Effects of bolus injection duration on perfusion estimates in dynamic CT and dynamic susceptibility contrast MRI.

Estimates of cerebral blood flow (CBF) and tissue mean transit time (MTT) have been shown to differ between dynamic CT perfusion (CTP) and dynamic susceptibility contrast MRI (DSC-MRI). This study investigates whether these discrepancies regarding CBF and MTT between CTP and DSC-MRI can be attributed to the different injection durations of these techniques. Five subjects were scanned using CTP and DSC-MRI. Region-wise estimates of CBF, MTT, and cerebral blood volume (CBV) were derived based on oscillatory index regularized singular value decomposition. A parametric model that reproduced the shape of measured time curves and characteristics of resulting perfusion parameter estimates was developed and used to simulate data with injection durations typical for CTP and DSC-MRI for a clinically relevant set of perfusion scenarios and noise levels. In simulations, estimates of CBF/MTT showed larger negative/positive bias and increasing variability for CTP when compared to DSC-MRI, especially for high CBF levels. While noise also affected estimates, at clinically relevant levels, the injection duration effect was larger. There are several methodological differences between CTP and DSC-MRI. The results of this study suggest that the injection duration is among those that can explain differences in estimates of CBF and MTT between these bolus tracking techniques.

Neurochemical properties measured by 1H magnetic resonance spectroscopy may predict cognitive behaviour therapy outcome in paediatric OCD: a pilot study.

To identify neurochemical factors measured pre-treatment that may predict cognitive behavioural treatment (CBT) outcome, aiming at understanding possible causes of poor CBT response. 1H magnetic resonance spectroscopy was used before treatment with CBT in treatment naïve 11-18 year-old patients with moderate-severe OCD. Diagnoses and assessment of OCD severity were based on semi-structured interviews. Linear mixed effects models were used to analyse the association between metabolite level and treatment outcome. Worse CBT outcome was associated with higher concentration of glutamine and glutamate combined (Glx) in middle cingulate cortex (MCC) (F = + 3.35, p = 0.004) and of N-acetylaspartate and N-acetylaspartylglutamate combined (tNAA) (F = + 2.59, p = 0.019). Also, we noted a tendency towards higher thalamic Glx concentration (F = + 1.91, p = 0.077) to be associated with worse CBT outcome. In general, the findings of the current pilot study are compatible with the hypothesis of an overweight of excitatory to inhibitory factors in brain circuits driving goal-directed behaviours (GDB). Higher MCC Glx and tNAA may be involved in the selection of GDB. A more detailed understanding of how these brain areas function in health and illness is needed.

Optimization of b-value schemes for estimation of the diffusion coefficient and the perfusion fraction with segmented intravoxel incoherent motion model fitting.

PURPOSE: Intravoxel incoherent motion (IVIM) modeling for estimation of the diffusion coefficient (D) and perfusion fraction (f) is increasingly popular, but no consensus on standard protocols exists. This study provides a framework for optimization of b-value schemes for reduced estimation uncertainty of D and f from segmented model fitting. THEORY: Analytical expressions for uncertainties of D and f from segmented model fitting were derived as Cramer-Rao lower bounds (CRLBs). METHODS: Optimized b-value schemes were obtained for 3 to 12 acquisitions and in the limit of infinitely many acquisitions through constrained minimization of the CRLBs, with b-values constrained to be 0 or 200 to 800 s/mm2 . The optimized b-value scheme with eight acquisitions was compared with b-values linearly distributed in the allowed range using simulations and in vivo liver data from seven healthy volunteers. RESULTS: All optimized b-value schemes contained exactly three unique b-values regardless of the total number of acquisitions (0, 200, and 800 s/mm2 ) with repeated acquisitions distributed approximately as 1:2:2. Compared with linearly distributed b-values, the variability of estimates of D and f was reduced by approximately 30% as seen both in simulations and in repeated in vivo measurements. CONCLUSION: The uncertainty of IVIM D and f estimates can be reduced by the use of optimized b-value schemes.

