Direct and indirect assessment of cancer metabolism explored by MRI

Magnetic resonance‐based approaches to obtain metabolic information on cancer have been explored for decades. Electron paramagnetic resonance (EPR) has been developed to pursue metabolic profiling and successfully used to monitor several physiologic parameters such as pO₂, pH, and redox status. All these parameters are associated with pathophysiology of various diseases. Especially in oncology, cancer hypoxia has been intensively studied because of its relationship with metabolic alterations, acquiring treatment resistance, or a malignant phenotype. Thus, pO₂ imaging leads to an indirect metabolic assessment in this regard. Proton electron double‐resonance imaging (PEDRI) is an imaging technique to visualize EPR by using the Overhauser effect. Most biological parameters assessed in EPR can be visualized using PEDRI. However, EPR and PEDRI have not been evaluated sufficiently for clinical application due to limitations such as toxicity of the probes or high specific absorption rate. Hyperpolarized (HP) ¹³C MRI is a novel imaging technique that can directly visualize the metabolic profile. Production of metabolites of the HP ¹³C probe delivered to target tissue are evaluated in this modality. Unlike EPR or PEDRI, which require the injection of radical probes, ¹³C MRI requires a probe that can be physiologically metabolized and efficiently hyperpolarized. Among several methods for hyperpolarizing probes, dissolution dynamic nuclear hyperpolarization is a widely used technique for in vivo imaging. Pyruvate is the most suitable probe for HP ¹³C MRI because it is part of the glycolytic pathway and the high efficiency of pyruvate‐to‐lactate conversion is a distinguishing feature of cancer. Its clinical applicability also makes it a promising metabolic imaging modality. Here, we summarize the applications of these indirect and direct MR‐based metabolic assessments focusing on pO₂ and pyruvate‐to‐lactate conversion. The two parameters are strongly associated with each other, hence the acquired information is potentially interchangeable when evaluating treatment response to oxygen‐dependent cancer therapies.     

Single‐shot single‐voxel lactate measurements using FOCI‐LASER and a multiple‐quantum filter

Measurement of tissue lactate using ¹H MRS is often confounded by overlap with intense lipid signals at 1.3 ppm. Single‐voxel localization using PRESS is also compromised by the large chemical shift displacement between voxels for the 4.1 ppm (–CH) resonance and the 1.3 ppm –CH₃ resonance, leading to subvoxels with signals of opposite phase and hence partial signal cancellation. To reduce the chemical shift displacement to negligible proportions, a modified semi‐LASER sequence was written (“FOCI‐LASER”, abbreviated as fLASER) using FOCI pulses to permit high RF bandwidth even with the limited RF amplitude characteristic of clinical MRI scanners. A further modification, MQF‐fLASER, includes a selective multiple‐quantum filter to detect lactate and reject lipid signals. The sequences were implemented on a Philips 3 T Achieva TX system. In a solution of brain metabolites fLASER lactate signals were 2.7 times those of PRESS. MQF‐fLASER lactate was 47% of fLASER (the theoretical maximum is 50%) but still larger than PRESS lactate. In oil, the main 1.3 ppm lipid peak was suppressed to less than 1%. Enhanced suppression was possible using increased gradient durations. The minimum detectable lactate concentration was approximately 0.5 mM. Coherence selection gradients needed to be at the magic angle to avoid large water signals derived from intermolecular multiple‐quantum coherences. In pilot patient measurements, lactate peaks were often observed in brain tumours, but not in cervix tumours; lipids were effectively suppressed. In summary, compared with PRESS, the fLASER sequence yields greatly superior sensitivity for direct detection of lactate (and equivalent sensitivity for other metabolites), while the single‐voxel single‐shot MQF‐fLASER sequence surpasses PRESS for lactate detection while eliminating substantial signals from lipids. This sequence will increase the potential for in vivo lactate measurement as a biomarker in targeted anti‐cancer treatments as well as in measurements of tissue hypoxia. Copyright © 2015 John Wiley & Sons, Ltd.     

