Experimental approaches to image localized human 31P NMR spectroscopy

Quantitative 1H Magnetic Resonance Imaging on Normal and Pathologic Rat Bones by Solid-State 1H ZTE Sequence with Water and Fat Suppression

BACKGROUND

Osteoporosis (OP) and osteomalacia (OM) are metabolic bone diseases characterized by mineral and matrix density changes. Quantitative bone matrix density differentiates OM from OP. MRI is a noninvasive and nonionizing imaging technique that can measure bone matrix density quantitatively in ex vivo and in vivo.

PURPOSE

To demonstrate water + fat suppressed 1H MRI to compute bone matrix density in ex vivo rat femurs in the preclinical model.

STUDY TYPE

Prospective.

ANIMAL MODEL

Fifteen skeletally mature female Sprague-Dawley rats, five per group (normal, ovariectomized (OVX), partially nephrectomized/vitamin D (Vit-D) deficient), 250-275 g, ∼15 weeks old.

FIELD STRENGTH/SEQUENCE

7T, zero echo time sequence with water + fat (VAPOR) suppression capability, μCT imaging, and gravimetric measurements.

ASSESSMENT

Cortical and trabecular bone segments from normal and disease models were scanned in the same coil along with a dual calibration phantom for quantitative assessment of bone matrix density.

STATISTICAL TESTS

ANOVA and linear regression were used for data analysis, with P-values <0.05 statistically significant.

RESULTS

The MRI-derived three-density PEG pellet densities have a strong linear relationship with physical density measures (r2 = 0.99). The Vit-D group had the lowest bone matrix density for cortical bone (0.47 ± 0.16 g cm-3), whereas the OVX had the lowest bone matrix density for trabecular bone (0.26 ± 0.04 g cm-3). Gravimetry results confirmed these MRI-based observations for Vit-D cortical (0.51 ± 0.07 g cm-3) and OVX trabecular (0.26 ± 0.03 g cm-3) bone groups.

DATA CONCLUSION

Rat femur images were obtained using a modified pulse sequence and a custom-designed double-tuned (1H/31P) transmit-receive solenoid-coil on a 7T preclinical MRI scanner. Phantom experiments confirmed a strong linear relation between MRI-derived and physical density measures and quantitative bone matrix densities in rat femurs from normal, OVX, and Vit-D deficient/partially nephrectomized animals were computed.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 2.

3D 31 P MR spectroscopic imaging of the human brain at 3 T with a 31 P receive array: An assessment of 1 H decoupling, T1 relaxation times, 1 H-31 P nuclear Overhauser effects and NAD. NMR IN BIOMEDICINE 2021;34:e4169. [PMID: 31518036 PMCID: PMC8244063 DOI: 10.1002/nbm.4169]

³¹ P MR spectroscopic imaging (MRSI) is a versatile technique to study phospholipid precursors and energy metabolism in the healthy and diseased human brain. However, mainly due to its low sensitivity, ³¹ P MRSI is currently limited to research purposes. To obtain 3D ³¹ P MRSI spectra with improved signal-to-noise ratio on clinical 3 T MR systems, we used a coil combination consisting of a dual-tuned birdcage transmit coil and a ³¹ P eight-channel phased-array receive insert. To further increase resolution and sensitivity we applied WALTZ4 ¹ H decoupling and continuous wave nuclear Overhauser effect (NOE) enhancement and acquired high-quality MRSI spectra with nominal voxel volumes of ~ 17.6 cm³ (effective voxel volume ~ 51 cm³ ) in a clinically relevant measurement time of ~ 13 minutes, without exceeding SAR limits. Steady-state NOE enhancements ranged from 15 ± 9% (γ-ATP) and 33 ± 3% (phosphocreatine) to 48 ± 11% (phosphoethanolamine). Because of these improvements, we resolved and detected all ³¹ P signals of metabolites that have also been reported for ultrahigh field strengths, including resonances for NAD⁺ , NADH and extracellular inorganic phosphate. T₁ times of extracellular inorganic phosphate were longer than for intracellular inorganic phosphate (3.8 ± 1.4s vs 1.8 ± 0.65 seconds). A comparison of measured T₁ relaxation times and NOE enhancements at 3 T with published values between 1.5 and 9.4 T indicates that T₁ relaxation of ³¹ P metabolite spins in the human brain is dominated by dipolar relaxation for this field strength range. Even although intrinsic sensitivity is higher at ultrahigh fields, we demonstrate that at a clinical field strength of 3 T, similar ³¹ P MRSI information content can be obtained using a sophisticated coil design combined with ¹ H decoupling and NOE enhancement.

Pros and cons of ultra-high-field MRI/MRS for human application

Magnetic resonance imaging and spectroscopic techniques are widely used in humans both for clinical diagnostic applications and in basic research areas such as cognitive neuroimaging. In recent years, new human MR systems have become available operating at static magnetic fields of 7 T or higher (≥300 MHz proton frequency). Imaging human-sized objects at such high frequencies presents several challenges including non-uniform radiofrequency fields, enhanced susceptibility artifacts, and higher radiofrequency energy deposition in the tissue. On the other side of the scale are gains in signal-to-noise or contrast-to-noise ratio that allow finer structures to be visualized and smaller physiological effects to be detected. This review presents an overview of some of the latest methodological developments in human ultra-high field MRI/MRS as well as associated clinical and scientific applications. Emphasis is given to techniques that particularly benefit from the changing physical characteristics at high magnetic fields, including susceptibility-weighted imaging and phase-contrast techniques, imaging with X-nuclei, MR spectroscopy, CEST imaging, as well as functional MRI. In addition, more general methodological developments such as parallel transmission and motion correction will be discussed that are required to leverage the full potential of higher magnetic fields, and an overview of relevant physiological considerations of human high magnetic field exposure is provided.

Assessing tissue metabolism by phosphorous-31 magnetic resonance spectroscopy and imaging: a methodology review

Many human diseases are caused by an imbalance between energy production and demand. Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide the unique opportunity for in vivo assessment of several fundamental events in tissue metabolism without the use of ionizing radiation. Of particular interest, phosphate metabolites that are involved in ATP generation and utilization can be quantified noninvasively by phosphorous-31 (31P) MRS/MRI. Furthermore, 31P magnetization transfer (MT) techniques allow in vivo measurement of metabolic fluxes via creatine kinase (CK) and ATP synthase. However, a major impediment for the clinical applications of 31P-MRS/MRI is the prohibitively long acquisition time and/or the low spatial resolution that are necessary to achieve adequate signal-to-noise ratio. In this review, current 31P-MRS/MRI techniques used in basic science and clinical research are presented. Recent advances in the development of fast 31P-MRS/MRI methods are also discussed.

