Calculation of sensitivity correction factors for surface coil MRS

Musculoskeletal spectroscopy

Magnetic resonance spectroscopy (MRS) of skeletal muscle has been successfully applied by physiologists over several decades, particularly for studies of high-energy phosphates (by (31)P-MRS) and glycogen (by (13)C-MRS). Unfortunately, the observation of these heteronuclei requires equipment that is typically not available on clinical MR scanners, such as broadband capability and a second channel for decoupling and nuclear Overhauser enhancement (NOE). On the other hand, (1)H-MR spectra of skeletal muscle can be acquired on many routine MR systems and also provide a wealth of physiological information. In particular, studies of intramyocellular lipids (IMCL) attract physiologists and endocrinologists because IMCL levels are related to insulin resistance and thus can lead to a better understanding of major health problems in industrial countries. The combination of (1)H-, (13)C-, and (31)P-MRS gives access to the major long- and short-term energy sources of skeletal muscle. This review summarizes the technical aspects and unique MR-methodological features of the different nuclei. It reviews clinical studies that employed MRS of one or more nuclei, or combinations of MRS with other MR modalities. It also illustrates that MR spectra contain additional physiological information that is not yet used in routine clinical applications.

Comparison of altered signal intensity, position, and morphology of the TMJ disc in MR images corrected for variations in surface coil sensitivity

OBJECTIVE

The purpose of this study is to evaluate the corrections of signal intensity of the temporomandibular joint (TMJ) disc caused by variations in sensitivity of the magnetic resonance imaging (MRI) surface coil, to compare the modified signal intensities of the posterior and anterior bands, and then to evaluate the relationship of the signal intensity difference to altered disc position and morphology in a group of TMJ patients.

STUDY DESIGN

MRI was performed on 96 joints. All patients underwent imaging in axial, coronal, and sagittal planes using fast-spin echo sequences (FSE). The images were taken in the closed, partially opened, and maximum opened mouth positions in 2 sequences. Classifications were made according to the position and morphology of the disc. TMJs were divided into normal, anterior disc displacement with reduction (ADDwR), anterior disc displacement without reduction (ADDwoR), and partial anterior disc displacement with reduction (PDDwR). Disc morphology was subdivided as biconcave, lengthened, biconvex, thick posterior band, and others (defined as folded and rounded). The correction of the inhomogeneous sensitivity of the surface coil was done with the original software. The signal intensities (SI) of the posterior band and anterior band of TMJ discs were measured. The correlations among the groups of TMJs and disc morphologies and SI were statistically analyzed by using Bonferroni/Dunn multicomparison method test.

RESULTS

Of the total number of joints studied with the help of MRI, 37 were normal, 12 exhibited ADDwR, 32 ADDwoR, and 9 PDDwR. The corrected MR images indicated that SI of the posterior bands were higher than the anterior band of the discs. It can also be concluded that the SI of the posterior bands increased significantly in the following order: normal, PDDwR, ADDwR, and ADDwoR, while there is no statistical difference in the SI of the anterior band of the discs. In ADDwR and ADDwoR, thick posterior band is the most common shape. In normal TMJ, the biconcave shape is identified as the most frequently encountered shape.

CONCLUSIONS

It was demonstrated that the SI of the posterior bands increase with the progress of internal derangement, and was found to be higher than that of the anterior band of the discs. It appears that disc degeneration starts from the posterior band of the disc.

Normal hepatic glycogen storage after fasting and feeding in children and adolescents with type 1 diabetes

OBJECTIVE

Hypoglycemia is the most important acute complication in patients with type 1 diabetes. Liver glycogen is an important storage form of glucose and thus important for maintaining glucose homeostasis. To test the hypothesis whether abnormal storage of glycogen in the liver is contributing to the risk of hypoglycemia in type 1 diabetic children and adolescents, liver glycogen was measured.

STUDY DESIGN

Hepatic glycogen concentrations were measured in 19 type 1 diabetic children and adolescents as well as in 19 age-matched controls, following overnight fasting and 4 h after two standardized meals. Hepatic glycogen was assessed by natural abundance 13C nuclear magnetic resonance spectroscopy (MRS).