Extraction of artefactual MRS patterns from a large database using non-negative matrix factorization

Despite the success of automated pattern recognition methods in problems of human brain tumor diagnostic classification, limited attention has been paid to the issue of automated data quality assessment in the field of MRS for neuro-oncology. Beyond some early attempts to address this issue, the current standard in practice is MRS quality control through human (expert-based) assessment. One aspect of automatic quality control is the problem of detecting artefacts in MRS data. Artefacts, whose variety has already been reviewed in some detail and some of which may even escape human quality control, have a negative influence in pattern recognition methods attempting to assist tumor characterization. The automatic detection of MRS artefacts should be beneficial for radiology as it guarantees more reliable tumor characterizations, as well as the development of more robust pattern recognition-based tumor classifiers and more trustable MRS data processing and analysis pipelines. Feature extraction methods have previously been used to help distinguishing between good and bad quality spectra to apply subsequent supervised pattern recognition techniques. In this study, we apply feature extraction differently and use a variant of a method for blind source separation, namely Convex Non-Negative Matrix Factorization, to unveil MRS signal sources in a completely unsupervised way. We hypothesize that, while most sources will correspond to the different tumor patterns, some of them will reflect signal artefacts. The experimental work reported in this paper, analyzing a combined short and long echo time ¹H-MRS database of more than 2000 spectra acquired at 1.5T and corresponding to different tumor types and other anomalous masses, provides a first proof of concept that points to the possible validity of this approach.     

Cancer metabolism in a snapshot: MRS(I)

The contribution of MRS(I) to the in vivo evaluation of cancer‐metabolism‐derived metrics, mostly since 2016, is reviewed here. Increased carbon consumption by tumour cells, which are highly glycolytic, is now being sampled by ¹³C magnetic resonance spectroscopic imaging (MRSI) following the injection of hyperpolarized [1‐¹³C] pyruvate (Pyr). Hot‐spots of, mostly, increased lactate dehydrogenase activity or flow between Pyr and lactate (Lac) have been seen with cancer progression in prostate (preclinical and in humans), brain and pancreas (both preclinical) tumours. Therapy response is usually signalled by decreased Lac/Pyr ¹³C‐labelled ratio with respect to untreated or non‐responding tumour. For therapeutic agents inducing tumour hypoxia, the ¹³C‐labelled Lac/bicarbonate ratio may be a better metric than the Lac/Pyr ratio. ³¹P MRSI may sample intracellular pH changes from brain tumours (acidification upon antiangiogenic treatment, basification at fast proliferation and relapse). The steady state tumour metabolome pattern is still in use for cancer evaluation. Metrics used for this range from quantification of single oncometabolites (such as 2‐hydroxyglutarate in mutant IDH1 glial brain tumours) to selected metabolite ratios (such as total choline to N‐acetylaspartate (plain ratio or CNI index)) or the whole ¹H MRSI(I) pattern through pattern recognition analysis. These approaches have been applied to address different questions such as tumour subtype definition, following/predicting the response to therapy or defining better resection or radiosurgery limits.     

Robust hyperpolarized 13C metabolic imaging with selective non‐excitation of pyruvate (SNEP)

In vivo metabolic imaging using hyperpolarized [1‐¹³C]pyruvate provides localized biochemical information and is particularly useful in detecting early disease changes, as well as monitoring disease progression and treatment response. However, a major limitation of hyperpolarized magnetization is its unrecoverable decay, due not only to T₁ relaxation but also to radio‐frequency (RF) excitation. RF excitation schemes used in metabolic imaging must therefore be able to utilize available hyperpolarized magnetization efficiently and robustly for the optimal detection of substrate and metabolite activities. In this work, a novel RF excitation scheme called selective non‐excitation of pyruvate (SNEP) is presented. This excitation scheme involves the use of a spectral selective RF pulse to specifically exclude the excitation of [1‐¹³C]pyruvate, while uniformly exciting the key metabolites of interest (namely [1‐¹³C]lactate and [1‐¹³C]alanine) and [1‐¹³C]pyruvate‐hydrate. By eliminating the loss of hyperpolarized [1‐¹³C]pyruvate magnetization due to RF excitation, the signal from downstream metabolite pools is increased together with enhanced dynamic range. Simulation results, together with phantom measurements and in vivo experiments, demonstrated the improvement in signal‐to‐noise ratio (SNR) and the extension of the lifetime of the [1‐¹³C]lactate and [1‐¹³C]alanine pools when compared with conventional non‐spectral selective (NS) excitation. SNEP has also been shown to perform comparably well with multi‐band (MB) excitation, yet SNEP possesses distinct advantages, including ease of implementation, less stringent demands on gradient performance, increased robustness to frequency drifts and B₀ inhomogeneity as well as easier quantification involving the use of [1‐¹³C]pyruvate‐hydrate as a proxy for the actual [1‐¹³C] pyruvate signal. SNEP is therefore a promising alternative for robust hyperpolarized [1‐¹³C]pyruvate metabolic imaging with high fidelity. Copyright © 2015 John Wiley & Sons, Ltd.     