Creatine in the central nervous system: From magnetic resonance spectroscopy to creatine deficiencies

Creatine (Cr) is an important organic compound acting as intracellular high-energy phosphate shuttle and in energy storage. While located in most cells where it plays its main roles in energy metabolism and cytoprotection, Cr is highly concentrated in muscle and brain tissues, in which Cr also appears to act in osmoregulation and neurotransmission. This review discusses the basis of Cr metabolism, synthesis and transport within brain cells. The importance of Cr in brain function and the consequences of its impaired metabolism in primary and secondary Cr deficiencies are also discussed. Cr and phosphocreatine (PCr) in living systems can be well characterized using in vivo magnetic resonance spectroscopy (MRS). This review describes how ¹H MRS allows the measurement of Cr and PCr, and how ³¹P MRS makes it possible to estimate the creatine kinase (CK) rate constant and so detect dynamic changes in the Cr/PCr/CK system. Absolute quantification by MRS using creatine as internal reference is also debated. The use of in vivo MRS to study brain Cr in a non-invasive way is presented, as well as its use in clinical and preclinical studies, including diagnosis and treatment follow-up in patients.

Mr Spectroscopy in Clinical Research

MR spectroscopy (MRS) offers unique possibilities for non-invasive evaluation of biochemistry in vivo. During recent years there has been a growing body of evidence from clinical research studies on human beings using 31P and 1H MRS. The results indicate that it is possible to evaluate phosphorous energy metabolism, loss of neurones, and lactate production in a large number of brain diseases. Furthermore, 31P and 1H MRS may be particularly clinically useful in evaluation of various disorders in skeletal muscle. In the heart 31P MRS seems at the moment the most suitable for evaluation of global affections of the myocardium. In the liver 31P MRS appears to be rather insensitive and non-specific, but absolute quantification of metabolite concentrations and using metabolic “stress models” may prove useful in the future. The clinical role of MRS in oncology is still unclear, but it may be useful for noninvasive follow-up of treatment.

Taken together, the evidence obtained so far certainly shows some trends for clinical applications of MRS. Methods are now available for the clinical research necessary for establishing routine clinical MRS examinations.

Selective maximization of (31)P MR spectroscopic signals of in vivo human brain metabolites at 3T

PURPOSE

To develop a short TR, short TE, large flip angle (LFA), in vivo (31)P MR spectroscopy (MRS) technique at 3T that selectively maximizes the signal-to-noise ratio (SNR) of long T(1) human brain metabolites implicated in bipolar disorder.

MATERIALS AND METHODS

Two pulse sequences were evaluated for efficiency. Slice profiles acquired with the scaled, sinc-shaped, radiofrequency (RF) LFA pulses were compared to those acquired with Shinnar-Le Roux (SLR) RF LFA pulses. The SLR-based LFA pulse sequence was used to maximize the inorganic phosphate signal in a phantom, after which volunteer metabolite signals were selectively maximized and compared to their correlates acquired with conventional spin-echo methods.

RESULTS

The comparison of slice profiles acquired with sinc-shaped RF LFA pulses vs. SLR RF LFA pulses showed that SLR-based pulse sequences, with their improved excitation and slice profiles, yield significantly better results. In vivo LFA spin-echo MRS implemented with SLR pulses selectively increased the (31)P MRS signal, by as much as 93%, of human brain metabolites that have T(1) times longer than the TR of the acquisition.

CONCLUSION

The data show that the LFA technique can be employed in vivo to maximize the signal of long T(1) (31)P brain metabolites at a given TE and TR. LFAs ranging between 120 degrees and 150 degrees are shown to maximize the (31)P signal of human brain metabolites at 3T.

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

T(1) measurement of (31)P metabolites at rest and during steady-state dynamic exercise using a clinical nuclear magnetic resonance scanner

This article illustrates some problems and possible solutions to determine the apparent spin-lattice relaxation time (T(1)) of the muscular (31)P metabolites at rest and during dynamic steady-state exercise using a clinical 1.5 T NMR scanner and a surface coil. T(1) was first estimated on a phosphates solution (phantom) using four different acquisition protocols, all based on the multiple-point "progressive saturation" method, and by fitting each data set with two different mathematical models. Subsequently, two of the four protocols and both models were used to estimate T(1) both at rest and during exercise on the calf muscles of 10 healthy volunteers. Experimental results obtained on the phantom showed that T(1) is greatly affected by the longest nominal explored repetition time (P<0.001) and by the mathematical model (P<0.001), ranging from 0.65+/-0.10 to 8.4+/-0.8 s. The two acquisition protocols applied on volunteers yielded significantly different T(1) (P<0.001), which were also rather different from the literature values for the same metabolites. Nevertheless, independently of the acquisition protocol and/or the fitting procedure, T(1) of all muscular phosphagens did not change statistically from rest to steady-state aerobic exercise.

In vivo31P NMR spectroscopy shows an increase in glycerophosphorylcholine concentration without alterations in mitochondrial function in the prefrontal cortex of medicated schizophrenic patients at rest

The (31)P NMR localised method was used to study the metabolism of phospholipid and high energy phosphate in the prefrontal cortex. The spectra were taken from patients with schizophrenia (11 males) receiving neuroleptic medication, and were compared to normal controls (15 males). Their spectral intensities were analysed using a non-linear least-squares method with a prior knowledge of the fixed chemical shifts and linewidths, leading to further resolution into resonances of glycerophosphorylethanolamine (GPE), glycerophosphorylcholine (GPC), phosphorylethanolamine (PE) and phosphorylcholine (PC). The metabolite concentrations were calculated referring to the spectral intensities of phosphate phantoms with known concentrations. T1 values of phantom and cerebrum were estimated from a series of localised inversion recovery spectra to correct for the signal saturation effects. The schizophrenic patients showed an increased concentration of GPC but not GPE, PE or PC. Furthermore, no difference was observed regarding the concentration of high-energy phosphates such as phosphocreatine, inorganic phosphate and ATP. The patients did not show any differences in mitochondrial function such as phosphorylation potential and the ratio of the rate of ATP synthesis. Thus, an increase in GPC concentration in the prefrontal cortex could be characteristic of the pathophysiology of schizophrenia with mild negative symptoms.

Relaxation times of 31P-metabolites in human calf muscle at 3 T

Localized (31)P-STEAM experiments were performed at 3 T to estimate relaxation times of phosphorus-containing metabolites in the human calf muscle in vivo. T(1) and T(2) times of PCr, P(i), and NTPs were measured in the resting calf muscle of healthy subjects by varying TR and TE. The localization performance of the (31)P-STEAM sequence was evaluated on a test object, resulting in a relative selection efficiency of 78 +/- 1% and contamination from outside the voxel of 0 +/- 2% under fully relaxed conditions. T(1) relaxation times (+/-SD, n = 5) of P(i), PCr, gamma-NTP, alpha-NTP, and beta-NTP obtained at 3 T are 5.2 +/- 1.0 s, 6.4 +/- 0.2 s, 4.5 +/- 0.3 s, 2.6 +/- 0.9 s, and 3.5 +/- 1.1 s, respectively. T(2) relaxation times (+/-SD, n = 6) of these metabolites are 148 +/- 17 ms, 334 +/- 30 ms, 78 +/- 13 ms, 55 +/- 7 ms, and 55 +/- 10 ms, respectively. Spin-lattice relaxation times established at 3 T are consistent with literature data at lower field strengths, whereas spin-spin relaxation times are lower. Several methodological considerations are discussed which may help improve quantification of metabolite concentrations in the human (calf) muscle in vivo by using localized noninvasive (31)P-MRS at 3 T, which is currently being tested for routine clinical applications.