Data-driven identification of tumor subregions based on intravoxel incoherent motion reveals association with proliferative activity.

PURPOSE: Intravoxel incoherent motion (IVIM) analysis gives information on tissue diffusion and perfusion and may thus have a potential for e.g. tumor tissue characterization. This work aims to study if clustering based on IVIM parameter maps can identify tumor subregions, and to assess the relevance of obtained subregions by histological analysis. METHODS: Fourteen mice with human neuroendocrine tumors were examined with diffusion-weighted imaging to obtain IVIM parameter maps. Gaussian mixture models with IVIM maps from all tumors as input were used to partition voxels into k clusters, where k = 2 was chosen for further analysis based on goodness of fit. Clustering was performed with and without the perfusion-related IVIM parameter D* , and with and without including spatial information. The validity of the clustering was assessed by comparison with corresponding histologically stained tumor sections. A Ki-67-based index quantifying the degree of tumor proliferation was considered appropriate for the comparison based on the obtained cluster characteristics. RESULTS: The clustering resulted in one class with low diffusion and high perfusion and another with slightly higher diffusion and low perfusion. Strong agreement was found between tumor subregions identified by clustering and subregions identified by histological analysis, both regarding size and spatial agreement. Neither D* nor spatial information had substantial effects on the clustering results. CONCLUSIONS: The results of this study show that IVIM parameter maps can be used to identify tumor subregions using a data-driven framework based on Gaussian mixture models. In the studied tumor model, the obtained subregions showed agreement with proliferative activity.

Impact of prior distributions and central tendency measures on Bayesian intravoxel incoherent motion model fitting.

PURPOSE: Bayesian model fitting has been proposed as a robust alternative for intravoxel incoherent motion (IVIM) model-fitting parameter estimation. However, consensus regarding choice of prior distribution and posterior distribution central tendency measure is needed. The aim of this study was to compare the quality of IVIM parameter estimates produced by different prior distributions and central tendency measures, and to gain knowledge about the effect of these choices. METHODS: Three prior distributions (uniform, reciprocal, and lognormal) and two measures of central tendency (mean and mode) found in the literature were studied using simulations and in vivo data from a tumor mouse model. RESULTS: Simulations showed that the uniform and lognormal priors were superior to the reciprocal prior, especially for the parameters D and f and clinically relevant SNR levels. The choice of central tendency measure had less effect on the results, but had some effects on estimation bias. Results based on simulations and in vivo data agreed well, indicating high validity of the simulations. CONCLUSIONS: Choice of prior distribution and central tendency measure affects the results of Bayesian IVIM parameter estimates. This must be considered when comparing results from different studies. The best overall quality of IVIM parameter estimates was obtained using the lognormal prior. Magn Reson Med 79:1674-1683, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Grey matter correlates of autistic traits in women with anorexia nervosa.

BACKGROUND: Patients with anorexia nervosa exhibit higher levels of behaviours typically associated with autism-spectrum disorder (ASD), but the neural basis is unclear. We sought to determine whether elevated autistic traits in women with anorexia nervosa may be reflected in cortical morphology. METHODS: We used voxel-based morphometry (VBM) to examine regional grey matter volumes in high-resolution MRI structural brain scans in women with anorexia nervosa and matched healthy controls. The Autism-spectrum Quotient (AQ) scale was used to assess autistic traits. RESULTS: Women with anorexia nervosa (n = 25) had higher AQ scores and lower bilateral superior temporal sulcus (STS) grey matter volumes than the control group (n = 25). The AQ scores correlated negatively with average left STS grey matter volume in women with anorexia nervosa. LIMITATIONS: We did not control for cognitive ability and examined only women with ongoing anorexia nervosa. CONCLUSION: Elevated autistic traits in women with anorexia nervosa are associated with morphometric alterations of brain areas linked to social cognition. This finding provides neurobiological support for the behavioural link between anorexia nervosa and ASD and emphasizes the importance of recognizing autistic traits in preventing and treating anorexia nervosa.

