Contrast-enhanced magnetic resonance imaging of central nervous system tumors: agents, mechanisms, and applications

Evolving Characteristics of Gadolinium-Based Contrast Agents for MR Imaging: A Systematic Review of the Importance of Relaxivity

Gadolinium-based contrast agents (GBCAs) are widely and routinely used to enhance the diagnostic performance of magnetic resonance imaging and magnetic resonance angiography examinations. T1 relaxivity (r1) is the measure of their ability to increase signal intensity in tissues and blood on T1-weighted images at a given dose. Pharmaceutical companies have invested in the design and development of GBCAs with higher and higher T1 relaxivity values, and "high relaxivity" is a claim frequently used to promote GBCAs, with no clear definition of what "high relaxivity" means, or general concurrence about its clinical benefit. To understand whether higher relaxivity values translate into a material clinical benefit, well-designed, and properly powered clinical studies are necessary, while mere in vitro measurements may be misleading. This systematic review of relevant peer-reviewed literature provides high-quality clinical evidence showing that a difference in relaxivity of at least 40% between two GBCAs results in superior diagnostic efficacy for the higher-relaxivity agent when this is used at the same equimolar gadolinium dose as the lower-relaxivity agent, or similar imaging performance when used at a lower dose. Either outcome clearly implies a relevant clinical benefit. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 3.

Non-invasive assessment of normal and impaired iron homeostasis in the brain

Strict iron regulation is essential for normal brain function. The iron homeostasis, determined by the milieu of available iron compounds, is impaired in aging, neurodegenerative diseases and cancer. However, non-invasive assessment of different molecular iron environments implicating brain tissue's iron homeostasis remains a challenge. We present a magnetic resonance imaging (MRI) technology sensitive to the iron homeostasis of the living brain (the r1-r2* relaxivity). In vitro, our MRI approach reveals the distinct paramagnetic properties of ferritin, transferrin and ferrous iron ions. In the in vivo human brain, we validate our approach against ex vivo iron compounds quantification and gene expression. Our approach varies with the iron mobilization capacity across brain regions and in aging. It reveals brain tumors' iron homeostasis, and enhances the distinction between tumor tissue and non-pathological tissue without contrast agents. Therefore, our approach may allow for non-invasive research and diagnosis of iron homeostasis in living human brains.

Imaging biomarkers associated with extra-axial intracranial tumors: a systematic review

Extra-axial brain tumors are extra-cerebral tumors and are usually benign. The choice of treatment for extra-axial tumors is often dependent on the growth of the tumor, and imaging plays a significant role in monitoring growth and clinical decision-making. This motivates the investigation of imaging biomarkers for these tumors that may be incorporated into clinical workflows to inform treatment decisions. The databases from Pubmed, Web of Science, Embase, and Medline were searched from 1 January 2000 to 7 March 2022, to systematically identify relevant publications in this area. All studies that used an imaging tool and found an association with a growth-related factor, including molecular markers, grade, survival, growth/progression, recurrence, and treatment outcomes, were included in this review. We included 42 studies, comprising 22 studies (50%) of patients with meningioma; 17 studies (38.6%) of patients with pituitary tumors; three studies (6.8%) of patients with vestibular schwannomas; and two studies (4.5%) of patients with solitary fibrous tumors. The included studies were explicitly and narratively analyzed according to tumor type and imaging tool. The risk of bias and concerns regarding applicability were assessed using QUADAS-2. Most studies (41/44) used statistics-based analysis methods, and a small number of studies (3/44) used machine learning. Our review highlights an opportunity for future work to focus on machine learning-based deep feature identification as biomarkers, combining various feature classes such as size, shape, and intensity. Systematic Review Registration: PROSPERO, CRD42022306922.

Comparison of contrast-enhanced T1-weighted imaging using DANTE-SPACE, PETRA, and MPRAGE: a clinical evaluation of brain tumors at 3 Tesla

Background

We aimed to compare the performance of three contrast-enhanced T1-weighted three-dimensional (3D) magnetic resonance (MR) sequences to detect brain tumors at 3 Tesla. The three sequences were: (I) delay alternating with nutation for tailored excitation sampling perfection with application-optimized contrasts using different flip angle evolution (DANTE-SPACE), (II) pointwise encoding time reduction with radial acquisition (PETRA), and (III) magnetization-prepared rapid acquisition with gradient echo (MPRAGE).

Methods

This study involved 77 consecutive patients, including 34 patients with known primary brain tumors and 43 patients suspected of intracranial metastases. All patients underwent each of the three sequences with comparable spatial resolution and acquisition time post-injection. Signal-to-noise ratios (SNRs) for gray matter (GM) and white matter (WM), contrast-to-noise ratios (CNRs) for lesion/GM, lesion/WM, and GM/WM were quantitatively compared. Two radiologists determined the total number of enhancing lesions by consensus. Intraclass correlation coefficients (ICCs) between the two radiologists for metastases presence, qualitative ratings for image quality, and acoustic noise level of each sequence were assessed.

Results

Among the three sequences, SNRs and CNRs between lesions and surrounding parenchyma were highest using DANTE-SPACE, but CNR_WM/GM was the lowest with DANTE-SPACE. SNRs for PETRA images were significantly higher than those for MPRAGE (P<0.001). CNRs between lesions and surrounding parenchyma were similar for PETRA and MPRAGE (P>0.05). Significantly more brain metastases were detected with DANTE-SPACE (n=94) compared with MPRAGE (n=71) and PETRA (n=72). The ICCs were 0.964 for MPRAGE, 0.975 for PETRA, and 0.973 for DANTE-SPACE. Qualitative scores for lesion imaging using DANTE-SPACE were significantly higher than those obtained with PETRA and MPRAGE (P=0.002 and P=0.004, respectively). The acoustic noise level for PETRA (64.45 dB) was significantly lower than that for MPRAGE (78.27 dB, P<0.01) and DANTE-SPACE (80.18 dB, P<0.01).

Conclusions

PETRA achieves comparable detection of brain tumors with MPRAGE and is preferred for depicting osseous metastases and meningeal enhancement. DANTE-SPACE with blood vessel suppression showed improved detection of cerebral metastases compared with MPRAGE and PETRA, which could be helpful for the differential diagnosis of tumors.