In vivo 31P magnetic resonance spectroscopy of human brain at 7 T: an initial experience

In vivo (31)P spectra were acquired from the human primary visual cortex at 7 T. The relaxation times of the cerebral metabolites, intracellular pH, rate constant (k(f)) of the creatine kinase (CK) reaction, and nuclear Overhauser enhancement (NOE) on the detected phosphorus moieties from irradiation of the water spins were measured from normal subjects. With a 5-cm-diameter surface coil, 3D (31)P chemical shift imaging was performed with a spatial resolution of 7.5 ml and an acquisition resolution of 8 min, resulting in a signal-to-noise ratio (SNR) for phosphocreatine (PCr) resonance of 32. The apparent T(1) and T(2) of PCr measured at 7 T were 3.37 +/- 0.29 s and 132.0 +/- 12.8 ms, respectively, which were considerably longer than those of adenosine triphosphate (ATP) (T(1): 1.02-1.27 s; T(2): 25-26 ms). The NOE measured in this study was 24.3% +/- 1.6% for PCr, and 10% for ATP. The k(f) measured in the human primary visual cortex was 0.24 +/- 0.03 s(-1). The results from this study suggest that ultra-high-field strength is advantageous for performing in vivo (31)P magnetic resonance spectroscopy (MRS) in the human brain.

The magnitude of signal errors introduced by ISIS in quantitative 31P MRS

It is well known that the quality of a quantitative 31P MRS measurement relies largely on the performance of the volume selection method, and that image selected in vivo spectroscopy (ISIS) suffers from contaminating signal caused mostly by T1 smearing. However, these signal errors and their magnitude are seldom addressed in clinical studies. The aim of this study was therefore to investigate the magnitude of signal errors in 31P MRS when using ISIS. The results from the measurements with a homogeneous head phantom are as follows: at low TR/T1 ratios the contamination increases rapidly, especially for small (<27 cm3) VOI sizes; at TR/T1=1, the signal from a 27 cm3 VOI was 20% too high, and from an 8 cm3 VOI 150% too high. The signal obtained from different VOI positions varied between 80 and 127%. The signal varied linearly with the 31P concentration in the object. However, a too high signal was obtained when the concentration was lower in the region of interest (inner container) than in the rest of the phantom. The agreement between the simulations and measurements shows that the results of this study are generally applicable to the measurement geometry and the ISIS experiment order rather than being specific for the MR system studied. The errors obtained both experimentally and in computer simulations are too large to be ignored in clinical studies using the ISIS pulse sequence.

Phosphorus-31 MR spectroscopy of normal adult human brain and brain tumours

Localized phosphorus-31 MR spectra were obtained in vivo in a large series of normal human brain tissue specimens of healthy volunteers (n=36) and various brain tumours (n=52). Tumour types examined included grade II and grade III gliomas (n=15 and n=1, respectively), glioblastomas (n=16) and meningeomas (n=12). An additional eight tumours were analysed during chemo- or radiotherapy. Spectra were acquired using a modified ISIS pulse sequence with a repetition time of 3 s. Voxel sizes ranged from 56 to 129 ml. The spectra were evaluated using a least-square variable projection (VARPRO) fitting procedure in the time domain, which allows semi-quantitative determination of relative metabolite concentrations. The measurements in normal cerebrum of healthy volunteers revealed the following results of metabolite signal intensity ratios: pH 7.04 (+/- 0.01), PCr/alpha-ATP 0.51 (+/- 0.03), P(i)/alpha-ATP 0.17 (+/-0.02), PCr/P(i) 2.09 (+/-0.12), PDE/alpha-ATP 3.65 (+/-0.13) and PME/alpha-ATP 0.41 (+/-0.04). Meningiomas showed the most obvious changes when compared with normal brain tissue. They are characterized by an alkaline environment (pH 7.16 +/- 0.03; p<0.005), a decrease in the phosphocreatine peak (p<0.0001) and significantly decreased phosphodiesters (p<0.0001). Glioblastomas also showed alkalization (pH 7.12 +/- 0.02; p<0.001) and a decrease in PDE/alpha-NTP (p<0.05), but no significant changes in PCr/alpha-NTP or PCr/Pi. In gliomas with low malignancy, less distinct changes could be detected with slight alkalization (pH 7.09 +/- 0.02; p<0.05) and more than a two-fold reduction in the PDE/alpha-NTP ratio (p<0.05). The spectra of brain tumours during chemo- and radiotherapy indicated clear but inconsistent influence of the therapy.

The performance of volume selection sequences for in vivo NMR spectroscopy: implications for quantitative MRS

Previous work has demonstrated that deficiencies in volume selection sequences used in magnetic resonance spectroscopy may compromise the quality of the spectra obtained. In this paper, further studies on the ISIS and PRESS sequences are presented. Under conditions of partial saturation, ISIS can exhibit serious contamination with extraneous signal, particularly when a small volume of interest (VOI) is selected. ISIS protocols should therefore use VOIs that are large relative to the target volume, and repetition times that are as long as practicable. In PRESS, contamination is found to be minimised by using a VOI that is small relative to the target volume, and to be independent of repetition time. PRESS performance is also independent of echo time, except when very short echo times are used. These results are consistent with previously published work on ISIS and PRESS, and it is now possible to establish generic features of these sequences and to understand the implications for quantitative spectroscopy. T(1)-weighting of contamination in ISIS can compromise both relative and absolute quantification techniques in several respects. Contamination in PRESS is largely independent of relaxation times and would be easier to model and correct for in the context of quantitative spectroscopy.

Extended ISIS sequences insensitive to T(1) smearing

Image selected in vivo spectroscopy (ISIS) is a volume selection method often used for in vivo (31)P MRS, since it is suitable for measurements of substances with short T(2). However, ISIS can suffer from significant signal contributions caused by T(1) smearing from regions outside the VOI. A computer model was developed to simulate this contamination. The simulation results for the ISIS experiment order implemented in our MR system (ISIS-0) were in agreement with results obtained from phantom measurements. A new extended ISIS experiment order (E-ISIS) was developed, consisting of four "optimal" ISIS experiment orders (ISIS-1 to ISIS-4) performed consecutively with dummy ISIS experiments in between. The simulation results show that contamination due to T(1) smearing is, effectively, eliminated with E-ISIS and is significantly lower than for ISIS-0 and ISIS-1. E-ISIS offers increased accuracy for quantitative and qualitative determination of substances studied using in vivo MRS. Hence, E-ISIS can be valuable for both clinical and research applications.