Comparison of methods for estimation of the intravoxel incoherent motion (IVIM) diffusion coefficient (D) and perfusion fraction (f).

OBJECTIVE: Intravoxel incoherent motion (IVIM) shows great potential in many applications, e.g., tumor tissue characterization. To reduce image-quality demands, various IVIM analysis approaches restricted to the diffusion coefficient (D) and the perfusion fraction (f) are increasingly being employed. In this work, the impact of estimation approach for D and f is studied. MATERIALS AND METHODS: Four approaches for estimating D and f were studied: segmented IVIM fitting, least-squares fitting of a simplified IVIM model (sIVIM), and Bayesian fitting of the sIVIM model using marginal posterior modes or posterior means. The estimation approaches were evaluated in terms of bias and variability as well as ability for differentiation between tumor and healthy liver tissue using simulated and in vivo data. RESULTS: All estimation approaches had similar variability and ability for differentiation and negligible bias, except for the Bayesian posterior mean of f, which was substantially biased. Combined use of D and f improved tumor-to-liver tissue differentiation compared with using D or f separately. DISCUSSION: The similar performance between estimation approaches renders the segmented one preferable due to lower numerical complexity and shorter computational time. Superior tissue differentiation when combining D and f suggests complementary biologically relevant information.

Grey matter correlates of autistic traits in women with anorexia nervosa

Background: Patients with anorexia nervosa exhibit higher levels of behaviours typically associated with autism-spectrum disorder (ASD), but the neural basis is unclear. We sought to determine whether elevated autistic traits in women with anorexia nervosa may be reflected in cortical morphology. Methods: We used voxel-based morphometry (VBM) to examine regional grey matter volumes in high-resolution MRI structural brain scans in women with anorexia nervosa and matched healthy controls. The Autism-spectrum Quotient (AQ) scale was used to assess autistic traits. Results: Women with anorexia nervosa (n = 25) had higher AQ scores and lower bilateral superior temporal sulcus (STS) grey matter volumes than the control group (n = 25). The AQ scores correlated negatively with average left STS grey matter volume in women with anorexia nervosa. Limitations: We did not control for cognitive ability and examined only women with ongoing anorexia nervosa. Conclusion: Elevated autistic traits in women with anorexia nervosa are associated with morphometric alterations of brain areas linked to social cognition. This finding provides neurobiological support for the behavioural link between anorexia nervosa and ASD and emphasizes the importance of recognizing autistic traits in preventing and treating ­anorexia nervosa.

1H magnetic resonance spectroscopy evidence for occipital involvement in treatment-naive paediatric obsessive-compulsive disorder.

OBJECTIVE: Obsessive-compulsive disorder (OCD) is a chronic psychiatric disorder leading to considerable distress and disability. Therapies are effective in a majority of paediatric patients, however, many only get partial response. It is therefore important to study the underlying pathophysiology of the disorder. METHODS: 1H magnetic resonance spectroscopy (MRS) was used to study the concentration of brain metabolites in four different locations (cingulate gyrus and sulcus, occipital cortex, thalamus and right caudate nucleus). Treatment-naive children and adolescents with OCD (13 subjects) were compared with a group of healthy age- and gender-matched subjects (11 subjects). Multivariate analyses were performed on the concentration values. RESULTS: No separation between controls and patients was found. However, a correlation between metabolite concentrations and symptom severity as measured with the Children's Yale-Brown Obsessive-Compulsive Scale (CY-BOCS) was found. Strongest was the correlation with the CY-BOCS obsession subscore and aspartate and choline in the caudate nucleus (positively correlated with obsessions), lipids at 2 and 0.9 ppm in thalamus, and occipital glutamate+glutamine, N-acetylaspartate and myo-inosytol (negatively correlated with obsessions). CONCLUSIONS: The observed correlations between 1H MRS and CY-BOCS in treatment-naive patients further supports an occipital involvement in OCD. The results are consistent with our previous study on adult OCD patients. The 1H MRS data were not supportive of a separation between the patient and control groups.