Modulation of Blood-Brain Barrier Permeability by Activating Adenosine A2 Receptors in Oncological Treatment

The blood–brain barrier (BBB) plays an important protective role in the central nervous system and maintains its homeostasis. It regulates transport into brain tissue and protects neurons against the toxic effects of substances circulating in the blood. However, in the case of neurological diseases or primary brain tumors, i.e., gliomas, the higher permeability of the blood-derived substances in the brain tissue is necessary. Currently applied methods of treatment for the primary brain neoplasms include surgical removal of the tumor, radiation therapy, and chemotherapy. Despite the abovementioned treatment methods, the prognosis of primary brain tumors remains bad. Moreover, chemotherapy options seem to be limited due to low drug penetration into the cancerous tissue. Modulation of the blood–brain barrier permeability may contribute to an increase in the concentration of the drug in the CNS and thus increase the effectiveness of therapy. Interestingly, endothelial cells in cerebral vessels are characterized by the presence of adenosine 2A receptors (A2AR). It has been shown that substances affecting these receptors regulate the permeability of the BBB. The mechanism of increasing the BBB permeability by A2AR agonists is the actin-cytoskeletal reorganization and acting on the tight junctions. In this case, the A2AR seems to be a promising therapy target. This article aims to assess the possibility of increasing the BBB permeability through A2AR agonists to increase the effectiveness of chemotherapy and to improve the results of cancer therapy.

Effect of contrast agent dosage on longitudinal relaxation time, signal and apparent tumor volume in glioblastoma at 9.4T

INTRODUCTION

Glioblastoma multiforme is the most frequent innate brain tumor and still yields an unfavorable prognosis of 15 months of survival after diagnosis. Animal models play an important role in the investigation of therapeutic strategies of brain tumors. Thereby, MRI is substantial to individual follow-up examination for therapeutic response. Contrast agent dosage at 1.5 and 3T MRI has been thoroughly tested, while there is hardly any data for 9.4T. Therefore, the aim of this study was to find the optimal contrast agent dosage at 9.4T for examination of T1 relaxation time and apparent tumor volume in an animal model.

MATERIAL AND METHODS

Six animals with a U-87 glioblastoma were part of this study. Scans were performed on a 9.4T MRI. The MRI protocol contained a standard T1w sequence, which was employed for tumor volumetry and signal intensity measurement after single, double and triple contrast agent dosage injections and a T2w sequence for volumetry of tumor and edema. From a T1 map, T1 relaxation times and tumor area were measured. Histologic tumor size measurements were also performed for two animals.

RESULTS

The mean apparent tumor volume in T1w MRI increased significantly with each additional contrast agent injection, mainly due to the increase of the hyperintense tumor rim. Volumetry based on T2w MRI resulted in a higher tumor volume than in T1w volumetry, whereas the tumor volume in T1w MRI approached the volume in T2w MRI with increasing contrast agent dosage. Histology revealed an apparent tumor volume that corresponded to the volume of the hypointense center in T1w MRI. In contrast, T1 time decrease and T1w signal increase occurred mainly in the tumor rim.

CONCLUSION

Increasing the contrast agent dosage led to an increase in apparent tumor volume. High-dose T1 MRI, however, overestimated the tumor volume that was determined by histology. Thereby, it can be concluded that standard contrast agent dosage is sufficient to visualize the core tumor volume in T1w MRI. Measurement of tumor volume after increasing contrast agent dosage depicts tumor core and edema, which can be due to diffusion or accumulation or both. Tumor core and edema, however, can be visualized by T2w MRI without need of additional contrast agent.

Barriers to Effective Drug Treatment for Brain Metastases: A Multifactorial Problem in the Delivery of Precision Medicine

The treatment of metastatic lesions in the brain represents a serious unmet medical need in the field of neuro-oncology. Even though many effective compounds have demonstrated success in treating peripheral (non-CNS) tumors with targeted agents, one aspect of this lack of success in the brain may be related to poor delivery of otherwise effective compounds. Many factors can influence the brain delivery of these agents, but one key barrier is a heterogeneously "leaky" BBB that expresses efflux transporters that limit the BBB permeability for many targeted agents. Future success in therapeutics for brain metastases must take into account the adequate delivery of "active, free drug" to the target, and may include combinations of targeted drugs that are appropriate to address each individual patient's tumor type. This review discusses some issues that are pertinent to precision medicine for brain metastases, using specific examples of tumor types that have a high incidence of brain metastases.

Application of Synthetic MRI for Direct Measurement of Magnetic Resonance Relaxation Time and Tumor Volume at Multiple Time Points after Contrast Administration: Preliminary Results in Patients with Brain Metastasis

Objective

The purpose of this study was to investigate the time-dependent effects of contrast medium on multi-dynamic, multi-echo (MDME) sequence in patients with brain metastases.

Materials and Methods

This study included 7 patients with 15 brain metastases who underwent magnetic resonance (MR) examination which included MDME sequences at 1 minute, 10 minutes and 20 minutes after contrast injection. Two volumes of interests, covering an entire tumor (whole tumor) and the enhancing portion of the tumor, were derived from post-contrast synthetic T1-weighted images. Statistical comparisons were performed for three different time delays for histogram parameters of the longitudinal relaxation rate (R₁) and the transverse relaxation rate (R₂), and lesion volumes.

Results

The mean and the median of R₁ and the mean of R₂ in both the whole tumor and the inner enhancing portion were larger on the 10 minutes delayed images than on the 1 minute or 20 minutes delayed images (mean of R₁ in the whole tumor on the 1 minute, 10 minutes, and 20 minutes delayed images: 1.26 ms, 1.39 ms, and 1.37 ms; mean of R₁ in the inner enhancing portion: 1.43 ms, 1.53 ms and 1.44 ms; all p < 0.017). The volumes of the whole tumor and the inner enhancing portion were significantly larger in the 10 minutes and 20 minutes delayed images than on the 1 minute delayed images (all p < 0.017).

Conclusion

Magnetic resonance relaxation times and the volumes of the whole tumor and the inner enhancing portion were measured larger on the 10 minutes or 20 minutes delayed images than on the 1 minute delayed images. The MDME sequence immediately after contrast injection cannot fully reflect the effects of gadolinium-based contrast agent leakage in the tissue.

Targeted Therapy and Immunotherapy Response Assessment with F-18 Fluorothymidine Positron-Emission Tomography/Magnetic Resonance Imaging in Melanoma Brain Metastasis: A Pilot Study

INTRODUCTION

This pilot study aimed at exploring the utility of the proliferation tracer F-18 fluorothymidine (FLT) and positron-emission tomography (PET)/magnetic resonance imaging (MRI) (FLT-PET/MRI) for early treatment monitoring in patients with melanoma brain metastasis (MBM) who undergo targeted therapy or immunotherapy.

MATERIAL AND METHODS

Patients with newly diagnosed MBM underwent baseline and follow-up FLT-PET/MRI scans at 3-4 weeks of targeted therapy or immunotherapy. Up to six measurable brain lesions ≥1.0 cm per subject, as identified on T1-weighted post-gadolinium images, were included for quantitative analyses. The maximum SUV of each lesion was divided by the mean SUV of the pons to obtain the SUV ratio (SUVR).

