Lac) is affected by modulation of its resonances due to homonuclear scalar couplings (J). A simple and convenient way to distinguish Lac from lipids is to set the TE to 1/J so that the Lac signal is inverted while other resonances (such as lipid) remain in-phase. However, at high field strengths, such as 3 Tesla or above, the modulation of the Lac signal is complicated by chemical shift effects that cause modulation patterns to vary within different subregions of the localized volume. Under some conditions the Lac signal may even disappear completely. In this note we introduce the concept of inner volume saturation (IVS), which makes use of high bandwidth spatial pulses to remove the signal corresponding to the regions of the localized volume that contribute unwanted modulation patterns. The method is described theoretically and demonstrated experimentally at 3 Tesla in a phantom and a patient with acute stroke. The phantom measurements indicate that virtually 100% of the Lac signal can be recovered using this method. The method should be feasible at magnetic fields above 3 Tesla, and may also be applied to other coupled spin systems in which modulation effects are important.

Aged↗

PubMed2006

Theoretical and experimental investigation of the VASO contrast mechanism.

Vascular space occupancy (VASO)-dependent functional MRI (fMRI) is a blood-nulling technique capable of generating microvascular cerebral blood volume (CBV)-weighted images. It is shown that at high magnetic field (3.0T) and high spatial resolution (1.89 x 1.89 x 3 mm(3)), the VASO signal changes are too large (6-7%) to originate from CBV effects alone. Additional contributions are investigated theoretically and experimentally as a function of MRI parameters (TR and TE), as well as the signal-to-noise ratio, (SNR) and spatial resolution. First, it is found that an arterial spin labeling (ASL) contribution causes large negative VASO signal changes at short TR. Second, even at high fMRI spatial resolution, CSF volume contributions (7-13%) cause VASO signal changes to become more negative, most noticeably at long TR and TE. Third, white matter (WM) effects reduce signal changes at lower spatial resolution. The VASO technique has been tested using different stimulus paradigms and field strengths (1-3), giving results consistent with comparable tasks investigated using BOLD and cerebral blood flow (CBF)-based techniques. Finally, simulations show that a mixture of fresh and steady-state blood may significantly alter signal changes at short TR (< or =3 s), permitting larger VASO signal changes than expected under pure steady-state conditions. Thus, many competing effects contribute to VASO contrast and care should be taken during interpretation.

Adult↗

PubMed2005

Suppression of strong coupling artefacts in J-spectra.

A major problem in J-resolved spectroscopy and many other experiments is the appearance of "artefacts" due to strong coupling. The signals giving rise to these artefacts follow the same coherence transfer pathways as the required signals, making it impossible to suppress them using phase cycling or pulsed field gradients. We present new methods for suppressing these artefacts, applicable to any experiment containing a J-resolved pulse sequence element, as well as to constant-time experiments.

Artifacts↗

PubMed2004

Development of a method for the measurement of long-range 13C-1H coupling constants from HMBC spectra.

This paper describes a number of improvements to a method, developed in this laboratory and described in J. Magn. Reson. 85 (1989) 111-113, which makes it possible to determine values of long-range 13C-1H coupling constants from heteronuclear multiple bond correlation (HMBC) spectra. First, it is shown how pulsed-field gradients can be introduced into the HMBC experiment without perturbing the form of the cross-peak multiplets; a one-dimensional version of the experiment is also described which permits the rapid measurement of a small number of couplings. Second, the experiment is modified so that one-bond and long-range cross-peaks can be separated, and so that the one-bond cross-peaks have more reliable intensities. Finally, it is shown how these one-bond cross-peaks can be used to advantage in the fitting procedure.

Algorithms↗