Impact of region-of-interest method on quantitative analysis of DTI data in the optic tracts.

BACKGROUND: To extract DTI parameters from a specific structure, a region of interest (ROI) must be defined. ROI selection in small structures is challenging; the final measurement results could be affected due to the significant impact of small geometrical errors. In this study the optic tracts were analyzed with the aim to assess differences in DTI parameters due to ROI method and to identify the most reliable method. METHODS: Images of 20 healthy subjects were acquired. Fractional anisotropy (FA) was extracted from the optic tracts by four different ROI methods. Manual tracing was performed in 1) the b0 image and 2) a T1-weighted image registered to the FA image. Semi-automatic segmentation was performed based on 3) tractography and 4) the FA-skeleton algorithm in the tract-based spatial statistics (TBSS) framework. Results were analyzed with regard to ROI method as well as to inter-scan, intra-rater and inter-rater reliability. RESULTS: The resulting FA values divided the ROI methods into two groups that differed significantly: 1) the FA-skeleton and the b0 methods showed higher FA values compared to 2) the tractography and the T1-weighted methods. The intra- and inter-rater variabilities were similar for all methods, except for the tractography method where the inter-rater variability was higher. The FA-skeleton method had a better reproducibility than the other methods. CONCLUSION: Choice of ROI method was found to be highly influential on FA values when the optic tracts were analyzed. The FA-skeleton method performed the best, yielding low variability and high repeatability.

In Patients With Idiopathic Normal Pressure Hydrocephalus Postoperative Cerebral Perfusion Changes Measured by Dynamic Susceptibility Contrast Magnetic Resonance Imaging Correlate With Clinical Improvement.

OBJECTIVE: To explore relationships between clinical improvement and relative cerebral blood flow (rCBF) changes after shunt-insertion in idiopathic normal pressure hydrocephalus (iNPH) as measured by dynamic susceptibility contrast magnetic resonance imaging. METHODS: In 20 idiopathic normal pressure hydrocephalus patients rCBF was measured preoperatively and 3 months postoperatively. Because of shunt-induced right-sided artefacts, evaluation was restricted to 12 left-sided cortical, subcortical, and periventricular regions of interest. Correlations between rCBF and clinical symptoms were analyzed in shunt responders. RESULTS: In responders, the postoperative regions of interest-based rCBF increase of 2% to 9% was significant in the parenchyma, the hippocampus, and the anterior periventricular white matter. Perfusion improvement in the cingulus, caudate head, and thalamus correlated with decreased disturbance in one or more of the domains neuropsychology, gait, balance, and total performance. CONCLUSIONS: Apparently, dynamic susceptibility contrast magnetic resonance imaging can measure postoperative perfusion changes in responders. Postoperatively, perfusion increase in some grey matter structures seems to determine the degree of clinical improvement.

Tractography of Meyer's loop for temporal lobe resection­validation by prediction of postoperative visual field outcome‬‬‬‬.