A comparison of quantitative methods for clinical imaging with hyperpolarized 13C‐pyruvate

Dissolution dynamic nuclear polarization (DNP) enables the metabolism of hyperpolarized ¹³C‐labelled molecules, such as the conversion of [1‐¹³C]pyruvate to [1‐¹³C]lactate, to be dynamically and non‐invasively imaged in tissue. Imaging of this exchange reaction in animal models has been shown to detect early treatment response and correlate with tumour grade. The first human DNP study has recently been completed, and, for widespread clinical translation, simple and reliable methods are necessary to accurately probe the reaction in patients. However, there is currently no consensus on the most appropriate method to quantify this exchange reaction. In this study, an in vitro system was used to compare several kinetic models, as well as simple model‐free methods. Experiments were performed using a clinical hyperpolarizer, a human 3 T MR system, and spectroscopic imaging sequences. The quantitative methods were compared in vivo by using subcutaneous breast tumours in rats to examine the effect of pyruvate inflow. The two‐way kinetic model was the most accurate method for characterizing the exchange reaction in vitro, and the incorporation of a Heaviside step inflow profile was best able to describe the in vivo data. The lactate time‐to‐peak and the lactate‐to‐pyruvate area under the curve ratio were simple model‐free approaches that accurately represented the full reaction, with the time‐to‐peak method performing indistinguishably from the best kinetic model. Finally, extracting data from a single pixel was a robust and reliable surrogate of the whole region of interest. This work has identified appropriate quantitative methods for future work in the analysis of human hyperpolarized ¹³C data. © 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.     

T1 nuclear magnetic relaxation dispersion of hyperpolarized sodium and cesium hydrogencarbonate‐13C

In vivo pH mapping in tissue using hyperpolarized hydrogencarbonate‐¹³C has been proposed as a method to study tumor growth and treatment and other pathological conditions related to pH changes. The finite spin–lattice relaxation times (T₁) of hyperpolarized media are a significant limiting factor for in vivo imaging. Relaxation times can be measured at standard magnetic fields (1.5 T, 3.0 T etc.), but no such data are available at low fields, where T₁ values can be significantly shorter. This information is required to determine the potential loss of polarization as the agent is dispensed and transported from the polarizer to the MRI scanner. The purpose of this study is to measure T₁ dispersion from low to clinical magnetic fields (0.4 mT to 3.0 T) of different hyperpolarized hydrogencarbonate formulations previously proposed in the literature for in vivo pH measurements. ¹³C–enriched cesium and sodium hydrogencarbonate preparations were hyperpolarized using dynamic nuclear polarization, and the T₁ values of different samples were measured at different magnetic field strengths using a fast field‐cycling relaxometer and a 3.0 T clinical MRI system. The effects of deuterium oxide as a dissolution medium for sodium hydrogencarbonate were also analyzed. This study finds that the cesium formulation has slightly shorter T₁ values compared with the sodium preparation. However, the higher solubility of cesium hydrogencarbonate‐¹³C means it can be polarized at greater concentration, using less trityl radical than sodium hydrogencarbonate‐¹³C. This study also establishes that the preparation and handling of sodium hydrogencarbonate formulations in relation to cesium hydrogencarbonate is more difficult, due to the higher viscosity and lower achievable concentrations, and that deuterium oxide significantly increases the T₁ of sodium hydrogencarbonate solutions. Finally, this work also investigates the influence of pH on the spin–lattice relaxation of cesium hydrogencarbonate‐¹³C measured over a pH range of 7 to 9 at 0.47 T.     

