in vivo mr spectroscopic imaging of the prostate from application to interpretation

Accepted Manuscript In vivo MR spectroscopic imaging of the prostate, from application to interpretation Nassim Tayari, Arend Heerschap, Tom W.J Scheenen, Thiele Kobus PII: S0003-2697(17)30060-X DOI: 10.1016/j.ab.2017.02.001 Reference: YABIO 12621 To appear in: Analytical Biochemistry Received Date: July 2016 Revised Date: 23 December 2016 Accepted Date: February 2017 Please cite this article as: N Tayari, A Heerschap, T.W.J Scheenen, T Kobus, In vivo MR spectroscopic imaging of the prostate, from application to interpretation, Analytical Biochemistry (2017), doi: 10.1016/j.ab.2017.02.001 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT Title: In vivo MR spectroscopic imaging of the prostate, from application to interpretation Department of Radiology and Nuclear Medicine (766) SC Radboud University Medical Center Nijmegen The Netherlands Phone : (31)-24-3618908 : (31)-24-3540866 TE D * Corresponding author: Nassim Tayari M AN U P.O Box 9101 Fax RI PT Authors: Nassim Tayari, MSc, 1* Arend Heerschap, PhD, Tom W.J Scheenen, PhD, and Thiele Kobus, PhD AC C EP E-mail : Nassim.Tayari@radboudumc.nl ACCEPTED MANUSCRIPT Abstract Proton magnetic resonance spectroscopic imaging (1H MRSI) enables non-invasive assessment of certain metabolites in the prostate gland Several studies have demonstrated RI PT that this metabolic information, in combination with anatomical information from T2weighted MR imaging significantly improves prostate cancer detection, localization and disease characterization The technology of H MRSI is continuously evolving with 31 P and 13 C MRSI of the SC improvements of hardware and acquisition methods Recently, M AN U prostate have regained new interest after a dormant period of decades This review focuses on recent technical progress of in vivo 1H MRSI of the prostate, in particular those that enhance clinical applicability at 3T with respect to commonly used techniques to examine the prostate These developments consist of higher magnetic field TE D strengths, and better MR coils and acquisition techniques Besides the improvements for 1H MRSI, the developments and opportunities for reviewed Finally, we briefly review 13 31 P and 13 C MRSI for the prostate are C MRS of the prostate, in particular the new EP possibilities with hyperpolarized substrates AC C Keywords: prostate cancer; magnetic resonance spectroscopic imaging; MRSI data acquisition, prostate metabolites ACCEPTED MANUSCRIPT Introduction MR imaging is increasingly being used in the clinical management of prostate cancer such as for the detection and localization of cancer tissue, and to assess the stage and RI PT aggressiveness of the disease This is currently performed by multi-parametric MR imaging (mpMRI) of the prostate, consisting of T2-weighted imaging to visualize prostate anatomy, complemented with one or more functional MR techniques including diffusion weighted SC imaging (DWI) and dynamic contrast enhanced imaging (DCE-MRI) [1] For the interpretation of mpMRI in cancer detection a reporting system called PIRADS has been developed, which M AN U assesses the likelihood of clinically significant disease [2, 3] As MR spectroscopic imaging (MRSI) has been demonstrated to be valuable in the diagnosis, localization and characterization of the disease [1, 4-9] it was included in the original PI-RADS, but not in the most recent version, PI-RADS 2.0, due to the low practicality of current 1H MRSI methods in TE D routine clinical use In particular non-standardized and sometimes rather long examinations, the need for in-house expertise, lack of standardized automated processing and adequate data display limit the application of prostate MRSI mainly to clinical research EP There are many recent developments that might lead to a more prominent role for MRSI in AC C prostate cancer management such as improved radiofrequency (RF) coils, faster and more robust acquisition schemes with a higher sensitivity, and more dedicated automatic processing software In addition, important advances on ultra-high field 7T MR systems enable to achieve higher spatial resolutions in 1H MRSI and to perform 3D 31P MRSI of the entire prostate Finally, the introduction of in vivo 13 C MR spectroscopic imaging of the human prostate using hyperpolarized compounds opens new possibilities for the characterization of prostate cancer ACCEPTED MANUSCRIPT Acknowledging the previously published review articles on clinical 1H MRSI of prostate cancer [4-8], here we will highlight (i) the recent technical developments in 1H MRSI, (ii) recent results on 1H and 31P MRSI at 7T MR systems, and (iii) 13C MRS applications in prostate RI PT cancer H MRSI of the prostate Since the first acquired 1H MR spectra of the prostate in 1990 [10], substantial progress has SC been made in the methodology with higher field strengths and improved coils and acquisition techniques These improvements make it possible to acquire MR spectra of M AN U voxels with sizes in the order of 0.5 cm3 with sufficient SNR and