RESULTS

Mean (+/- SEM) fasting hepatic glycogen concentrations measured in arbitrary units (au) were similar in type 1 diabetic subjects and controls (4.98 +/- 0.36 vs. 4.48 +/- 0.33 au; p = 0.31). Both groups presented with an increase in liver glycogen concentrations 4 h after the standardized meals (diabetic subjects 5.70 +/- 0.37 au, p = 0.01; controls 5.78 +/- 0.47 au, p < 0.01). Hepatic glycogen accumulation after feeding was 19.1% in diabetic children and adolescents compared with 35.8% in controls, but this difference did not reach significance.

CONCLUSION

In children and adolescents with moderately controlled type 1 diabetes, hepatic glycogen stores after fasting and feeding are comparable to those of matched controls.

19F-MRS studies of fluorinated drugs in humans

The use of 19F-NMR as a noninvasive probe to measure directly the pharmacokinetics of drugs at their target (effector) site(s) is illustrated in this article by human studies with 5-fluorouracil (5-FU). This drug, and several of its metabolites, have been measured in vivo in animals and in patients using standard clinical MRI systems. Using a pharmacokinetic imaging approach the parameter that can be measured most readily is the tumoral t(1/2) of 5-FU. Patients whose tumoral t(1/2) of 5-FU is equal to/greater than 20 min are designated as "trappers", and those whose tumoral t(1/2) of 5-FU is less are nontrappers. Trapping of 5-FU in tumors is a necessary, albeit not a sufficient condition, for response. Problems associated with the technical aspects of these measurements have been discussed, as well as how modulators and other agents will affect the tumoral t(1/2) of 5-FU. The rationale for the biological processes underlying the fate of 5-FU in humans has been illustrated with the use of a 12 compartment model, where several of the steps have been discussed and the consequences of their inhibition/stimulation related to the noninvasive studies that can be performed with modulators of the action of 5-FU. These 19F-NMR studies have now been extended to other fluoropyrimidines, some of which are prodrugs of 5-FU, and others where the fluorine atoms are on the ribose ring. These studies also reveal information that has both scientific and clinical significance. The studies presented here illustrate some of the potential and some of the usefulness of 19F-MRS in patient management and in drug development. It is a technique that has proven itself.

Measurements of human breast cancer using magnetic resonance spectroscopy: a review of clinical measurements and a report of localized 31P measurements of response to treatment

A review of the literature has shown that in human breast tumours, large signals from phosphomonoesters (PME) and phosphodiesters (PDE) are evident. In serial measurements in 19 patients with breast cancer, a decrease in PME was significantly associated with a stable or responding disease (p = 0.017), and an increase in PME was associated with disease progression. Extract studies have shown PME to comprise of phosphoethanolamine (PEth) and phosphocholine (PCho), with the PEth to PCho ratio ranging from 1.3 to 12. The PCho content of high grade tumours was found to be higher than low grade tumours. In some animal models, changes in PCho have been shown to correlate with indices of cellular proliferation, and spheroid studies have shown a decrease in PCho content in spheroids with smaller growth fractions. A serial study of 25 patients with advanced primary breast tumours undergoing hormone, chemotherapy or radiotherapy treatments, showed that in this heterogenous group there were significant changes in metabolites that were seen during the first 3 weeks (range 2-4 weeks) of treatment, that correlated with volume change over this period, employed here as a measure of response. Changes in PME (p = 0.003), total phosphate (TP) (p = 0.008) and total nucleoside tri-phosphate (TNTP) (p = 0.02) over 3 (+/-1) weeks were significantly associated with response, as were the levels of PME (p<0.001), PDE (p = 0.01), TP (p = 0.001) and TNTP (p = 0.007) at week 3 (+/-1). PME at week 3 (+/-1) was also significantly associated with the best volume response to treatment (p = 0.03). A reproducibility analysis of results from the observation of normal breast metabolism in four volunteers showed a mean coefficient of variation of 25%, after correcting for changes resulting from the menstrual cycle. Reproducibility studies in four patients with breast cancer showed a mean coefficient of variation of 33%, with the reproducibility being better in patients measured on different days (difference in TP was -6%) compared with those measured on the same day (difference in TP was -29%).