BACKGROUND: Postoperative visual field defects are common after temporal lobe resection because of injury to the most anterior part of the optic radiation, Meyer's loop. Diffusion tensor tractography is a promising technique for visualizing the optic radiation preoperatively. The aim of this study was to assess the anatomical accuracy of Meyer's loop, visualized by the two most common tractography methods­deterministic (DTG) and probabilistic tractography (PTG)­in patients who had undergone temporal lobe resection. METHODS: Eight patients with temporal lobe resection for temporal lobe pathology were included. Perimetry and diffusion tensor imaging were performed pre- and postoperatively. Two independent operators analyzed the distance between the temporal pole and Meyer's loop (TP-ML) using DTG and PTG. Results were compared to each other, to data from previously published dissection studies and to postoperative perimetry results. For the latter, Spearman's rank correlation coefficient (r(s)) was used. RESULTS: Median preoperative TP-ML distances for nonoperated sides were 42 and 35 mm, as determined by DTG and PTG, respectively. TP-ML assessed with PTG was a closer match to dissection studies. Intraclass correlation coefficients were 0.4 for DTG and 0.7 for PTG. Difference between preoperative TP-ML (by DTG and PTG, respectively) and resection length could predict the degree of postoperative visual field defects (DTG: r(s) = -0.86, p < 0.05; PTG: r(s) = -0.76, p < 0.05). CONCLUSION: Both DTG and PTG could predict the degree of visual field defects. However, PTG was superior to DTG in terms of reproducibility and anatomical accuracy. PTG is thus a strong candidate for presurgical planning of temporal lobe resection that aims to minimize injury to Meyer's loop.

Cerebral perfusion measured by dynamic susceptibility contrast MRI is reduced in patients with idiopathic normal pressure hydrocephalus.

PURPOSE: To demonstrate in idiopathic normal pressure hydrocephalus (iNPH) patients by dynamic susceptibility contrast MRI a reduced preoperative cerebral blood flow (CBF) which correlates with the severity of clinical symptoms and predicts shunt outcome. MATERIALS AND METHODS: In cortical, subcortical, periventricular regions and along peri-and paraventricular profiles absolute perfusion values were estimated by multi-slice DSC MRI in 21 iNPH patients and 16 age-matched healthy individuals (HI). Relative CBF (rCBF), calculated with the occipital cortex as internal reference, was used for comparison between groups and for correlation analysis between regional rCBF and symptoms or outcome. RESULTS: iNPH patients showed significantly decreased rCBF in the basal medial frontal cortex, hippocampus, lentiform nucleus, periventricular white matter (PVWM), central grey matter and the global parenchyma as compared to HI. iNPH patients with higher preoperative rCBF in the PVWM performed better in clinical tests. A lower overall preoperative function resulted in a more obvious recovery after shunt insertion. Shunt-responders had higher rCBF values in the basal medial frontal cortex than non-responders. CONCLUSION: DSC MRI perfusion is a potentially useful diagnostic tool in iNPH and perfusion based criteria might be possible predictors of shunt response.

31P MR spectroscopy to evaluate the efficacy of hepatic artery embolization in the treatment of neuroendocrine liver metastases.

BACKGROUND: It is common to treat patients with metastatic disease from gastrointestinal neuroendocrine (NE) tumors with surgical reduction to prolong survival. This can be combined with hepatic arterial embolization (HAE) and medical treatment to reduce hormonal symptoms. Today there are no rapid and reliable methods to evaluate the efficacy of HAE in the treatment of neuroendocrine liver metastasis. PURPOSE: To investigate metabolic changes in hepatic metastases of NE tumors following HAE, and to establish if there are any early spectral patterns that might indicate therapeutic efficacy based on in vivo (31)P MRS data. MATERIAL AND METHODS: Volume selective (31)P MRS was used to study 11 patients with disseminated NE tumors with regional lymph nodes and bilobar liver metastases. Measurements were performed before and 1 and 3 days after HAE. RESULTS: Non-responders had significantly higher PME/Pi and αNTP/ΣNTP ratios than the responders before HAE (P < 0.05). Three days after HAE, non-responders still had significantly higher αNTP/ΣNTP than the responders did (P < 0.05). We also observed trends for increased PME ratios 3 days after HAE, decreased ATP-levels, and liberated Pi in responders. CONCLUSION: This (31)P-MRS study showed significant differences in PME/Pi and αNTP/ΣP ratios between responders and non-responders on the day before HAE, which is an interesting finding that may reflect intrinsic properties of the tumor tissue. We also observed trends for cell membrane renewal and increased energy consumption in responders after HAE. These results demonstrate potentials for (31)P-MRS to predict individual responsiveness prior to HAE.