Point-wise measurements of MRS volume selection performance are insensitive to magnetic susceptibility effects of phantom materials

The purpose was to analyse magnetic susceptibility effects on accuracy of point-wise measurements of signal profiles in the assessment of MRS volume selection performance. An existing phantom design consisting of a sphere with a movable signal source was used for the investigation. The influence from the phantom on magnetic field homogeneity was measured with phase sensitive 1H imaging and 31P spectroscopy on a 1.5 T whole body MR system. The susceptibility effects for such a phantom design can be separated in 1/ A variation in the background magnetic field, which is caused by the stationary structures and has a significant influence on spatial accuracy. 2/ A magnetic field distortion, which is caused by the movable signal source and has very little influence on accuracy. The spatial inaccuracy due to susceptibility effects in this phantom, was 0.03 mm for positions of the signal source covering a 40-mm VOI. Susceptibility effects from the movable signal source were substantial but had very little influence on spatial accuracy. Still, improvements of this phantom design are possible. Point-wise measurements using a phantom with a movable signal source is inherently insensitive to susceptibility effects from the signal source and permits accurate signal profile measurements of high spatial (sub-mm) resolution.

Single-shot, three-dimensional "non-echo" localization method for in vivo NMR spectroscopy

Metabolite signals with short T(1) or T(2) are difficult to localize with full sensitivity. This limitation was overcome with the development and implementation of a single-shot, complete three-dimensional "non-echo" localization method with reduced sensitivity to spatial B(1) variation, which is suitable for measuring signals with very short T(1) or T(2), e.g., the (13)C NMR signals of glycogen. The proposed method is based on a T(1)-optimized outer volume suppression scheme using pulses of the hyperbolic secant type applied at different power levels, which is robust over a fivefold range of T(1). Strong lipid, muscle glycogen, and glucose signals originating outside the rat brain were suppressed. Signals of glycogen, aspartate, glutathione, GABA C4, N-acetyl aspartate as well as the C3 and C4 signals of glutamate and glutamine with resolved homonuclear (13)C-(13)C coupling were fully resolved in vivo at 9.4 Tesla using higher-order shimming. The method can be extended to other nuclei and to localized MRS of humans.

Proline metabolism in the wild-type and in a salt-tolerant mutant of nicotiana plumbaginifolia studied by (13)C-nuclear magnetic resonance imaging

To obtain insight into the link between proline (Pro) accumulation and the increase in osmotolerance in higher plants, we investigated the biochemical basis for the NaCl tolerance of a Nicotiana plumbaginifolia mutant (RNa) that accumulates Pro. Pro biosynthesis and catabolism were investigated in both wild-type and mutant lines. (13)C-Nuclear magnetic resonance with [5-(13)C]glutamate (Glu) as the Pro precursor was used to provide insight into the mechanism of Pro accumulation via the Glu pathway. After 24 h under 200 mM NaCl stress in the presence of [5-(13)C]Glu, a significant enrichment in [5-(13)C]Pro was observed compared with non-stress conditions in both the wild type (P2) and the mutant (RNa). Moreover, under the same conditions, [5-(13)C]Pro was clearly synthesized in higher amounts in RNa than in P2. On the other hand, measurements of enzyme activities indicate that neither the biosynthesis via the ornithine pathway, nor the catabolism via the Pro oxidation pathway were affected in the RNa mutant. Finally, the regulatory effect exerted by Pro on its biosynthesis was evaluated. In P2 plantlets, exogenous Pro markedly reduced the conversion of [5-(13)C]Glu into [5-(13)C]Pro, whereas Pro feedback inhibition was not detected in the RNa plantlets. It is proposed that the origin of tolerance in the RNa mutant is due to a mutation leading to a substantial reduction of the feedback inhibition normally exerted in a wild-type (P2) plant by Pro at the level of the Delta-pyrroline-5-carboxylate synthetase enzyme.

In vivo evidence for axonal dysfunction remote from focal cerebral demyelination of the type seen in multiple sclerosis

To test for axonal damage or dysfunction in white matter tracts remote from acute demyelinating lesions, we used brain proton magnetic resonance spectroscopic imaging to measure changes in N-acetyl aspartate (NAA), an index of neuronal integrity, in the white matter of the normal-appearing hemisphere of three patients with large, solitary brain demyelinating lesions of the type seen early in multiple sclerosis. During the acute phase of their disease, all patients showed normal ratios of NAA to creatine (Cr) resonance intensity throughout the hemisphere contralateral to the lesion. However, on examination 1 month later, all of the patients showed abnormally low NAA/Cr resonance intensity ratios (reduction of NAA/Cr by 22-35%) in voxels of the contralateral hemisphere which were homologous to the demyelinating lesion. Other voxels in the normal-appearing hemisphere showed normal NAA relative resonance intensities. The decrease in NAA/Cr in voxels of the normal-appearing hemispheres resolved in all patients after 6 months, with a time course similar to that observed for NAA from voxels within the lesions. We conclude that effects of damage or dysfunction to axons traversing inflammatory lesions can be transmitted over long distances in the normal-appearing white matter. Such remote, secondary effects may be an expression of dysfunction of axons in projection pathways or of the reorganization of functional pathways seen in brains recovering from an acute injury.

T1 measurements of 31P metabolites in resting and exercising human gastrocnemius/soleus muscle at 1.5 Tesla

This study has measured the apparent 31P T1 times at 1.5 T in human gastrocnemius/soleus muscle groups at rest, during exercise, and during recovery from a 90-sec submaximal plantar flexion exercise. T1 times were measured with a 10-sec time resolution in 11 normally active volunteers using a surface coil with a nine-point progressive saturation technique. A two-point short repetition time technique was also used. Both techniques showed similar trends in the apparent T11 times of all the 31P metabolites at rest, during exercise, and during recovery. For the nine-point progressive saturation technique, the apparent T1 of PCr and beta-ATP decreased approximately 20% to a steadystate value (P = 0.027 and P = 0.004, respectively). The two-point short repetition time technique demonstrated a 10% reduction in the apparent T1 of PCr and beta-ATP Both techniques demonstrated an apparent T1 increase of 58% for inorganic phosphate at the beginning of exercise (P<0.0001) and a return to resting value during the 90-sec submaximal isometric contraction. Neither technique showed any significant differences between resting and exercising T1 times of the alpha-ATP and gamma-ATP resonances (P = 0.06 and P>0.40, respectively).

Metabolism of the dimethyl ester of [2,3-(13)C]succinic acid in rat hepatocytes

Hepatocytes prepared from overnight fasted rats were incubated for 120 min in the presence of the dimethyl ester of [2,3-(13)C]succinic acid (10 mM). The identification and quantification of 13C-enriched metabolites in the incubation medium were performed by a novel computational strategy for the deconvolution of NMR spectra with multiplet structures and constraints. The generation of 13C-labelled metabolites, including succinate, fumarate, malate, lactate, alanine, aspartate and glucose accounted for about half of the initial amount of the ester present in the incubation medium. A fair correlation was observed between the experimental abundance of each 13C-labelled glucose isotopomer and the corresponding values derived from a model for the metabolism of [2,3-(13)C]succinate. Newly formed glucose was more efficiently labelled in the carbon C5 than C2, as well as the carbon C6 than C1, supporting the concept that D-glyceraldehyde-3-phosphate may undergo enzyme-to-enzyme channelling between glyceraldehyde-3-phosphate dehydrogenase and phosphofructoaldolase.