Quantification of phosphorus metabolites from chemical shift imaging spectra with corrections for point spread effects and B1 inhomogeneity

A method is described for quantifying phosphorus metabolites in tissue using spectra localized with surface coils and chemical shift imaging (CSI) and assuming that metabolites are uniformly distributed within a well-defined volume. An analytical expression is developed that yields a single numerical correction factor that takes into account the excitation and receiver profiles of the coil, T1 saturation, and point spread effects associated with Fourier transformation of CSI data. An external phosphorus standard is used to calibrate instrument gain and the B1 profile of the coil. For spherical samples, point spread effects can modulate the signal intensities of three-dimensional CSI spectra from -32% to +54%, depending on the voxel size. Measurements of phantoms of known concentrations showed systematic variations of +/- 10% and random errors of +/- 5%. We have used this method to measure the concentration of phosphocreatine in the thigh muscle of normal volunteers.

A simple phantom to locate the origin of MRI ghost artefacts

A simple pyramidal tube phantom has been designed to allow accurate location of a selected slice along the desired axis of a magnetic resonance scanner and to provide a check of the quality of slice selection. It is particularly helpful in locating the origin of any out-of-slice ghost artefacts. The geometry of the phantom allows the slice positions to be calculated readily.

Dual surface coil with high-B1 homogeneity for deep organ MR imaging

The theory and construction of a dual surface coil which provides good B1 homogeneity and sensitivity in a defined volume of interest is described. The probe comprises two coaxial rings, of different diameters and in different planes, which carry opposing currents of different values. Current in the second ring compensates for the roll-off of the B1 field associated with a single surface coil. Coupling between the rings and a third matching ring is by mutual inductance only. A comparison to a traditional surface coil with practical application to pig brain imaging at a field strength of 7 Tesla is shown.

Retrospective correction of surface coil MR images using an automatic segmentation and modeling approach

The use of surface coils in magnetic resonance imaging offers significant improvements in the signal-to-noise ratio over volume coils for many applications. However, the inhomogeneous reception profile of surface coils hampers their usefulness by introducing significant nonuniformities or intensity variations which can vary by greater than six-fold across the sample. In this study, we evaluated an automatic technique for retrospective correction of intensity variations observed in a high-resolution surface coil MR image of the rat brain obtained using an adiabatic magnetic resonance imaging sequence. The images are shown to have a coefficient of variation less than 12% following application of this correction algorithm. This image intensity correction technique can be applied retrospectively to all data sets and corrects both sample/patient dependent effects (e.g. attenuation of overlying tissue) or sample independent effects (e.g. coil geometry or position). This approach should also prove valuable in improving regions of interest analysis, volume histograms and thresholding techniques.

Quantification of phosphorus metabolites in human calf muscle and soft-tissue tumours from localized MR spectra acquired using surface coils

Metabolite concentrations determined from MR spectra provide more specific information than peak area ratios. This paper presents a method of quantification that allows metabolite concentrations to be determined from in vivo 31P MR spectra acquired using a surface coil and ISIS localization. Corrections for the effects of B1 field inhomogeneity produced by surface coils are based on a measured and calibrated spatial sensitivity field map for the coil. Account is taken of imperfections in pulse performance, coil loading effects and relaxation effects, the latter making use of published metabolite relaxation times. The technique is demonstrated on model solutions. The concentrations of the main 31P metabolites in normal human calf muscle measured using this method are [PCr] = 26.9 +/- 4.1 mM; [Pi] = 3.6 +/- 1.2 mM; [NTP] = 6.8 +/- 1.8 mM. Quantification of spectra acquired from soft-tissue tumours in patients both pre- and post-treatment showed that changes in metabolite concentrations are more sensitive to metabolic changes than changes in peak area ratios.