RESULTS

Five enrolled subjects underwent the baseline FLT-PET/MRI study in which the MBM showed a median size of 1.7 cm (range 1.0-2.9) and increased metabolic activity with SUVR of 9.9 (range 3.2-18.4). However, only two subjects (cases #1 and #2) returned for a follow-up scan. At baseline, a total of 22 lesions were analyzed in all five subjects, which showed a median size of 1.7 cm (range 1.0-2.9) and median SUVR of 9.9 (range 3.2-18.4). At follow-up, case #1 was a 55-year-old man who received targeted BRAF inhibitor and MEK inhibitor therapy with dabrafenib and trametinib. Fused PET/MRI data of six measured lesions demonstrated a significant reduction in MBM proliferative activity (median -68%; range -38 to -77%) and size (median -23%; range -4 to -55%) at three weeks of therapy. Nevertheless, the subject eventually progressed and died 13 months after therapy initiation. Case #2 was a 36-year-old man who received immunotherapy with nivolumab and ipilimumab. The five measured MBM lesions showed a mixed response at both proliferative and morphologic imaging at 1-month follow-up. Some lesions demonstrated interval decrease while others interval increase in proliferative activity with a median -44% (range -77 to +68%). On MRI, the size change was +7% (range -64 to +50%). The therapy was switched to dabrafenib and trametinib, which led to a partial response. The patient is still alive 16 months following therapy initiation.

CONCLUSION

The five cases presented show the potential benefit of hybrid FLT-PET/MRI for the diagnosis of MBM and treatment monitoring of targeted therapy and immunotherapy. However, further studies are required to assess their complementary role in distinguishing true progression from pseudoprogression.

Brain Distribution of a Novel MEK Inhibitor E6201: Implications in the Treatment of Melanoma Brain Metastases

Clinically meaningful efficacy in the treatment of brain tumors, including melanoma brain metastases (MBM), requires selection of a potent inhibitor against a suitable target, and adequate drug distribution to target sites in the brain. Deregulated constitutive signaling of mitogen-activated protein kinase (MAPK) pathway has been frequently observed in melanoma, and mitogen-activated protein/extracellular signal-regulated kinase (MEK) has been identified to be an important target. E6201 is a potent synthetic small-molecule MEK inhibitor. The purpose of this study was to evaluate brain distribution of E6201, and examine the impact of active efflux transport at the blood-brain barrier on the central nervous system (CNS) exposure of E6201. In vitro studies utilizing transfected Madin-Darby canine kidney II (MDCKII) cells indicate that E6201 is not a substrate of P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp). In vivo studies also suggest a minimal involvement of P-gp and Bcrp in E6201's brain distribution. The total concentrations in brain were higher than in plasma, resulting in a brain-to-plasma AUC ratio (Kp) of 2.66 in wild-type mice. The brain distribution was modestly enhanced in Mdr1a/b^-/-, Bcrp1^-/-, and Mdr1a/b^-/-Bcrp1^-/- knockout mice. The nonspecific binding of E6201 was higher in brain compared with plasma. However, free-drug concentrations in brain following 40 mg/kg intravenous dose reach levels that exceed reported in vitro half-maximal inhibitory concentration (IC₅₀) values, suggesting that E6201 may be efficacious in inhibiting MEK-driven brain tumors. The brain distribution characteristics of E6201 make it an attractive targeted agent for clinical testing in MBM, glioblastoma, and other CNS tumors that may be effectively targeted with inhibition of MEK signaling.

Is it Safe to Use Gadolinium-Based Contrast Agents in Mri?

Gadolinium-based contrast agents have greatly expanded the capability of magnetic resonance imaging and have been used extensively in neuroradiology over the past 30 years. When initially developed they were thought to be relatively harmless; it was later discovered they are associated with nephrogenic systemic fibrosis and should be used with caution in certain patient groups, especially those with renal failure. Lately it has been found that the use of these contrast agents may result in deposition of gadolinium in the brain even in patients with an intact blood-brain barrier. While this has not been shown to be associated with any clinical effects, a precautionary approach has been advised by the regulatory authorities. Here we review the development of the gadolinium contrast agents, their use and the advice related to this new information.

Role of FDG-PET/MRI, FDG-PET/CT, and Dynamic Susceptibility Contrast Perfusion MRI in Differentiating Radiation Necrosis from Tumor Recurrence in Glioblastomas

BACKGROUND AND PURPOSE

To compare the utility of quantitative PET/MRI, dynamic susceptibility contrast (DSC) perfusion MRI (pMRI), and PET/CT in differentiating radiation necrosis (RN) from tumor recurrence (TR) in patients with treated glioblastoma multiforme (GBM).

METHODS

The study included 24 patients with GBM treated with surgery, radiotherapy, and temozolomide who presented with progression on imaging follow‐up. All patients underwent PET/MRI and pMRI during a single examination. Additionally, 19 of 24 patients underwent PET/CT on the same day. Diagnosis was established by pathology in 17 of 24 and by clinical/radiologic consensus in 7 of 24. For the quantitative PET/MRI and PET/CT analysis, a region of interest (ROI) was drawn around each lesion and within the contralateral white matter. Lesion to contralateral white matter ratios for relative maximum, mean, and median were calculated. For pMRI, lesion ROI was drawn on the cerebral blood volume (CBV) maps and histogram metrics were calculated. Diagnostic performance for each metric was assessed using receiver operating characteristic curve analysis and area under curve (AUC) was calculated.

RESULTS

In 24 patients, 28 lesions were identified. For PET/MRI, relative mean ≥ 1.31 resulted in AUC of .94 with both sensitivity and negative predictive values (NPVs) of 100%. For pMRI, CBV max ≥3.32 yielded an AUC of .94 with both sensitivity and NPV measuring 100%. The joint model utilizing r‐mean (PET/MRI) and CBV mode (pMRI) resulted in AUC of 1.0.

CONCLUSION

Our study demonstrates that quantitative PET/MRI parameters in combination with DSC pMRI provide the best diagnostic utility in distinguishing RN from TR in treated GBMs.

Drug delivery to melanoma brain metastases: Can current challenges lead to new opportunities?