Metabolism of hyperpolarized 13C‐acetoacetate to β‐hydroxybutyrate detects real‐time mitochondrial redox state and dysfunction in heart tissue

Mitochondrial dysfunction is considered to be an important component of many metabolic diseases yet there is no reliable imaging biomarker for monitoring mitochondrial damage in vivo. A large prior literature on inter‐conversion of β‐hydroxybutyrate and acetoacetate indicates that the process is mitochondrial and that the ratio reflects a specifically mitochondrial redox state. Therefore, the conversion of [1,3‐¹³C]acetoacetate to [1,3‐¹³C]β‐hydroxybutyrate is expected to be sensitive to the abnormal redox state present in dysfunctional mitochondria. In this study, we examined the conversion of hyperpolarized (HP) ¹³C‐acetoacetate (AcAc) to ¹³C‐β‐hydroxybutyrate (β‐HB) as a potential imaging biomarker for mitochondrial redox and dysfunction in perfused rat hearts. Conversion of HP‐AcAc to β‐HB was investigated using ¹³C magnetic resonance spectroscopy in Langendorff‐perfused rat hearts in four groups: control, global ischemic reperfusion, low‐flow ischemic, and rotenone (mitochondrial complex‐I inhibitor)‐treated hearts. We observed that more β‐HB was produced from AcAc in ischemic hearts and the hearts exposed to complex I inhibitor rotenone compared with controls, consistent with the accumulation of excess mitochondrial NADH. The increase in β‐HB, as detected by ¹³C MRS, was validated by a direct measure of tissue β‐HB by ¹H nuclear magnetic resonance in tissue extracts. The redox ratio, NAD⁺/NADH, measured by enzyme assays of homogenized tissue, also paralleled production of β‐HB from AcAc. Transmission electron microscopy of tissues provided direct evidence for abnormal mitochondrial structure in each ischemic tissue model. The results suggest that conversion of HP‐AcAc to HP‐β‐HB detected by ¹³C‐MRS may serve as a useful diagnostic marker of mitochondrial redox and dysfunction in heart tissue in vivo.     

T2 relaxation times of 13C metabolites in a rat hepatocellular carcinoma model measured in vivo using 13C‐MRS of hyperpolarized [1‐13C]pyruvate

A single‐voxel Carr‐Purcell‐Meibloom‐Gill sequence was developed to measure localized T₂ relaxation times of ¹³C‐labeled metabolites in vivo for the first time. Following hyperpolarized [1‐¹³C]pyruvate injections, pyruvate and its metabolic products, alanine and lactate, were observed in the liver of five rats with hepatocellular carcinoma and five healthy control rats. The T₂ relaxation times of alanine and lactate were both significantly longer in HCC tumors than in normal livers (p < 0.002). The HCC tumors also showed significantly higher alanine signal relative to the total ¹³C signal than normal livers (p < 0.006). The intra‐ and inter‐subject variations of the alanine T₂ relaxation time were 11% and 13%, respectively. The intra‐ and inter‐subject variations of the lactate T₂ relaxation time were 6% and 7%, respectively. The intra‐subject variability of alanine to total carbon ratio was 16% and the inter‐subject variability 28%. The intra‐subject variability of lactate to total carbon ratio was 14% and the inter‐subject variability 20%. The study results show that the signal level and relaxivity of [1‐¹³C]alanine may be promising biomarkers for HCC tumors. Its diagnostic values in HCC staging and treatment monitoring are yet to be explored. Copyright © 2010 John Wiley & Sons, Ltd.     

Stratification of the aggressiveness of prostate cancer using pre‐biopsy multiparametric MRI (mpMRI)

Risk stratification, based on the Gleason score (GS) of a prostate biopsy, is an important decision‐making tool in prostate cancer management. As low‐grade disease may not need active intervention, the ability to identify aggressive cancers on imaging could limit the need for prostate biopsies. We assessed the ability of multiparametric MRI (mpMRI) in pre‐biopsy risk stratification of men with prostate cancer. One hundred and twenty men suspected to have prostate cancer underwent mpMRI (diffusion MRI and MR spectroscopic imaging) prior to biopsy. Twenty‐six had cancer and were stratified into three groups based on GS: low grade (GS ≤ 6), intermediate grade (GS = 7) and high grade (GS ≥ 8). A total of 910 regions of interest (ROIs) from the peripheral zone (PZ, range 25–45) were analyzed from these 26 patients. The metabolite ratio [citrate/(choline + creatine)] and apparent diffusion coefficient (ADC) of voxels were calculated for the PZ regions corresponding to the biopsy cores and compared with histology. The median metabolite ratios for low‐grade, intermediate‐grade and high‐grade cancer were 0.29 (range: 0.16, 0.61), 0.17 (range: 0.13, 0.32) and 0.13 (range: 0.05, 0.23), respectively (p = 0.004). The corresponding mean ADCs (×10^–3 mm²/s) for low‐grade, intermediate‐grade and high‐grade cancer were 0.99 ± 0.08, 0.86 ± 0.11 and 0.69 ± 0.12, respectively (p < 0.0001). The combined ADC and metabolite ratio model showed strong discriminatory ability to differentiate subjects with GS ≤ 6 from subjects with GS ≥ 7 with an area under the curve of 94%. These data indicate that pre‐biopsy mpMRI may stratify PCa aggressiveness noninvasively. As the recent literature data suggest that men with GS ≤ 6 cancer may not need radical therapy, our data may help limit the need for biopsy and allow informed decision making for clinical intervention. Copyright © 2015 John Wiley & Sons, Ltd.     