A Longitudinal Study of Medial Temporal Lobe Volumes in Graves Disease.

CONTEXT: Neuropsychiatric symptoms are common features of Graves disease (GD) in hyperthyroidism and after treatment. The mechanism behind these symptoms is unknown, but reduced hippocampal volumes have been observed in association with increased thyroid hormone levels. OBJECTIVE: This work aimed at investigating GD influence on regional medial temporal lobe (MTL) volumes. METHODS: Sixty-two women with newly diagnosed GD underwent assessment including magnetic resonance (MR) imaging in hyperthyroidism and 48 of them were followed up after a mean of 16.4 ±â€…4.2 SD months of treatment. Matched thyroid-healthy controls were also assessed twice at a 15-month interval. MR images were automatically segmented using multiatlas propagation with enhanced registration. Regional medial temporal lobe (MTL) volumes for amygdalae and hippocampi were compared with clinical data and data from symptom questionnaires and neuropsychological tests. RESULTS: Patients had smaller MTL regions than controls at inclusion. At follow-up, all 4 MTL regions had increased volumes and only the volume of the left amygdala remained reduced compared to controls. There were significant correlations between the level of thyrotropin receptor antibodies (TRAb) and MTL volumes at inclusion and also between the longitudinal difference in the levels of free 3,5,3'-triiodothyronine and TRAb and the difference in MTL volumes. There were no significant correlations between symptoms or test scores and any of the 4 MTL volumes. CONCLUSION: Dynamic alterations in the amygdalae and hippocampi in GD reflect a previously unknown level of brain involvement both in the hyperthyroid state of the condition and after treatment. The clinical significance, as well as the mechanisms behind these novel findings, warrant further study of the neurological consequences of GD.

Brain structural and functional correlates to defense-related inhibition of muscle sympathetic nerve activity in man.

An individual's blood pressure (BP) reactivity to stress is linked to increased risk of hypertension and cardiovascular disease. However, inter- and intra-individual BP variability makes understanding the coupling between stress, BP reactivity, and long-term outcomes challenging. Previous microneurographic studies of sympathetic signaling to muscle vasculature (i.e. muscle sympathetic nerve activity, MSNA) have established a neural predictor for an individual's BP reactivity during short-lasting stress. Unfortunately, this method is invasive, technically demanding, and time-consuming and thus not optimal for widespread use. Potential central nervous system correlates have not been investigated. We used MagnetoEncephaloGraphy and Magnetic Resonance Imaging to search for neural correlates to sympathetic response profiles within the central autonomic network and sensorimotor (Rolandic) regions in 20 healthy young males. The main correlates include (a) Rolandic beta rebound and an anterior cingulate cortex (ACC) response elicited by sudden stimulation and (b) cortical thickness in the ACC. Our findings highlight the involvement of the ACC in reactions to stress entailing peripheral sympathetic responses to environmental stimuli. The Rolandic response furthermore indicates a surprisingly strong link between somatosensory and autonomic processes. Our results thus demonstrate the potential in using non-invasive neuroimaging-based measures of stress-related MSNA reactions, previously assessed only using invasive microneurography.

Degraded water suppression in small volume ¹H MRS due to localised shimming.