Melanoma has a high propensity to metastasize to the brain, and patients with melanoma brain metastases (MBM) have an extremely poor prognosis. The recent approval of several molecularly-targeted agents (e.g., BRAF, MEK inhibitors) and biologics (anti-CTLA-4, anti-PD-1 and anti-PD-L1 antibodies) has brought new hope to patients suffering from this formerly untreatable and lethal disease. Importantly, there have been recent reports of success in some clinical studies examining the efficacy of both targeted agents and immunotherapies that show similar response rates in both brain metastases and extracranial disease. While these studies are encouraging, there remains significant room for improvement in the treatment of MBM, given the lack of durable response and the development of resistance to current therapies. Critical questions remain regarding mechanisms that lead to this lack of durable response and development of resistance, and how those mechanisms may differ in systemic sites versus brain metastases. One issue that may not be fully appreciated is that the delivery of several small molecule molecularly-targeted therapies to the brain is often restricted due to active efflux at the blood-brain barrier (BBB) interface. Inadequate local drug concentrations may be partially responsible for the development of unique patterns of resistance at metastatic sites in the brain. It is clear that there can be local, heterogeneous BBB breakdown in MBM, as exemplified by contrast-enhancement on T1-weighted MR imaging. However, it is possible that the successful treatment of MBM with small molecule targeted therapies will depend, in part, on the ability of these therapies to penetrate an intact BBB and reach the protected micro-metastases (so called "sub-clinical" disease) that escape early detection by contrast-enhanced MRI, as well as regions of tumor within MRI-detectable metastases that may have a less compromised BBB. The emergence of resistance in MBM may be related to several diverse, yet interrelated, factors including the distinct microenvironment of the brain and inadequate brain penetration of targeted therapies to specific regions of tumor. The tumor microenvironment has been ascribed to play a key role in steering the course of disease progression, by dictating changes in expression of tumor drivers and resistance-related signaling mechanisms. Therefore, a key issue to consider is how changes in drug delivery, and hence local drug concentrations within a metastatic microenvironment, will influence the development of resistance. Herein we discuss our perspective on several critical questions that focus on many aspects relevant to the treatment of melanoma brain metastases; the answers to which may lead to important advances in the treatment of this devastating disease.

Quantitative T1-mapping detects cloudy-enhancing tumor compartments predicting outcome of patients with glioblastoma

Contrast enhancement of glioblastomas (GBM) is caused by the decrease in relaxation time, T1. Here, we demonstrate that the quantitative measurement of T1 (qT1) discovers a subtle enhancement in GBM patients that is invisible in standard MRI. We assessed the volume change of this “cloudy” enhancement during radio‐chemotherapy and its impact on patients’ progression‐free survival (PFS). We enrolled 18 GBM patients in this observational, prospective cohort study and measured 3T‐MRI pre‐ and post contrast agent with standard T1‐weighted (T1w) and with sequences to quantify T1 before radiation, and at 6‐week intervals during radio‐chemotherapy. We measured contrast enhancement by subtracting pre from post contrast contrast images, yielding relative signal increase ∆T1w and relative T1 shortening ∆qT1. On ∆qT1, we identified a solid and a cloudy‐enhancing compartment and evaluated the impact of their therapy‐related volume change upon PFS. In ∆qT1 maps cloudy‐enhancing compartments were found in all but two patients at baseline and in all patients during therapy. The qT1 decrease in the cloudy‐enhancing compartment post contrast was 21.64% versus 1.96% in the contralateral control tissue (P < 0.001). It was located at the margin of solid enhancement which was also seen on T1w. In contrast, the cloudy‐enhancing compartment was visually undetectable on ∆T1w. A volume decrease of more than 21.4% of the cloudy‐enhancing compartment at first follow‐up predicted longer PFS (P = 0.038). Cloudy‐enhancing compartment outside the solid contrast‐enhancing area of GBM is a new observation which is only visually detectable with qT1‐mapping and may represent tumor infiltration. Its early volume decrease predicts a longer PFS in GBM patients during standard radio‐chemotherapy.

Time-delayed contrast-enhanced MRI improves detection of brain metastases: a prospective validation of diagnostic yield

The radiological detection of brain metastases (BMs) is essential for optimizing a patient's treatment. This statement is even more valid when stereotactic radiosurgery, a noninvasive image guided treatment that can target BM as small as 1-2 mm, is delivered as part of that care. The timing of image acquisition after contrast administration can influence the diagnostic sensitivity of contrast enhanced magnetic resonance imaging (MRI) for BM. Investigate the effect of time delayed acquisition after administration of intravenous Gadavist® (Gadobutrol 1 mmol/ml) on the detection of BM. This is a prospective IRB approved study of 50 patients with BM who underwent post-contrast MRI sequences after injection of 0.1 mmol/kg Gadavist® as part of clinical care (time-t0), followed by axial T1 sequences after a 10 min (time-t1) and 20 min delay (time-t2). MRI studies were blindly compared by three neuroradiologists. Single measure intraclass correlation coefficients were very high (0.914, 0.904 and 0.905 for time-t0, time-t1 and time-t2 respectively), corresponding to a reliable inter-observer correlation. The delayed MRI at time-t2 delayed sequences showed a significant and consistently higher diagnostic sensitivity for BM by every participating neuroradiologist and for the entire cohort (p = 0.016, 0.035 and 0.034 respectively). A disproportionately high representation of BM detected on the delayed studies was located within posterior circulation territories (compared to predictions based on tissue volume and blood-flow volumes). Considering the safe and potentially high yield nature of delayed MRI sequences, it should supplement the standard MRI sequences in all patients in need of precise delineation of their intracranial disease.

Manganese-Enhanced Magnetic Resonance Imaging as a Diagnostic and Dispositional Tool after Mild-Moderate Blast Traumatic Brain Injury

Traumatic brain injury (TBI) caused by explosive munitions, known as blast TBI, is the signature injury in recent military conflicts in Iraq and Afghanistan. Diagnostic evaluation of TBI, including blast TBI, is based on clinical history, symptoms, and neuropsychological testing, all of which can result in misdiagnosis or underdiagnosis of this condition, particularly in the case of TBI of mild-to-moderate severity. Prognosis is currently determined by TBI severity, recurrence, and type of pathology, and also may be influenced by promptness of clinical intervention when more effective treatments become available. An important task is prevention of repetitive TBI, particularly when the patient is still symptomatic. For these reasons, the establishment of quantitative biological markers can serve to improve diagnosis and preventative or therapeutic management. In this study, we used a shock-tube model of blast TBI to determine whether manganese-enhanced magnetic resonance imaging (MEMRI) can serve as a tool to accurately and quantitatively diagnose mild-to-moderate blast TBI. Mice were subjected to a 30 psig blast and administered a single dose of MnCl2 intraperitoneally. Longitudinal T1-magnetic resonance imaging (MRI) performed at 6, 24, 48, and 72 h and at 14 and 28 days revealed a marked signal enhancement in the brain of mice exposed to blast, compared with sham controls, at nearly all time-points. Interestingly, when mice were protected with a polycarbonate body shield during blast exposure, the marked increase in contrast was prevented. We conclude that manganese uptake can serve as a quantitative biomarker for TBI and that MEMRI is a minimally-invasive quantitative approach that can aid in the accurate diagnosis and management of blast TBI. In addition, the prevention of the increased uptake of manganese by body protection strongly suggests that the exposure of an individual to blast risk could benefit from the design of improved body armor.