Focus on the glycerophosphocholine pathway in choline phospholipid metabolism of cancer

Activated choline metabolism is a hallmark of carcinogenesis and tumor progression, which leads to elevated levels of phosphocholine and glycerophosphocholine in all types of cancer tested so far. Magnetic resonance spectroscopy applications have played a key role in detecting these elevated choline phospholipid metabolites. To date, the majority of cancer‐related studies have focused on phosphocholine and the Kennedy pathway, which constitutes the biosynthesis pathway for membrane phosphatidylcholine. Fewer and more recent studies have reported on the importance of glycerophosphocholine in cancer. In this review article, we summarize the recent literature on glycerophosphocholine metabolism with respect to its cancer biology and its detection by magnetic resonance spectroscopy applications.     

Methodological and physiological test–retest reliability of 13C‐MRS glycogen measurements in liver and in skeletal muscle of patients with type 1 diabetes and matched healthy controls

Glycogen is a major substrate in energy metabolism and particularly important to prevent hypoglycemia in pathologies of glucose homeostasis such as type 1 diabetes mellitus (T1DM). ¹³C‐MRS is increasingly used to determine glycogen in skeletal muscle and liver non‐invasively; however, the low signal‐to‐noise ratio leads to long acquisition times, particularly when glycogen levels are determined before and after interventions. In order to ease the requirements for the subjects and to avoid systematic effects of the lengthy examination, we evaluated if a standardized preparation period would allow us to shift the baseline (pre‐intervention) experiments to a preceding day. Based on natural abundance ¹³C‐MRS on a clinical 3 T MR system the present study investigated the test–retest reliability of glycogen measurements in patients with T1DM and matched controls (n = 10 each group) in quadriceps muscle and liver. Prior to the MR examination, participants followed a standardized diet and avoided strenuous exercise for two days. The average coefficient of variation (CV) of myocellular glycogen levels was 9.7% in patients with T1DM compared with 6.6% in controls after a 2 week period, while hepatic glycogen variability was 13.3% in patients with T1DM and 14.6% in controls. For comparison, a single‐session test–retest variability in four healthy volunteers resulted in 9.5% for skeletal muscle and 14.3% for liver. Glycogen levels in muscle and liver were not statistically different between test and retest, except for hepatic glycogen, which decreased in T1DM patients in the retest examination, but without an increase of the group distribution. Since the CVs of glycogen levels determined in a “single session” versus “within weeks” are comparable, we conclude that the major source of uncertainty is the methodological error and that physiological variations can be minimized by a pre‐study standardization. For hepatic glycogen examinations, familiarization sessions (MR and potentially strenuous interventions) are recommended. Copyright © 2016 John Wiley & Sons, Ltd.     

Manganese‐enhanced MRI (MEMRI) via topical loading of Mn2+ significantly impairs mouse visual acuity: a comparison with intravitreal injection

Manganese‐enhanced MRI (MEMRI) with topical loading of MnCl₂ provides optic nerve enhancement comparable to that seen by intravitreal injection. However, the impact of this novel and non‐invasive Mn²⁺ loading method on visual function requires further assessments. The objective of this study is to determine the optimal topical Mn²⁺ loading dosage for MEMRI and to assess visual function after MnCl₂ loading. Intravitreal administration was performed to compare the two approaches of MnCl₂ loading. Twenty‐four hours after topical loading of 0, 0.5, 0.75, and 1 M MnCl₂, T₁‐weighted, T2‐weighted, diffusion tensor imaging and visual acuity (VA) assessments were performed to determine the best topical loading dosage for MEMRI measurements and to assess the integrity of retinas and optic nerves. Mice were perfusion fixed immediately after in vivo experiments for hematoxylin and eosin and immunohistochemistry staining. Topical loading of 1 M MnCl₂ damaged the retinal photoreceptor layer with no detectable damage to retina ganglion cell layers or prechiasmatic optic nerves. For the topical loading, 0.75 M MnCl₂ was required to see sufficient enhancement of the optic nerve. At this concentration the visual function was significantly affected, followed by a slow recovery. Intravitreal injection (0.25 μL of 0.2 M MnCl₂) slightly affected VA, with full recovery a day later. To conclude, intravitreal MnCl₂injection provides more reproducible results with less adverse side‐effects than topical loading. Copyright © 2014 John Wiley & Sons, Ltd.     