OBJECT: High quality, artifact free, ¹H MRS require a homogeneous magnetic field over the volume of interest (VOI) and an effective global water suppression (WS). However, the shim coils act globally and, hence, affect the resonance frequency over the entire object, making the frequency selective WS spatially selective. Unsuppressed water signal may cause spurious echo artifacts. AIM: Study and visualise the effect localised shimming has on WS. MATERIALS AND METHODS: A water suppression imaging (WSI) scan for visualising regions unaffected by the WS was designed and applied in vivo for two different VOI localisations, at two field strengths. Two shim optimisation methods, and first and second order shims were compared. In addition, shim settings for six VOI localisations were retrospectively investigated. RESULTS: The WSI-scan effectively visualised the spatial extent of the WS. The coverage decreased when the shim was optimised on a smaller VOI. Second order shims accentuated the problem, resulting, on average, in a WS coverage of only 35% of the head volume. CONCLUSION: Localised shimming can result in large regions of unsuppressed water, which can lead to spurious echo artifacts in the spectrum. To help overcome these problems globally optimised shims can be used during WS.

Erratum: Reliable phase-contrast flow volume magnetic resonance measurements are feasible without adjustment of the velocity encoding parameter.

[This corrects the article DOI: 10.1117/1.JMI.7.6.063502.].

Contrast agent influences MRI phase-contrast flow measurements in small vessels.

Contrast-enhanced MR angiography is often combined with phase contrast (PC) flow measurement to answer a particular clinical question. The contrast agent that is administered during contrast-enhanced MR angiography may still be present in the blood during the consecutive PC flow measurement. The aim of this work was to evaluate the influence of contrast agent on PC flow measurements in small vessels. For that purpose, both in vivo measurements and computer simulations were performed. The dependence of the PC flow quantification on the signal amplitude difference between blood and stationary background tissue for various vessel sizes was characterized. Results show that the partial-volume effect strongly affects the accuracy of the PC flow quantification when the imaged vessel is small compared to the spatial resolution. A higher blood-to-background-contrast level during imaging significantly increases the partial-volume effect and thereby reduces the accuracy of the flow quantification. On the other hand, a higher blood-to-background-contrast level facilitated the segmentation of the vessel for flow rate determination. PC flow measurements should therefore be performed after contrast agent administration in large vessels, but before contrast agent administration in small vessels.

Anterior to posterior hippocampal MRS metabolite difference is mainly a partial volume effect.

BACKGROUND: The concentration of N-acetylaspartate (NAA) in hippocampus, as measured with magnetic resonance spectroscopy (MRS), and the ratio of NAA/(choline (Cho) + creatine (Cr)) are valuable tools in the lateralization of temporal lobe epilepsy (TLE). MRS of hippocampus is also increasingly used to study certain psychiatric and degenerative diseases. However, the reliability of such measurements of hippocampus has been questioned. PURPOSE: To re-evaluate MRS imaging data from prior control subjects with regard to variation of metabolite concentrations in hippocampus from anterior to posterior and the partial volume (R)contribution to the measurements from adjacent tissue. MATERIAL AND METHODS: Twelve healthy subjects, mean age 33 years, were studied with MRS imaging. The measurement volume was angled along the temporal horns and metabolite concentration images were reconstructed at the MR system. Regions of interest (ROIs) in the anterior, medial, and posterior parts of both hippocampi were evaluated. Signal normalization to the total MRS signal from all ROIs permitted pooling of individual data with different and unknown signal scaling. One subject was re-examined with a high resolution three-dimensional (3D) volume of the brain for evaluation of partial volumes in the MRS examination. RESULTS: Overall, there were significantly lower concentrations of NAA in the anterior parts, and of (Cho+Cr) in the posterior parts, while the NAA/(Cho+Cr) ratio in the posterior parts of the mesial temporal lobes was significantly higher. Hippocampus accounted for one-half, one-third, and one-quarter of the anterior, middle, and posterior ROIs, respectively. The NAA/(Cho+Cr) ratio thus showed a reverse relationship to the relative volume of hippocampal tissue within the ROI. CONCLUSION: Metabolite concentrations in the mesial temporal lobe obtained with MRS imaging represent the mean value of hippocampus and a considerable amount of adjacent tissue. To assess the hippocampus alone, an actual voxel well below 1 cm(3) and a sub-centimeter slice thickness are required.