spectral resolution to detect metabolites throughout the entire prostate in a clinically feasible measurement time MR Hardware Field strength TE D The first in vivo 1H spectra of the prostate were obtained at a field strength of 2T [10] Nowadays instruments with field strengths of 1.5T and 3T are commonly used, along with EP some initial experiments at 7T Higher magnetic field strengths offer increased spectral resolution and higher signal-to-noise ratio (SNR), but are challenged by the absorption of RF AC C by the tissue, and by increased susceptibility variations causing faster signal attenuation of the free induction decay, adversely affecting SNR and spectral line widths A comparison for MRSI between 1.5T and 3T showed an SNR improvement by a factor of [11] This increase in SNR enables a higher spatial and/or temporal resolution At higher spatial resolution SNR is relatively enhanced due to reduced intra-voxel de-phasing [11] No comparison in SNR or spatial/temporal resolution between 3T and 7T in prostate is available yet, but such studies ACCEPTED MANUSCRIPT of the brain suggest a more accurate assessment of metabolites levels at higher field strength [12, 13] The increased frequency dispersion at higher fields improves the separation between metabolite resonances, but also requires pulses with more RF power deposition, reaching or RI PT exceeding the maximum specific absorption rate (SAR) limit To stay within SAR-limits, an increase in repetition time (TR) might be necessary, which results in longer acquisition times Clever acquisition schemes to deal with these challenges will be discussed below SC RF coils M AN U MR(S)I of the prostate is generally performed with an integrated body coil for excitation together with external phased array coils and/or endorectal coil for signal reception The use of an endorectal coil is recommended for MRSI at 1.5T, but optional at 3T, due to its proximity to the prostate The spectral quality of MRSI data at 1.5T with endorectal coil is TE D comparable to that at 3T without endorectal coil except for the voxels close to the endorectal coil, which have a higher SNR [14] At 3T a mild but significant improvement in prostate cancer localization using 1H MSRI was observed when an endorectal coil was used EP compared to no endorectal coil [15] However, no difference in cancer localization AC C performance was observed at 1.5T between both coil configurations [16] As 7T MR-systems lack an integrated body coil, local coils are required that can both transmit and receive RF-signals The first 1H MR spectra of the prostate at 7T were obtained with a transmit/receive endorectal loop-coil [17, 18] A loop coil provides high sensitivity for adjacent tissue of interest, but suffers from transmit field (B1+) and receive field (B1-) inhomogeneity To compensate for RF field inhomogeneity, adiabatic RF pulses have been applied [16], providing a uniform RF field over the region of interest (see pulse sequence section) Others have developed an external transmit/receive 8-channel phased array coil for ACCEPTED MANUSCRIPT prostate imaging at 7T [19] As the 1H wavelength is short at 7T, destructive interferences can occur To generate an homogenous and constructive B1+-field in the prostate with this setup, B1+-shimming is required [19] During B1+-shimming, the phase of each transmitchannel is optimized in order to have a constructive B1+ in the prostate Next to receiving RI PT with an external array coil for 1H MRSI, these 8-channel coils can also be combined with a receive-only endorectal coil [20, 21] The use of an endorectal coil has some issues: the positioning is time consuming, demands a SC level of expertise, and can be uncomfortable for patients Most often, an endorectal coil M AN U with an inflatable balloon is used, which brings the coil close to the prostate With a dualchannel inflatable coil the SNR and image quality is increased compared to common single loop endorectal coil [22] Note however, an inflatable balloon changes the shape and size of the prostate significantly, which can cause difficulties if the images are used for subsequent TE D treatment [23] Recently, a rigid reusable dual-channel endorectal coil became available that provided an increased SNR and image quality up to cm from the coil compared to the Acquisition Pulse sequence EP single loop inflatable coil [24] AC C Initially, merely the field of view of the endorectal coil was used to localize prostate spectra [10] Subsequently, in vivo single voxel MRS of the prostate was performed [25, 26] Due to the multi-focal nature of prostate cancer, single voxel MRS is inadequate for prostate applications and MR spectroscopic imaging was introduced [27-31] Using phase-encoding in three directions and weighted elliptical k-space sampling, spectra of the entire prostate were obtained in 8-15 minutes with nominal voxel sizes down to 0.4 cc at a field strength of 1.5T [31] ACCEPTED MANUSCRIPT Volume localization in the prostate was first performed using point-resolved spectroscopy (PRESS) [25] and stimulated echo acquisition mode (STEAM) [26], of which the former is commonly used in the clinic to acquire prostate spectra (Figure 1A) The inter-pulse timing and magnetic field strength affect the spectral appearance of some compounds detectable M AN U SC RI PT in prostate spectra [7], which will be discussed in more detail in the section on metabolites TE D Figure 1: Schematic diagrams of RF pulses in 3D prostate MRSI with MEGA water and lipid suppression pulses A) PRESS: Localization is performed with a 90° excitation pulse and two conventional 180° refocusing pulses The echo time (TE) is defined as two times (τ2) Before excitation, outer volume saturation (OVS) pulses saturate peri-prostatic lipid signals B) sLASER: After excitation with a conventional slice selective excitation pulse, the signal is refocused with two pairs of slice-selective low-power adiabatic refocusing pulses (WURST(16,4) modulated GOIA pulses) In 2009, the use of adiabatic pulses for localization by adiabatic selective refocusing (LASER) EP was introduced for prostate MRSI [17, 32] They have better slice profiles, reducing outer volume signal contamination, are less sensitive to B1+ inhomogeneities, and have large AC C bandwidths, thus diminishing chemical shift displacement artifacts However, the pulses are RF power-demanding and need to be played out in pairs to achieve a homogeneous phase distribution over the selected slice To lower RF power deposition, GOIA (gradientmodulated offset independent adiabaticity) pulses [33] are used that require less RF to reach adiabaticity [34], and slice-selective excitation is performed by a single conventional 90° excitation pulse: the semi-LASER (sLASER) sequence (Figure 1B) [35, 36] At 3T, the ACCEPTED MANUSCRIPT measurement time for 3D MRSI using sLASER with GOIA pulses can be reduced to about minutes without an endorectal coil with a nominal voxel resolution of × × mm [37] The transmit/receive endorectal coil used for 3D 1H MRSI at 7T has a very inhomogeneous RI PT B1 This can be addressed by LASER sequences which are insensitive to B1+ inhomogeneities [17] As the excitation pulse in the sLASER sequence is non-adiabatic, sequences were introduced using either a composite adiabatic slice-selective excitation (cLASER) or a non- SC slice-selective adiabatic excitation (nsLASER), allowing for shorter TEs, whilst maintaining the adiabatic spin excitation [38, 39] However, long repetition times were required due to high M AN U RF power deposition and SAR limitations, leading to long measurement times These SARissues were addressed in a feasibility study at 7T using external phased array coils and a double spin-echo with asymmetric slice selective excitation pulses and a pair of spectralspatial pulses The spectral-spatial pulses excite and refocus only the metabolites of interest TE D and eliminate the need for additional water or lipid suppression pulses [20] The potential of 3D MRSI at 7T in prostate cancer requires further research EP As prostate MRS measurements may suffer from movement artifacts, motion reduction is essential This can be tackled by several approaches such as limiting bowel movement using AC C anti-peristaltic drugs and the application of a navigator [40] MRSI data can be measured faster by simultaneous sampling in spatial and spectral dimensions, e.g by traversing kspace in several short spiral trajectories within one read-out period [41, 42] Besides the time-gain of this approach, the spiral readouts start at the center of the k-space, which enables correcting for motion induced phase variations [42] Spiral k-space acquisitions are an attractive flexible alternative to a Cartesian sampling grid for prostate MRSI [43] Suppression of contaminating lipid and other signals: ACCEPTED MANUSCRIPT The prostate is surrounded by lipid tissue, which may cause large resonances in the ppm– range close to those of citrate, which is one of the important prostate metabolites Great care should be taken to prevent lipid signal contamination in prostate spectra To achieve this several techniques have been used: outer volume saturation (OVS), additional pulses for RI PT lipid and water suppression, frequency selective excitation or refocusing, and k-space apodization SC OVS slabs can be placed around the prostate (Figure 2) to pre-saturate signals from periprostatic lipids The signals of the excited spins are crushed with dephasing gradients M AN U Conventional OVS bands are optimized to compensate for poor edge profiles, B1 field inhomogeneity and chemical shift errors [44] Very selective saturation (VSS) pulses have a reduced B1 and T1 dependency [44] The saturation slabs are usually positioned manually; however, in conformal voxel MRS, the assignment of spatial saturation planes is optimized TE D by automatic placement, orientation, timing and flip angle setting of VSS pulses around the excitation volume based on the shape of the prostate [45, 46] To facilitate clinical use of prostate MRSI, automation of certain steps such as prostate volume segmentation, field of EP view and 3D volume selection and OVS placement are warranted