Refined modelling of the short‐T2 signal component and ensuing detection of glutamate and glutamine in short‐TE,localised, 1H MR spectra of human glioma measured at 3 T

Short‐TE ¹H MRS has great potential for brain cancer diagnostics. A major difficulty in the analysis of the spectra is the contribution from short‐T₂ signal components, mainly coming from mobile lipids. This complicates the accurate estimation of the spectral parameters of the resonance lines from metabolites, so that a qualitative to semi‐quantitative interpretation of the spectra dominates in practice. One solution to overcome this difficulty is to measure and estimate the short‐T₂ signal component and to subtract it from the total signal, thus leaving only the metabolite signals. The technique works well when applied to spectra obtained from healthy individuals, but requires some optimisation during data acquisition. In the clinical setting, time constraints hardly allow this. Here, we propose an iterative estimation of the short‐T₂ signal component, acquired in a single acquisition after measurement of the full spectrum. The method is based on QUEST (quantitation based on quantum estimation) and allows the refinement of the estimate of the short‐T₂ signal component after measurement. Thus, acquisition protocols used on healthy volunteers can also be used on patients without further optimisation. The aim is to improve metabolite detection and, ultimately, to enable the estimation of the glutamine and glutamate signals distinctly. These two metabolites are of great interest in the characterisation of brain cancer, gliomas in particular. When applied to spectra from healthy volunteers, the new algorithm yields similar results to QUEST and direct subtraction of the short‐T₂ signal component. With patients, up to 12 metabolites and, at least, seven can be quantified in each individual brain tumour spectrum, depending on the metabolic state of the tumour. The refinement of the short‐T₂ signal component significantly improves the fitting procedure and produces a separate short‐T₂ signal component that can be used for the analysis of mobile lipid resonances. Thus, in brain tumour spectra, distinct estimates of signals from glutamate and glutamine are possible. Copyright © 2016 John Wiley & Sons, Ltd.     

Imaging chemical exchange saturation transfer (CEST) effects following tumor‐selective acidification using lonidamine

Increased lactate production through glycolysis in aerobic conditions is a hallmark of cancer. Some anticancer drugs have been designed to exploit elevated glycolysis in cancer cells. For example, lonidamine (LND) inhibits lactate transport, leading to intracellular acidification in cancer cells. Chemical exchange saturation transfer (CEST) is a novel MRI contrast mechanism that is dependent on intracellular pH. Amine and amide concentration‐independent detection (AACID) and apparent amide proton transfer (APT*) represent two recently developed CEST contrast parameters that are sensitive to pH. The goal of this study was to compare the sensitivity of AACID and APT* for the detection of tumor‐selective acidification after LND injection. Using a 9.4‐T MRI scanner, CEST data were acquired in mice approximately 14 days after the implantation of 10⁵ U87 human glioblastoma multiforme (GBM) cells in the brain, before and after the administration of LND (dose, 50 or 100 mg/kg). Significant dose‐dependent LND‐induced changes in the measured CEST parameters were detected in brain regions spatially correlated with implanted tumors. Importantly, no changes were observed in T₁‐ and T₂‐weighted images acquired before and after LND treatment. The AACID and APT* contrast measured before and after LND injection exhibited similar pH sensitivity. Interestingly, LND‐induced contrast maps showed increased heterogeneity compared with pre‐injection CEST maps. These results demonstrate that CEST contrast changes after the administration of LND could help to localize brain cancer and monitor tumor response to chemotherapy within 1 h of treatment. The LND CEST experiment uses an anticancer drug to induce a metabolic change detectable by endogenous MRI contrast, and therefore represents a unique cancer detection paradigm which differs from other current molecular imaging techniques that require the injection of an imaging contrast agent or tracer. Copyright © 2015 John Wiley & Sons, Ltd.