Adiabatic water suppression using frequency selective excitation

A new method for B1-insensitive water suppression using adiabatic RF pulses is described. The transition zone of the inversion profile of adiabatic full passage (AFP) pulses is used for frequency-selective excitation followed by dephasing of the excited water with magnetic field gradients. Several improvements of AFP pulses, which also have implications for other applications, are described. The technique was evaluated by simulations based on the Bloch equations (including relaxation), in vitro experiments and an in vivo verification on neonatal and adult rat brain.

Signal profile measurements of single- and double-volume acquisitions with image-selected in vivo spectroscopy for 31P magnetic resonance spectroscopy

The volume-selection performance was studied for single- and double-volume-of-interest (VOI) acquisition with the volume-selection method image-selected in vivo spectroscopy for 31P magnetic resonance spectroscopy. High-resolution signal profiles were measured using a phantom simulating a brain. Inside the phantom there was a small, remotely controlled, movable signal source filled with ortho-phosphoric acid. Signal profiles of the VOI were measured in three perpendicular directions for 1VOI (single VOI) and 2VOI (double VOI) acquisition. The measured signal profiles for both acquisitions were very similar, but they showed a discrepancy with regard to the intended VOI (iVOI). The transition regions were on average 3.8 mm and the average full width at half maximum of the signal profile was 30 mm for an iVOI size of 30*30*30 (mm3). No displacement was observed in the signal profiles. To avoid overlapping signal profiles, the minimum separation between two iVOIs was found to be 10 mm in our magnetic resonance (MR) system. A substantial negative signal contribution from regions outside the iVOI was measured in the y-direction for 1VOI acquisition and one of the two VOIs in 2VOI acquisition. The other VOI in 2VOI acquisition exhibited only minor contamination. The measurements presented underline the importance of detailed knowledge on the volume selection performance in in vivo magnetic resonance spectroscopy.

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.

Evaluation of 31P metabolite differences in human cerebral gray and white matter

31P NMR is commonly used to study brain energetics in health and disease. Due to sensitivity constraints, the NMR measurements are typically made in volumes that do not contain pure gray or white matter. For accurate evaluation of abnormalities in brain metabolite levels, it is necessary to consider the differences in normal levels of 31P metabolites in gray and white matter. In this study, voxels from a three-dimensional spectroscopic image acquisition were analyzed for their dependence on tissue type to assess differences in metabolite levels between gray and white matter. Specifically, gray matter was found to have significantly higher ratios of phosphocreatine (PCr) to gamma-ATP and PCr to the total 31P metabolite signal, whereas pH and the ratio of PCr to inorganic phosphate (Pi) were found to differ insignificantly between gray and white matter. Thus, tissue type can be an important factor to consider for alterations in bioenergetics by 31P NMR spectroscopic studies of the brain.

A new method for absolute quantitation of MRS metabolites

A new method for absolute quantitation of MRS spectra is presented. This method is not based on a reference peak, derived from a real NMR signal, but rather on a synthesized NMR reference produced by an electronic device, transmitted by a broad-band antenna to avoid quality factor variations. This signal is therefore received at the same time as the sample signal. The reference line produced is stable in time (maximum variation lower than 2%) and allows precise and accurate measurement of absolute concentrations (mean error lower than 3%) in vitro and in vivo.

The total entropy for evaluating 31P-magnetic resonance spectra of the liver in healthy volunteers and patients with metastases

RATIONALE AND OBJECTIVES

The authors describe the clinical status of liver tissue with only a single numerical quantity (total entropy) derived from spectroscopic data of 31P-magnetic resonance (MR) spectra.

METHODS

Twenty-four patients with liver metastases and 20 volunteers were investigated with image-guided volume selective 31P-MR spectroscopy on a 1.5-T whole body scanner. From each in vivo 31P-MR spectrum, the ratios of phosphomonoester (PME)/beta-adenosine triphosphate (ATP), inorganic phosphate (Pi)/beta-ATP and phosphodiester (PDE)/ beta-ATP and the total entropy (H*) were calculated. Mean values and standard deviations were determined and significance of the differences were tested with Student's t test.

RESULTS

For patients, the H* = 4.7 +/- 4.3, PME/beta-ATP 0.72 +/- 0.28, Pi/beta-ATP = 1.00 +/- 0.39, PDE/beta-ATP = 1.68 +/- 0.59. For the volunteers, H* = 7.6 +/- 2.5, PME/beta-ATP = 0.39 +/- 0.15, Pi/beta-ATP = 0.90 +/- 0.19, PDE/beta-ATP = 1.25 +/- 0.28. The total entropy of patients' spectra showed significantly lower values compared with those of volunteers. PME/beta-ATP and PDE/beta-ATP of the patients increased and differed significantly from volunteer data.

CONCLUSIONS

It was demonstrated that the results of in vivo 31P-MR spectroscopy may be described with a single criterion by means of the total entropy.

Statistics for investigation of multimodal MR imaging data and an application to multiple sclerosis patients

Magnetic resonance spectroscopy can image axonal damage specifically based on changes in N-acetyl aspartate (NAA), a neuronal marker. We have developed statistical methods for multimodal analysis of MR spectroscopic images. These methods, which are extensions of mixed-effect models, have allowed us to quantify differences in images from different subgroups of patients with multiple sclerosis (MS) and to determine the dependence of chemical pathology on clinical disability, duration of disease and lesions on T2-weighted MRI. Statistical power was improved by using all reliable resonance intensities in the spectroscopic images while taking into consideration the intra-subject correlations. We studied 17 normal subjects, 14 patients with relapsing remitting (RR) MS and 21 patients with chronic progressive (CP) MS. The ratio of resonance intensities of N-acetylaspartate over creatine (Cr) was found to be significantly lower than normal in normal appearing white matter (NAWM) of both RR and CP patients (19.6% in RR, 28.8% in CP), NAA/Cr was decreased even more in MS plaques than in NAWM (44.2% in RR, 17.7% in CP), NAA/Cr was correlated with clinical disability (p < 0.02) and disease duration (p < 0.1). Our results suggest that, in this setting, MRS reflects accumulated neuronal loss or damage and can be used as a measure of disease severity. The methods developed provide opportunities to evaluate the relationship between inflammation, demyelination, axonal loss and clinical disability in future studies.