Correlation of (18)F-fluoroethyl tyrosine positron-emission tomography uptake values and histomorphological findings by stereotactic serial biopsy in newly diagnosed brain tumors using a refined software tool

Background

Magnetic resonance imaging (MRI) is the standard neuroimaging method to diagnose neoplastic brain lesions, as well as to perform stereotactic biopsy surgical planning. MRI has the advantage of providing structural anatomical details with high sensitivity, though histological specificity is limited. Although combining MRI with other imaging modalities, such as positron-emission tomography (PET), has proven to increment specificity, exact correlation between PET threshold uptake ratios (URs) and histological diagnosis and grading has not yet been described.

Objectives

The aim of this study was to correlate exactly the histopathological criteria of the biopsy site to its PET uptake value with high spatial resolution (mm³), and to analyze the diagnostic value of PET using the amino acid O-(2-[¹⁸F]fluoroethyl)-l-tyrosine (¹⁸F-FET) PET in patients with newly diagnosed brain lesions in comparison to histological findings obtained from stereotactic serial biopsy.

Patients and methods

A total of 23 adult patients with newly diagnosed brain tumors on MRI were enrolled in this study. Subsequently to diagnoses, all patients underwent a ¹⁸F-FET PET-guided stereotactic biopsy, using an original newly developed software module, which is presented here. Conventional MRI, stereotactic computed tomography series, and ¹⁸F-FET PET images were semiautomatically fused, and hot-spot detection was performed for target planning. UR was determined using the uptake value from the biopsy sites in relation to the contralateral frontal white matter. UR values ≥1.6 were considered positive for glioma. High-grade glioma (HGG) was suspected with URs ≥3.0, while low-grade glioma (LGG) was suspected with URs between 1.6 and 3.0. Stereotactic serial biopsies along the trajectory at multiple sites were performed in millimeter steps, and the FET URs for each site were correlated exactly with a panel of 27 different histopathological markers. Comparisons between FET URs along the biopsy trajectories and the histological diagnoses were made with Pearson product-moment correlation coefficients. Analysis of variance was performed to test for significant differences in maximum UR between different tumor grades.

Results

A total of 363 biopsy specimens were taken from 23 patients by stereotactic serial biopsies. Histological examination revealed eight patients (35%) with an LGG: one with a World Health Organization (WHO)-I lesion and seven with a WHO-II lesion. Thirteen (57%) patients revealed an HGG (two with a WHO-III and three with a WHO-IV tumor), and two patients (9%) showed a process that was neither HGG nor LGG (group X or no-grade group). The correlation matrix between histological findings and the UR revealed five strong correlations. Low cell density in tissue samples was found to have a significant negative correlation with the measured cortical uptake rate (r=−0.43, P=0.02), as well as moderate cell density (r=−0.48, P=0.02). Pathological patterns of proliferation (r=0.37, P=0.04), GFAP (r=0.37, P=0.04), and Olig2 (r=0.36, P=0.05) showed a significant positive correlation with cortical URs. Analysis of variance tests showed a significant difference between the LGG and the HGG groups (F=8.27, P<0.002), but no significant differences when differentiating between the X group and the HGG (P=0.2)/LGG (P=0.8) groups, nor between the no-grade group and the WHO-I group.

Conclusion

¹⁸F-FET PET is a valuable tool, as it allows the differentiation of HGGs from LGGs. Its use is not limited to preoperative evaluation; it may also refine biopsy targeting and improve tumor delimitation for radiotherapy. Histology is still necessary, and remains the gold standard for definitive diagnosis of brain lesions.

Use of contrast-enhanced fluid-attenuated inversion recovery sequence to detect brain lesions in dogs and cats

Background

The diagnostic value of a contrast‐enhanced T2‐weighted FLAIR sequence (ceFLAIR) in brain imaging is unclear.

Hypothesis/Objectives

That the number of brain lesions detected with ceFLAIR would be no greater than the sum of lesions detected with nFLAIR and ceT1W sequence.

Animals

One hundred and twenty‐nine animals (108 dogs and 21 cats) undergoing magnetic resonance imaging (MRI) of the head between July 2010 and October 2011 were included in the study.

Methods

A transverse ceFLAIR was added to a standard brain MRI protocol. Presence and number of lesions were determined based on all available MRI sequences by 3 examiners in consensus and lesion visibility was evaluated for nFLAIR, ceFLAIR, and ceT1W sequences.

Results

Eighty‐three lesions (58 intra‐axial and 25 extra‐axial) were identified in 51 patients. Five lesions were detected with nFLAIR alone, 2 with ceT1W alone, and 1 with ceFLAIR alone. Significantly higher numbers of lesions were detected using ceFLAIR than nFLAIR (76 versus 67 lesions; P = 0.04), in particular for lesions also detected with ceT1W images (53 versus 40; P =.01). There was no significant difference between the number of lesions detected with combined nFLAIR and ceT1W sequences compared to those detected with ceFLAIR (82 versus 76; P =.25).

Conclusion and Clinical Importance

Use of ceFLAIR as a complementary sequence to nFLAIR and ceT1W sequences did not improve the detection of brain lesions and cannot be recommended as part of a routine brain MRI protocol in dogs and cats with suspected brain lesions.

Molecular imaging of fibrin in a breast cancer xenograft mouse model

RATIONALE AND OBJECTIVES

Fibrin deposition has been indicated within the stroma of a majority of solid tumors. Here we assess the feasibility of using the established fibrin-specific probe EP-2104R for noninvasive imaging of fibrin in the context of breast cancer.

METHODS

EP-2104R, untargeted gadopentetate dimeglumine (Gd-DTPA), and a newly synthesized nonfibrin binding control linear peptide (CLP) were compared using steady-state and dynamic contrast-enhanced magnetic resonance imaging in a breast cancer xenograft mouse model at 9.4 T.

RESULTS

EP-2104R transiently enhanced both tumor core and tumor periphery, but only the enhancement in the tumor periphery persisted even 90 minutes after EP-2104R administration. However, untargeted Gd-DTPA and CLP are not retained in the tumor periphery. The half-life of EP-2104R in the tumor periphery (103 ± 18 minutes) is significantly longer (P < 0.05) than that of either Gd-DTPA (29.6 ± 2.4 minutes) or CLP (42.4 ± 1.5 minutes), but the rate of clearance is similar for all the 3 probes from the tumor core. The presence of high concentrations of fibrin in the tumor periphery was corroborated using immunohistochemistry with a fibrin-specific antibody.

