An assessment of the safety of rapidly changing magnetic fields in the rabbit: Implications for NMR imaging

The effects of rapidly changing magnetic fields on the cardiac and respiratory functions of anaesthetised rabbits have been investigated. No changes were observed using pulsed fields with peak rates of change of 60 T/sec. The implications of these experiments for the safety of NMR imaging are discussed.     

Magnetization transfer contrast (MTC) in FLASH MR imaging

Magnetization transfer between bound and free protons was used as a source of contrast in high speed MR imaging using the FLASH technique. Contrast in FLASH MR images was found to depend upon the reduced magnetization and the spin lattice relaxation rate of free protons in the presence of bound proton radio-frequency saturation. MTC FLASH imaging was thus used to estimate the variation with saturation frequency of free proton spin-lattice relaxation during magnetization transfer.     

Investigation of cerebral ischemia using magnetization transfer contrast (MTC) MR imaging

The effects of cerebral ischemia in rat brain were monitored as a function of time using proton MR imaging. Spinspin relaxation time (T₂), proton density, and magnetization transfer contrast (MTC) were measured by MR imaging at various time intervals during a 1-week period following the induction of ischemic damage. Ischemic injury was characterized by a maximization of both T₂ value and MTC appearance at 24 hr postischemic injury. These changes were accompanied by a gradual increase in MR observable water density over the first few days of ischemia. A reduction in the magnetization exchange rate between “free” and “bound” water protons as measured by MTC imaging is at least partially responsible for the elevation in T₂ values observed during ischemia, and may accompany breakdown of cellular structure.     

Effects of radiation therapy on the human normal brain (white matter) visualized by MR imaging

Sequential MR imaging could provide information related to the pathological changes due to the application of external cytotoxic agents such as radiotherapy on the central nervous system. This paper describes the results of our attempt to demonstrate short-term changes associated with normal brain during and immediately following radiotherapy when the whole brain is irradiated for malignant conditions. No observable changes were found in the normal brain parenchyma in any of the patients (n = 8) in T₁-, T₂-, and proton-density-weighted images in the sequential scans in the first and second weeks and immediately following radiotherapy. Also, no changes were observed in the normal brain appearance at 2 mo (n = 6), up to 6 mo (n = 1), and up to 15 mo (n = 1) after completion of radiotherapy.     

Frequency-dependent C(V) spectroscopy of the hole system in InAs quantum dots

The hole system in InAs quantum dots was investigated by frequency-dependent capacitance–voltage spectroscopy. Up to eight distinct charging peaks could be observed and the energy difference between the individual peaks could be estimated. All charging peaks decrease with increasing measurement frequency; however, the lower the energy of the hole level the stronger the decrease. A comparison with the results of the electron system in similar quantum dots yields that for all hole levels the effective mass in the barrier is much larger than in the electron system.     

The oral administration of MnCl2: A potential alternative to IV injection for tissue contrast enhancement in magnetic resonance imaging

Mn⁺² (as MnCl₂) was administered to rabbits intravenously and orally (a route of administration which based upon our previous experiments in rats⁷ promises to give selective hepatobiliary enhancement with less systemic toxicity). Nuclear magnetic relaxation dispersion or T₁ (NMRD) was performed on selected tissues (heart, liver, kidney, serum, and bile) in both animal groups to examine possible qualitative and semiquantitative differences in T₁ relaxation at equivalent sacrifice times. One animal was given an oral dose of MnCl₂ (620 micromoles/kg) and imaged sequentially (T₁ weighted sequence, .12T) for 30 minutes. The NMRD curves for organ tissues show an increase in relaxation efficacy in the 10–20MHz range characteristic of Mn-macromolecular complexes and are similar irrespective of the route of administration. The lack of increased relaxation enhancement for bile in this frequency range reflects cleavage of this complex upon excretion. Decreased overall relaxation in the liver is observed when oral Mn⁺² is compared to IV Mn⁺² due to the small fraction of administered dose that is absorbed. However, the images document a significant increase in the intensity of liver signal after the oral dose. We suspect this dose may ultimately be adjusted downward to give selective hepatobiliary effects.     

The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.