Phosphorus metabolites in different muscles of the rat leg by 31P image-selected in vivo spectroscopy

The difference in concentration of phosphorylated metabolites in muscles with different fiber composition was studied in vivo by localized 31P nuclear magnetic resonance spectroscopy in the rat hindlimb 120-160 microliters volumes were selected in regions containing the soleus and gastrocnemius muscles. Concentrations of phosphocreatine (PCr), adenosine triphosphate and inorganic phosphate (Pi) were determined and intracellular pH was calculated in the respective muscle groups. The highest level of PCr was found in the gastrocnemius muscle, containing 30.7 mmoles/dm3 tissue compared to 22.3 mmoles/dm3 in the soleus muscle. Pi was significantly lower in gastrocnemius (1.9 mmoles/dm3) than in soleus (3.2 mmoles/dm3). The ATP concentration was 6.7 and 6.4 mmoles/dm3 and pH was determined to 7.11 and 7.09 in the gastrocnemius and soleus muscle, respectively. Our NMR data show that it is possible to measure high-energy phosphates with precision in small localized volumes with the ISIS method using a Helmholtz coil. Earlier biochemical data are confirmed by these in vivo NMR results. Localized in vivo 31P NMR spectroscopy can contribute to the understanding of the underlying mechanisms of several metabolic events in different regions of the tissue. The method can be used for future studies of varying ischemia tolerance, varying degrees of adaptation to exercise with regard to oxidative capacity, and pH compartmentation in muscles with different fiber composition.

Reproducibility of human cardiac 31P-NMR spectroscopy

The reproducibility of the phosphocreatine to adenosine triphosphate ratio (PCr/ATP) was assessed from cardiac phosphorus-31 (31P) NMR spectra of the human left ventricle acquired with three different localization techniques. Cardiac 31P-NMR spectra (n = 68) were obtained at rest from 16 healthy subjects with three-dimensional (3D) image selected in vivo spectroscopy (ISIS), 1D spectroscopic imaging (SI), or with a combination of 2D ISIS and the 1D SI technique (ISIS + SI). The average PCr/ATP ratios were 1.41 +/- 0.20 for ISIS + SI and 1.31 +/- 0.19 for ISIS and were in the lower range of values obtained in previous studies, mainly because of a lower saturation correction factor for the cardiac PCr/ATP ratio. The SI experiment yielded an average PCr/ATP value of 0.98 +/- 0.20, significantly lower as compared to the correct values obtained with ISIS + SI and ISIS (p < 0.001), underscoring the need for 3D localization to avoid contamination of the NMR signal by liver tissue. Intersubject standard deviations of the PCr/ATP ratio were comparable to values reported previously. For all three localization techniques the absolute intra-examination differences in PCr/ATP (0.06 for ISIS to 0.15 for ISIS + SI) were significantly smaller (p approximately 0.03) than inter-examination differences (0.24 for ISIS to 0.29 for ISIS + SI). Therefore, consecutive acquisition of cardiac 31P-NMR spectra from the same patient during a single examination, e.g. under various cardiac loading conditions, appears to be a reliable approach for metabolic evaluation of heart disease.

Simultaneous localized 1H STEAM/31P ISIS spectroscopy in vivo

A sequence for simultaneous acquisition of 1H STEAM and 31P ISIS spectra is described, and 1H and 31P spectra obtained simultaneously from the same volume of interest in both a phantom and a volunteer are presented. The STEAM and ISIS parts of the sequence use a common gradient scheme that is also used during the localized shimming process, partially compensating for eddy current effects. It is demonstrated that this method of simultaneous multinuclear spectroscopy does not compromise the localization performance of the sequence.

Quantitation of phosphorus metabolites in newborn human brain using internal water as reference standard

A new method for noninvasive, in vivo quantitation of cerebral phosphorus (31P) metabolites is described. The technique employs point-resolved spectroscopy (PRESS) to obtain both 31P-metabolite and proton (1H) water spectra: brain water is used as an internal concentration reference. Spin-spin relaxation times (T2s) of cerebral 31P metabolites are much longer than the minimum echo time (TE) usable on a spectrometer equipped with actively shielded gradient coils. With short-TE (approximately 10 ms) 31P PRESS, T2 relaxation is minimal and phase modulation of the nucleotide triphosphate (NTP) multiplets can be accounted for 1H water spectra were acquired using several TEs so that extra- and intracellular water signals could be separated from that due to cerebrospinal fluid. Prior calibration of the 31P and 1H spectrometer channels and an assumed brain-water concentration enabled estimations of metabolite concentrations. Using this method, mean 31P metabolite concentrations in the brains of eight normal infants of gestational plus postnatal age 34 to 39 wk were: phosphomonoester (PME) 5.6 (SD 0.9); inorganic phosphate 1.4 (0.4); mobile phosphodiester 2.3 (0.6); phosphocreatine 2.9 (0.3); nucleotide triphosphate 2.8 (0.6); and total mobile phosphate 21.4 (2.8) mmol/kg wet.

Influence of different fasting periods on P-31-MR-spectroscopy of the liver in normals and patients with liver metastases

The purpose of this study was to determine the influence of different fasting periods on the in vivo P-31-MR spectroscopy of the healthy liver and patients with liver metastases. Image-guided localized P-31-MRS was performed in 24 patients with liver metastases and in 20 healthy volunteers. The spectra were obtained with a whole body scanner operating at 1.5 T using a surface coil. The P-31-MRS was performed after a fasting period of 3-5 h (group 1) and after overnight fasting (group 2). The PME/beta-NTP, PDE/beta-NTP and Pi/beta-NTP were calculated from P-31-MR spectra and were compared in relation to the nutrition status of the volunteers and patients. The PME/beta-NTP and PDE/beta-NTP were significantly increased in spectra of patients with metastases. There were no significant changes in the ratios of phosphorus metabolites in healthy liver tissue or in liver metastases after a fasting period of 3-5 h as compared with overnight fasting.

Chemical pathology of acute demyelinating lesions and its correlation with disability

We report the chemical pathological changes on magnetic resonance spectroscopic images of 4 patients, each of whom had a single large demyelinating plaque. The patients were followed from soon after the onset of the symptoms for a minimum of 7 months to a maximum of 3 1/2 years. We observed increases in the relative resonance intensities of choline-containing compounds, lactate, and myo-inositol inside the lesion acutely. Decreases in relative resonance intensities of N-acetylaspartate and creatine were seen both in and around the magnetic resonance imaging-detected lesions. In all patients neurological deficits improved and creatine, lactate, and myo-inositol resonance intensities normalized during the follow-up. Choline compounds recovered more slowly and were still abnormally high in 1 patient after 7 months. Partial recovery of the N-acetylaspartate resonance was seen for all patients. Evaluation of the relationships between indices of cerebral chemical pathology, brain lesion volumes, and functional disability showed highly significant negative correlations between N-acetylaspartate resonance intensities and both brain lesion volumes (r = -0.80, p < 0.0001) and clinical disability (r = -0.73, p < 0.0001). As N-acetylaspartate is localized solely in neurons in the adult central nervous system, our results suggest that neuronal dysfunction may be a proximate mechanism of disability even in inflammatory disorders primarily affecting myelin and oligodendroglial cells.