CONCLUSIONS

The persistent enhancement observed in the tumor periphery with EP-2104R is likely a result of its fibrin-specific binding rather than its size and demonstrates the feasibility of EP-2104R for molecular imaging of fibrin in tumor stroma.

Examination of blood-brain barrier (BBB) integrity in a mouse brain tumor model

The present study evaluates, both functionally and biochemically, brain tumor-induced alterations in brain capillary endothelial cells. Brain tumors were induced in Balb/c mice via intracranial injection of Lewis Lung carcinoma cells into the right hemisphere of the mouse brain using stereotaxic apparatus. Blood-brain barrier (BBB) permeability was assessed at various stages of tumor development, using both radiolabeled tracer permeability and magnetic resonance imaging with gadolinium diethylene-triamine-pentaacetate contrast enhancement (Gad-DTPA). The expression of the drug efflux transporter, P-glycoprotein (P-gp), in the BBB at various stages of tumor development was also evaluated by Western blot and immunohistochemistry. Median mouse survival following tumor cell injection was 17 days. The permeability of the BBB to (3)H-mannitol was similar in both brain hemispheres at 7 and 10 days post-injection. By day 15, there was a twofold increase in (3)H-mannitol permeability in the tumor bearing hemispheres compared to the non-tumor hemispheres. Examination of BBB permeability with Gad-DTPA contrast enhanced MRI indicated cerebral vascular permeability changes were confined to the tumor area. The permeability increase observed at the later stages of tumor development correlated with an increase in cerebral vascular volume suggesting angiogenesis within the tumor bearing hemisphere. Furthermore, the Gad-DPTA enhancement observed within the tumor area was significantly less than Gad-DPTA enhancement within the circumventricular organs not protected by the BBB. Expression of P-gp in both the tumor bearing and non-tumor bearing portions of the brain appeared similar at all time points examined. These studies suggest that although BBB integrity is altered within the tumor site at later stages of development, the BBB is still functional and limiting in terms of solute and drug permeability in and around the tumor.

Contrast-enhanced MR imaging in neuroimaging

MR imaging without and with gadolinium-based contrast agents (GBCAs) is an important imaging tool for defining normal anatomy and characteristics of lesions. GBCAs have been used in contrast-enhanced MR imaging in defining and characterizing lesions of the central nervous system for more than 20 years. The combination of unenhanced and GBCA-enhanced MR imaging is the clinical gold standard for the noninvasive detection and delineation of most intracranial and spinal lesions. MR imaging has a high predictive value that rules out neoplasm and most inflammatory and demyelinating processes of the central nervous system.

Quantification of blood-to-brain transfer rate in multiple sclerosis

Blood-brain barrier (BBB) disruption visualized in lesions by MRI is a major biomarker of disease activity in multiple sclerosis (MS). However, in MS, destruction occurs to a variable extent in lesions as well as in gray matter (GM) and in the normal appearing white matter (NAWM). A method to quantify the BBB disruption in lesions as well as in non-lesion areas would be useful for assessment of MS progression and treatments. The objective of this study was to quantify the BBB transfer rate (Ki) in WM lesions, in the NAWM, and in the full-brain of MS patients. Thirteen MS patients with active lesions and 10 healthy controls with age and gender matching were recruited for full-brain and WM Ki studies. Dynamic contrast-enhanced MRI (DCEMRI) scans were conducted using T1 mapping with partial inversion recovery (TAPIR), a fast T1 mapping technique, following administration of a quarter-dose of the contrast agent Gadolinium-DTPA (Gd-DTPA). The Patlak modeling technique was used to derive a voxel-based map of Ki. In all patients contrast-enhanced lesions, quantified by Ki maps, were observed. Compared with controls, patients with MS exhibited an increase in mean Ki of the full-brain (P-value<0.05) but no significant difference in mean Ki of NAWM. The identified increase in full-brain Ki of MS patients suggests a global vascular involvement associated with MS disease. The lack of observed significant decrease in Ki in NAWM suggests lower involvement of WM vasculature than full-brain vasculature in MS. Ki maps constructed from time series data acquired by DCEMRI provide additional information about BBB that could be used for evaluation of vascular involvement in MS and monitoring treatment effectiveness.

Review of MR elastography applications and recent developments

The technique of MR elastography (MRE) has emerged as a useful modality for quantitatively imaging the mechanical properties of soft tissues in vivo. Recently, MRE has been introduced as a clinical tool for evaluating chronic liver disease, but many other potential applications are being explored. These applications include measuring tissue changes associated with diseases of the liver, breast, brain, heart, and skeletal muscle including both focal lesions (e.g., hepatic, breast, and brain tumors) and diffuse diseases (e.g., fibrosis and multiple sclerosis). The purpose of this review article is to summarize some of the recent developments of MRE and to highlight some emerging applications.

Efficacy and Safety of Gadobutrol (1.0 M) versus Gadopentetate Dimeglumine (.5 M) for Enhanced Mri of Cns Lesions: A Phase Iii, Multicenter, Single-blind, Randomized Study in Chinese Patients

The aim of this study was to compare the efficacy and safety of macrocyclic gadobutrol (1.0 M) with linear gadopentetate dimeglumine (0.5 M) for contrast-enhanced magnetic resonance imaging (MRI) of central nervous system (CNS) lesions in Chinese patients (N = 147) with known or suspected CNS lesions, who were enrolled in this single-blind, randomized, parallel-group study. Three blinded independent readers evaluated all efficacy variables. The primary efficacy variable was the difference between the two agents for the change in contrast-to-noise ratio (CNR) between non-enhanced and contrast-enhanced scans of lesions. Secondary outcomes included mean change in number of lesions detected before and after contrast enhancement, diagnostic confidence, and safety and tolerability parameters. Gadobutrol was non-inferior to gadopentetate dimeglumine in respect to the difference in the mean change in CNR (6.94; 95% confidence interval [CI] lower limit: -3.90; predefined maximum 95% CI lower limit: -6.52). The mean change in the number of CNS lesions detected was greater with gadobutrol versus gadopentetate dimeglumine (1.2 vs. 0.2 lesions). Diagnostic confidence was classified as ‘high’ for more patients with gadobutrol versus gadopentetate dimeglumine by the investigators (58.8% vs. 55.4%) and by the three blinded readers (63.6% vs. 55.7%, 23.7% vs. 18.0% and 81.7% vs. 81.0%). Both agents were well tolerated by participating patients. We concluded that in Chinese patients with CNS lesions, gadobutrol (1.0 M) was as effective and well tolerated in contrast-enhanced MRI as gadopentetate dimeglumine (0.5 M). Gadobutrol provided improved visualization of CNS lesions compared with gadopentetate dimeglumine, with a comparable tolerability profile.