Fat suppression at 7T using a surface coil: application of an adiabatic half-passage chemical shift selective radiofrequency pulse

The selective suppression of fat using chemical shift selective (CHESS) sinc, gaussian presaturation, or binomial radiofrequency pulses are widely implemented techniques in magnetic resonance imaging. For applications wherein transmitter coils that generate inhomogeneous magnetic (B1), fields are used (e.g., surface coils), adiabatic radiofrequency pulses that are less susceptible to spatial variations in B1 amplitude will improve the spatial homogeneity of spin excitation angle. Herein, we describe the application of an adiabatic half-passage hyperbolic secant CHESS pulse suitable for acquiring fat-suppressed magnetic resonance images with surface coils on high-field systems. Images obtained from a water/fat phantom and from the abdominal region of a rat are presented indicating excellent suppression of fat signal from the entire coil-sensitive volume.

Reversible decreases in N-acetylaspartate after acute brain injury

N-Acetylaspartate (NAA), which constitutes the major proportion of the dominant resonance in proton MR spectra of brain, is localized in mature brain exclusively in neurons and neuronal processes. A decrease in NAA has been observed in many cerebral pathologies and has usually been interpreted as an index of irreversible neuronal loss. The authors report a follow-up study of six patients with acute brain damage (four from demyelinating lesion and two from mitochondrial encephalopathy with lactic acidosis and stroke-like episodes [MELAS]). All patients underwent serial MR spectroscopy examinations. The four patients with acute demyelinating lesions initially showed decreases in NAA in the centers of the lesions that ranged between 34-72% of values from homologous brain volumes in the other hemisphere. All four patients subsequently showed substantial recovery of NAA as their clinical status improved. The two patients with MELAS syndrome had large decreases of NAA signal (50% and 20% of normal values, respectively) from their occipital lobe lesions during the acute stroke-like episodes. After the acute phase of the illness a progressive increase of NAA in the same volumes was seen in both patients (to 76% and 60% of normal values, respectively). These results demonstrate that significant recovery of NAA can occur after acute brain damage. The potential contribution of reversible neuronal dysfunction (as well as neuronal loss) must be considered in the interpretation of decreases in the NAA resonance associated with acute brain pathology.

Signal profile measurements for evaluation of the volume-selection performance of ISIS

High-resolution signal profiles obtained with a test phantom were used in this study to evaluate the volume-selection performance of an implementation of ISIS (Image Selected In vivo Spectroscopy). The phantom simulated the brain with regard to volume and loading of coil. A remotely controlled, movable signal source inside the phantom was filled with orthophosphoric acid. Signal profiles of the volume of interest (VOI) were measured in three perpendicular directions. Special interest was focused on the transition zones, the position of the profiles, and the effects of off-resonance and T1 smearing. The transition zones were on average 5.6 mm wide and the full width at half maximum (FWHM) was 35 mm for a VOI of 40 x 40 x 40 mm3. The positions of the centre of the signal profiles were x = 3.2, y = -0.7 and z = 3.3 mm off-centre. The deviation of the volume position could be explained by off-resonance effects during imaging and spectroscopy. These data illustrate the importance of detailed knowledge of the volume-selection performance when attempting precision measurements using image-guided in vivo MRS.

Evaluation of volume selection methods in in vivo MRS. Design of a new test phantom

In vivo MR spectroscopy (MRS) requires some kind of volume selection method to be able to measure the signal from a selected part of the body. To be able to interpret the spectra correctly, the quality of the volume selection must be investigated for each new MRS application using phantom measurements. A new phantom, especially suitable for precision measurements of the volume selection performance, is presented. It contains a small, remotely controlled signal source placed inside a larger vessel. This principle can be applied to various body regions, coil types and nuclei. The measurement conditions are close to the clinical situation. The phantom does not have to be repositioned during a signal profile measurement and the signal contribution from each point along the profile is determined regarding sign and amplitude.

In vivo 31P MRS: absolute concentrations, signal-to-noise and prior knowledge

Absolute metabolite concentrations have been estimated for nucleoside triphosphate and P(i) from in vivo 31P MR measurements using ISIS localization in a rat tumour model, and the results have been compared to those obtained from acid extracts of the tumours. The aim of the experiment was to assess the performance of four different spectral analysis techniques used for absolute quantitation. The spectral analysis techniques used were two frequency domain methods (peak area integration and Lorentzian fitting--FITSPEC) and two time domain methods (VARPRO and HLSVD). The spectra were acquired in blocks so that the degradation in performance of the four spectral analysis methods with decreasing signal-to-noise ratio (SNR) could be compared and referenced. This and the inclusion of a sophisticated method incorporating prior knowledge yields a more realistic and comprehensive protocol than previously published comparisons. The results suggest that VARPRO is the method of choice for quantitative analysis of tumour 31P MR spectra, giving the most reliable results at low SNR.

Leukoencephalopathy with swelling and a discrepantly mild clinical course in eight children

An identical syndrome of cerebral leukoencephalopathy and megalencephaly with infantile onset was discovered in 8 children, including 2 siblings. Neurological findings were initially normal or near normal, despite megalencephaly and magnetic resonance imaging (MRI) evidence of severe white matter affection. Slowly progressive ataxia and spasticity developed, while intellectual functioning was preserved for years after onset of the disorder. MRI characteristics included diffuse abnormality in signal intensity and swelling of the cerebral hemispheral white matter with cyst-like spaces in the frontoparietal and anterior-temporal subcortical areas. MR spectra were relatively mildly abnormal. Screening for inborn errors, especially those that cause either megalencephaly or white matter disease or both was negative. A distinguishing feature of the present disorder is the apparently severe abnormality of the cerebral white matter as demonstrated by MRI, which contrasts with the remarkably slow course of functional deterioration.

NOE enhancements and T1 relaxation times of phosphorylated metabolites in human calf muscle at 1.5 Tesla

Nuclear Overhauser effect (NOE) enhancements and relaxation times of 31P metabolites in human calf were measured in 12 volunteers (4 men and 8 women) at 1.5 T using a dual tuned four-ring birdcage. The NOE enhancements of inorganic phosphate (Pi), phosphocreatine (PCr), gamma-, alpha-, and beta-nucleoside triphosphate (NTP) from 19 measurements were 0.51 +/- 0.10, 0.64 +/- 0.03, 0.53 +/- 0.03, 0.56 +/- 0.08, and 0.47 +/- 0.05, respectively. The relaxation times were independent of proton irradiation and from 23 measurements were 3.49 +/- 0.35, 4.97 +/- 0.58, 4.07 +/- 0.36, 2.90 +/- 0.25, and 3.61 +/- 0.25 s for Pi, PCr, gamma-, alpha-, and beta-NTP, respectively. No significant differences between gender and age were observed for either NOE enhancements or relaxation times. Also, among nine volunteers, we observed no significant differences in T1 between the coupled and decoupled cases.

Calculation of sensitivity correction factors for surface coil MRS

Quantification of MRS signals obtained with surface coils is difficult due to the inhomogeneous response of these coils. This inhomogeneity results in the measured signal from a defined volume of interest (VOI) being spatially dependent. To account for the sensitivity variation with position from the surface coil, we have developed a method of calculating correction factors for defined VOIs based on an experimentally obtained 3D sensitivity coil map. These factors may then be applied to spectra obtained from these VOIs to accurately take into consideration the varying coil sensitivity resulting in a reduction of measured signal. This method is demonstrated here to be able to correct for the inhomogeneity of surface coils over a range of two coil radii to within 4% accuracy.

Clinical applicability of human in vivo localized phosphorus-31 magnetic resonance spectroscopy of bone and soft tissue tumors

BACKGROUND

Magnetic resonance imaging (MRI) is of restricted value for the in vivo characterization of tumor types. The applicability of phosphorus-31 (31P) magnetic resonance spectroscopy (MRS) in the diagnosis of bone and soft tissue tumors is unknown.

METHODS

A total of 191 consecutive patients (85 females and 106 males; mean age 41 years, range 1-80) with a well-defined bone or soft tissue tumor on MRI were analyzed for additional 31P spectroscopy. Histology and/or cytology was obtained from all tumors. Because of low sensitivity of the 31P nucleus and the contamination of surrounding tissue, only large, superficially located tumors accessible to the surface coil could be accepted for MRS.

RESULTS

Twenty-one patients (11%) could be included in the study. From this remaining group only 12 studies (57%) produced spectra with well resolved phosphorus peaks and an acceptable signal-to-noise ratio. However, these spectra did not allow differentiation between the benign and malignant nature of the lesions. The other 9 studies showed spectra with poor signal intensities and/or poorly defined peaks, making tumor differentiation impossible.

CONCLUSION

Only 6% of the bone and soft tissue tumors produced well defined spectra, which implies that localized 31P MRS cannot be considered as a routine technique in the diagnostic and treatment evaluation of bone and soft tissue tumors.

Assessment of absolute metabolite concentrations in human tissue by 31P MRS in vivo. Part I: Cerebrum, cerebellum, cerebral gray and white matter

Absolute metabolite concentrations were determined in four different brain regions using phosphorus magnetic resonance spectroscopy (31P MRS) on 10 healthy adult volunteers. Localized spectra were collected simultaneously from the cerebellum and the cerebrum and, later, from deep white matter and cortical gray matter by means of a two-volume ISIS pulse sequence and a Helmholtz-type RF-coli. Each brain spectrum was quantified with a calibration spectrum from a head-shaped simulation phantom. A time-domain fitting routine was used to process the fully relaxed data. Several metabolite concentrations (mmol/liter) differed significantly between the cerebrum and the cerebellum (PME = 3.2 +/- 0.3 and 4.0 +/- 0.6, PCr = 2.9 +/- 0.3 and 3.9 +/- 0.4, NTP = 2.9 +/- 0.2 and 2.6 +/- 0.2, respectively) and between cortical gray matter and deep white matter (PME = 3.1 +/- 0.4 and 4.3 +/- 0.8, PDE = 10.1 +/- 2.5 and 14.2 +/- 2.6, respectively). The concentration of free magnesium ion was found to be similar in all four brain regions (0.53 +/- 0.21 mmol/liter) but the intracellular pH was significantly higher in the cerebellum (7.04 +/- 0.03) than in the cerebrum (6.99 +/- 0.02).

Saturation correction in human cardiac 31P MR spectroscopy at 1.5 T

This study was conducted to verify the validity of using saturation factors obtained from unlocalized 31P spectra containing both chest wall and heart muscle signals for correcting human heart muscle phosphocreatine/beta-adenosine triphosphate (PCr/beta-ATP) ratios. Saturation factors and T1 relaxation times were determined from 31P magnetic resonance spectra of human chest wall and heart muscle simultaneously in healthy volunteers using one-dimensional spectroscopic imaging in combination with a two-dimensional ISIS sequence by using adiabatic 180 degrees inversion and adiabatic 90 degrees excitation pulses at 1.5 T. Blood corrected saturation factors for PCr/beta-ATP at a TR of 2.4 s were significantly different in heart muscle and chest wall muscle, 1.30 +/- 0.25 and 1.73 +/- 0.31, respectively (p < 0.05). T1 values for PCr and beta-ATP in heart muscle were 4.28 +/- 0.72 and 2.99 +/- 0.52 and in chest wall muscle 6.82 +/- 1.07 and 3.39 +/- 0.48, respectively. The T1(PCr)/T1(beta-ATP) ratios in chest wall and heart muscle were not identical. The mean PCr/beta-ATP ratios in heart and chest wall muscle of six healthy volunteers were 1.23 +/- 0.17 and 3.71 +/- 0.53, respectively.

Comparison of calibration strategies for the in vivo determination of absolute metabolite concentrations in the human brain by 31P MRS

Cerebral concentrations of phosphorus metabolites can be assessed non-invasively by 31P MRS provided the metabolite signals are calibrated with the signal of a standard of known concentration. The reliability of the concentration estimates depends mainly on the strategy of calibration. Three strategies were compared by assessing the concentrations both in a test dummy and in the brain of volunteers. The first strategy utilized tissue water as an internal heteronuclear concentration standard. The second and third strategies used as phosphorus solution as an external homonuclear standard; this solution was either put into a reference bottle placed on top of the head or into a simulation phantom measured instead of the head. Localization was always achieved with the ISIS pulse sequence. The two external homonuclear strategies achieved a higher accuracy (mean error approximately 5%) and reproducibility (mean SD approximately 8%) of the concentration estimates than the internal heteronuclear strategy (mean error approximately 11%; mean SD approximately 15%).

ATP production rates as a function of force level in the human gastrocnemius/soleus using 31P MRS

Net forward adenosine triphosphate (ATP) production rates were calculated from 31P nuclear magnetic resonance spectroscopy (MRS) kinetic data collected with 1-8 s time resolution during isometric voluntary contractions of the human gastrocnemius/soleus muscle group. Volume normalized muscle output (Newtons/ml) was then divided by the calculated net ATP use (mM/s) to estimate the metabolic economy (ME) (Newtons.s/mumol ATP). The ATP production rates from anaerobic glycolysis (An Gly) and creatine kinase (CK) reactions are approximately half of the oxidative phosphorylation ATP production rates (Ox Phos) at the end of 90 s of isometric contractions for a series of force levels. However, ME was independent of the force level in these exercises. The correlation between MVC of healthy, trained subjects, and the maximal cross-sectional area of the gastrocnemius/soleus shows an average of 21.2 +/- 4.6 Newtons/cm2 (mean +/- SD, N = 15). This was measured using a foot pedal with a transducer measurement point 10.6 cm above the heel. [ADP] versus Ox Phos ATP production rate fits a Michaelis-Menten kinetic control equation with an offset (underestimation of Ox Phos) of 0.3 mM/s and a Km = 27 microM for ADP and a Vmax = 1.0 mM/s. This suggests that [ADP] is the controlling factor for mitochondrial function at the end of a 90 to 120 s isometric contraction in normal subjects at any force level. Quantitative measurements of the phosphorus metabolite concentrations were obtained from 20 individuals and these data are also reported.