In vivo bio-imaging using chlorotoxin-based conjugates

Surgical resection remains the primary component of cancer therapy. The precision required to successfully separate cancer tissue from normal tissue relies heavily on the surgeon's ability to delineate the tumor margins. Despite recent advances in surgical guidance and monitoring systems, intra-operative identification of these margins remains imprecise and directly influences patient prognosis. If the surgeon had improved tools to distinguish these margins, tumor progression and unacceptable morbidity could be avoided. In this article, we review the history of chlorotoxin and its tumor specificity and discuss the research currently being generated to target optical imaging agents to cancer tissue.

Feasibility of pseudocontinuous arterial spin labeling at 7 T with whole-brain coverage

Object

We studied the feasibility of pseudocontinuous arterial spin labeling (pCASL) at 7 T.

Materials and methods

Simulations were performed to find the optimal labeling parameters for pCASL, with particular attention to the maximum-allowed specific absorption rate (SAR). Subsequently, pCASL experiments (four volunteers) were performed to find the B1 efficiency at the labeling position with and without high-permittivity pads placed around the head, and to study the optimal labeling duration (four separate volunteers). Finally, feasibility of whole-brain pCASL imaging was tested.

Results

Simulations showed that a lower B1 efficiency should be compensated by a lower effective flip angle of the labeling, a moderately shorter labeling duration, and a longer repetition time. B1 efficiency in the internal carotid arteries just below the carotid siphon was approximately 55% and 35% with and without high-permittivity pads, respectively. In vivo experiments showed an optimal labeling duration of 1,500 ms, although longer labeling durations up to 2,500 ms resulted in similar signal-to-noise efficiency. Whole-brain pCASL imaging was demonstrated in a single volunteer.

Conclusion

Despite decreased B1 efficiency, sufficient labeling efficiency can be achieved for whole-brain pCASL at 7 T with high-permittivity pads. However, image quality is still limited compared with 3 T, probably due to imaging instabilities, and further research is needed to elucidate this.

MR imaging of neoplastic central nervous system lesions: review and recommendations for current practice

MR imaging is the preferred technique for the diagnosis, treatment planning, and monitoring of patients with neoplastic CNS lesions. Conventional MR imaging, with gadolinium-based contrast enhancement, is increasingly combined with advanced, functional MR imaging techniques to offer morphologic, metabolic, and physiologic information. This article provides updated recommendations to neuroradiologists, neuro-oncologists, neurosurgeons, and radiation oncologists on the practical applications of MR imaging of neoplastic CNS lesions in adults, with particular focus on gliomas, based on a review of the clinical trial evidence and personal experiences shared at a recent international meeting of experts in neuroradiology, neuro-oncology, neurosurgery, and radio-oncology.

Tissue concentration of systemically administered antineoplastic agents in human brain tumors

The blood-brain-barrier (BBB) limits the penetration of many systemic antineoplastic therapies. Consequently, many agents may be used in clinical studies and clinical practice though they may not achieve therapeutic levels within the tumor. We sought to compile the currently available human data on antineoplastic drug concentrations in brain and tumor tissue according to BBB status. A review of the literature was conducted for human studies providing concentrations of antineoplastic agents in blood and metastatic brain tumors or high-grade gliomas. Studies were considered optimal if they reported simultaneous tissue and blood concentration, multiple sampling times and locations, MRI localization, BBB status at sampling site, tumor histology, and individual subject data. Twenty-Four studies of 19 compounds were included. These examined 18 agents in contrast-enhancing regions of high-grade gliomas, with optimal data for 2. For metastatic brain tumors, adequate data was found for 9 agents. Considerable heterogeneity was found in the measurement value, tumor type, measurement timing, and sampling location within and among studies, limiting the applicability of the results. Tissue to blood ratios ranged from 0.054 for carboplatin to 34 for mitoxantrone in high-grade gliomas, and were lowest for temozolomide (0.118) and etoposide (0.116), and highest for mitoxantrone (32.02) in metastatic tumors. The available data examining the concentration of antineoplastic agents in brain and tumor tissue is sparse and limited by considerable heterogeneity. More studies with careful quantification of antineoplastic agents in brain and tumor tissue is required for the rational development of therapeutic regimens.

Advanced Contrast-Enhanced MR Imaging of the CNS

One of the most frequent uses of magnetic resonance imaging (MRI) since its introduction has been in the assessment of the CNS for neoplasm. In recent years there has been a substantial improvement in the MR protocol for tumors that includes the use of functional imaging techniques. As shown in multiple experimental and clinical studies an optimized use of high quality contrast media and the introduction of these functional MRI methods has improved the detection and delineation of CNS tumors. This results not only in more confident diagnoses, but also in a substantially improved differential diagnostic process. The article reviews and summarizes the technical advances in functional techniques and their impact on the assessment of cerebral pathologies, namely brain tumors, and gives practical information on how to optimize sequence parameters to achieve the optimal tissue and pathology contrast.

High-relaxivity contrast-enhanced magnetic resonance neuroimaging: a review

Evaluation of brain lesions using magnetic resonance imaging (MRI) provides information that is critical for accurate diagnosis, prognosis, therapeutic intervention and monitoring response. Conventional contrast-enhanced MR neuroimaging using gadolinium (Gd) contrast agents primarily depicts disruption of the blood-brain barrier, demonstrating location and extent of disease, and also the morphological details at the lesion site. However, conventional imaging results do not always accurately predict tumour aggressiveness. Advanced functional MRI techniques such as dynamic contrast-enhanced perfusion-weighted imaging utilise contrast agents to convey physiological information regarding the haemodynamics and neoangiogenic status of the lesion that is often complementary to anatomical information obtained through conventional imaging. Most of the Gd contrast agents available have similar T1 and T2 relaxivities, and thus their contrast-enhancing capabilities are comparable. Exceptions are gadobenate-dimeglumine, Gd-EOB-DTPA, Gadobutrol and gadofosveset, which, owing to their transient-protein-binding capability, possess almost twice (and more) the T1 and T2 relaxivities as other agents at all magnetic field strengths. Numerous comparative studies have demonstrated the advantages of the increased relaxivity in terms of enhanced image contrast, image quality and diagnostic confidence. Here we summarise the benefits of higher relaxivity for the most common neuroimaging applications including MRI, perfusion-weighted imaging and MRA for evaluation of brain tumours, cerebrovascular disease and other CNS lesions.

Determination of the venous output function from MR signal phase: feasibility for quantitative DCE-MRI in human brain

For dynamic contrast-enhanced MRI studies in the human brain, it is useful to measure the venous output function (VOF). The purpose of this work was to explore the feasibility of measuring the VOF using the MR signal phase (in absolute units of gadolinium concentration) in the superior sagittal sinus. Phantom experiments were performed to validate the technique for different superior sagittal sinus angles (theta = 0-48 degrees relative to the main magnetic field), different curvatures (straight or radius = 45 mm), and different spatial resolutions (2.2-5.5 mm, to study partial-volume effects). Additionally, the technique was tested on three patients. The phantom experimental results (echo time = 5.5 ms, theta <or= 21 degrees) agreed with theoretical predictions to within 10%. For the patient studies, the measured VOFs had reasonable amplitude and shape characteristics and the patients' superior sagittal sinus angles (<15 degrees) and curvatures (radii approximately 40 mm) were within the range explored with phantoms. Our results suggest that partial-volume contributions to the VOF will be <5% and that the VOF can be evaluated in vivo to within 10% error. In conclusion, it is highly feasible to use MR signal phase to measure the VOF in the superior sagittal sinus for human dynamic contrast-enhanced MRI.

Superparamagnetic iron oxide nanoparticles: diagnostic magnetic resonance imaging and potential therapeutic applications in neurooncology and central nervous system inflammatory pathologies, a review

Superparamagnetic iron oxide nanoparticles have diverse diagnostic and potential therapeutic applications in the central nervous system (CNS). They are useful as magnetic resonance imaging (MRI) contrast agents to evaluate: areas of blood-brain barrier (BBB) dysfunction related to tumors and other neuroinflammatory pathologies, the cerebrovasculature using perfusion-weighted MRI sequences, and in vivo cellular tracking in CNS disease or injury. Novel, targeted, nanoparticle synthesis strategies will allow for a rapidly expanding range of applications in patients with brain tumors, cerebral ischemia or stroke, carotid atherosclerosis, multiple sclerosis, traumatic brain injury, and epilepsy. These strategies may ultimately improve disease detection, therapeutic monitoring, and treatment efficacy especially in the context of antiangiogenic chemotherapy and antiinflammatory medications. The purpose of this review is to outline the current status of superparamagnetic iron oxide nanoparticles in the context of biomedical nanotechnology as they apply to diagnostic MRI and potential therapeutic applications in neurooncology and other CNS inflammatory conditions.

Recent progress towards development of effective systemic chemotherapy for the treatment of malignant brain tumors

Systemic chemotherapy has been relatively ineffective in the treatment of malignant brain tumors even though systemic chemotherapy drugs are small molecules that can readily extravasate across the porous blood-brain tumor barrier of malignant brain tumor microvasculature. Small molecule systemic chemotherapy drugs maintain peak blood concentrations for only minutes, and therefore, do not accumulate to therapeutic concentrations within individual brain tumor cells. The physiologic upper limit of pore size in the blood-brain tumor barrier of malignant brain tumor microvasculature is approximately 12 nanometers. Spherical nanoparticles ranging between 7 nm and 10 nm in diameter maintain peak blood concentrations for several hours and are sufficiently smaller than the 12 nm physiologic upper limit of pore size in the blood-brain tumor barrier to accumulate to therapeutic concentrations within individual brain tumor cells. Therefore, nanoparticles bearing chemotherapy that are within the 7 to 10 nm size range can be used to deliver therapeutic concentrations of small molecule chemotherapy drugs across the blood-brain tumor barrier into individual brain tumor cells. The initial therapeutic efficacy of the Gd-G5-doxorubicin dendrimer, an imageable nanoparticle bearing chemotherapy within the 7 to 10 nm size range, has been demonstrated in the orthotopic RG-2 rodent malignant glioma model. Herein I discuss this novel strategy to improve the effectiveness of systemic chemotherapy for the treatment of malignant brain tumors and the therapeutic implications thereof.

MRI for identification of progression in brain tumors: from morphology to function

For monitoring of brain tumors, it is crucial to identify progression or treatment failure early during follow-up to change treatment schemes and, thereby, optimize patient outcome. In the past years, several areas within the field of magnetic resonance (MR) have seen considerable advances: modern contrast media, advanced morphologic approaches and several functional techniques, for example, in the visualization of tumor perfusion or tumor cell metabolism. This review presents these recent advances by introducing the different techniques and outlining their benefit for identification of progression in brain tumors, with a focus on gliomas, metastases and meningiomas. After radiotherapy, MR spectroscopy helps to more accurately discriminate between radiation necrosis and glioma progression. In low-grade gliomas, perfusion MR techniques enable a more sensitive detection of anaplastic transformation than conventional MRI. Modern contrast media, as well as diffusion tensor imaging, allow for an improved tumor delineation and assessment of tumor extension. We will also highlight the biological background of these techniques, their applicability and current limitations. In conclusion, modern MRI techniques have been developed that are on the doorstep to be integrated in clinical routine.

Magnetic resonance imaging of neural circuits

A major goal of modern MRI research is to be able to image neural circuits in the central nervous system. Critical to this mission is the ability to describe a number of important parameters associated with neural circuits. These parameters include neural architecture, functional activation of neural circuits, anatomical and functional connectivity of neural circuits, and factors that might alter neural circuits, such as trafficking of immune cells and brain precursor cells in the brain. Remarkably, a variety of work in human and animal brains has demonstrated that all these features of neural circuits can be visualized with MRI. In this Article we provide a brief summary of the new directions in neural imaging research, which should prove useful in future analyses of normal and pathological human brains and in studies of animal models of neurological and psychiatric disorders. At present, few MRI data characterizing the neural circuits in the heart are available, but in this Article we discuss the applicable present developments and the prospects for the future.

Neurosurgical implications of mannitol accumulation within a meningioma and its peritumoral region demonstrated by magnetic resonance spectroscopy: case report

Mannitol is widely considered the hyperosmolar therapy of choice in routine neurosurgical practice for the reduction of intracranial pressure (ICP). The authors present a unique case of a patient with a large meningioma treated with mannitol, in which mannitol accumulation within the tumor and its surrounding parenchyma was shown using in vivo magnetic resonance spectroscopy (MRS). This rare appearance of mannitol on MRS was characterized by a wide-based peak at 3.8 ppm, which remained detectable several hours after the last dose. These findings provide the first in vivo evidence in support of the prevailing theory that mannitol leakage into the peritumoral edematous region may contribute to rebound increases in ICP and suggest that this phenomenon has the potential to occur in extraaxial tumors. Judicious use of mannitol in the setting of elevated ICP due to tumor may be indicated to avoid potentially deleterious side effects caused by its accumulation.