AC C Additionally, dual-frequency selective MEGA pulses have been incorporated in prostate MRSI to suppress lipid and water resonances (Figure 1) The RF pulses selectively refocus water and lipid signals and are surrounded by crusher gradients to dephase the water and lipid spins while those of the metabolites of interest remain unaffected [47] MEGA pulses are used in 3D 1H MRSI sequences such as PRESS [31], GOIA-sLASER [36], cLASER, nsLASER [38] ACCEPTED MANUSCRIPT Phosphomonoesters and diesters Next to the energy balance related metabolites, 31 P MR spectra of the prostate contain resonances of compounds participating in phospholid metabolism These resonances belong to the phosphomonoesters (PME), i.e phosphocholine (PC) and phosphoethanolamine (PE), RI PT and their glycerol derivatives, the phosphodiesters (PDE) glycerophosphocholine (GPC) and glycerophosphoethanolamine (GPE) They are involved in signalling pathways, lipoprotein metabolism and the synthesis of phospatidylcholine and phosphadylethanolamine, two SC major cell membrane compounds [104] The metabolites are of interest in oncology as M AN U membrane phospholipid metabolism is altered in cancer PC, PE, GPC and GPE resonate around 5.6, 6.8, 2.8 and 3.2 ppm, respectively The 31P-atoms in these molecules are hetero-nuclear coupled to the protons in adjacent methylene group with a coupling constant around 6-7 Hz [105] At lower field strengths without 1H TE D decoupling, the two PMEs and two PDEs peaks are not resolved, but with the increased spectral resolution of 7T they can be distinguished from each other The T1 relaxation times of these metabolites at T vary between 5.9 and 8.3 s [99] EP Inorganic phosphate/ tissue pH AC C Pi is formed during the hydrolysis of ATP in ADP It is involved in many metabolic processes and is essential in glycolysis and to synthesize nucleic acids, phospholipids and proteins The resonance frequency Pi in cell is around ppm, but the exact frequency strongly depends on pH As the pKa of Pi is about 6.9, which is close to the range of physiological pH values, the chemical shift of Pi measured relative to the PCr signal, can be employed to determine tissue pH In 31P spectra of the prostate, one or two resonances in the 5.0-ppm region were observed [97-99, 101], most likely both belonging to Pi The observation of two 22 ACCEPTED MANUSCRIPT Pi resonances could point towards two compartments in the prostate with different pHs, calculated to be ~7.1 and ~7.6 [99, 101] This hypothesis of two compartments is strengthened by the observation that both resonances have a different T1 relaxation time: 6.5 s for the low pH and 4.6 s for the high pH resonance [99] Interestingly, there was a trend RI PT towards a higher ratio of Pi (pH~7.6) over Pi (pH~7.1) for cancerous tissue compared to normal tissue Further research is needed to unravel the underlying physiology of the two SC resonances and their value in prostate cancer characterization Processing and interpretation of 31P MRSI M AN U Acquisition of 31P MRSI is commonly done by a pulse-acquire scheme, which simplifies postprocessing of the spectra with respect to j-coupling evolution for coupled resonances However, there is a small time delay between the pulse and acquisition to facilitate phase encoding, which requires a first-order phase correction A detailed fitting procedure for TE D quantification of the spectra in Metabolite Report (work-in-progress package of Siemens Healthcare) was described by Lagemaat [99] Both Lorentzian (PCr, PC, PE, GPC, GPE) and Gaussian (ATP and Pi) lineshapes were used to fit the metabolites and heteronuclear EP coupling was incorporated for PMEs and PDEs AC C There is limited information available on the interpretation of 31P MR prostate data AT 1.5T an increase in the PME/PCr ratio was observed in cancer compared to non-cancer tissue [95, 96] However, these results could not be reproduced at higher field strength As localization was poor in that initial study, these spectra might have suffered from signal contamination of PCr from adjacent muscle tissue in small prostates, leading to unrepresentative ratios Results at 7T showed quite some overlap between the 31 P metabolite levels in cancer and normal tissue, which may be due to partial volume effects as the MR spectroscopy voxels were relatively large (~5.1 cm3) compared to the small size of most tumours An increased 23 ACCEPTED MANUSCRIPT spatial resolution is needed to fully evaluate the potential of these metabolites as biomarker for prostate cancer Interestingly, GPC and GPE were nearly absent in the 7T 31P spectra of RI PT non-cancerous tissue, but more often observed in cancerous tissue [101] 13 C MRSI of the prostate The first in vivo MR spectra of the human prostate were obtained bynatural abundance 13C 13 C is low (~1% of all natural carbon), 700 M AN U other signals, Cit As the natural abundance of SC MRS [106] At a field strength of 1.5 T, the authors obtained spectra that contained, among averages were needed to record this spectrum With the increasing interest in 1H MRS, which also detects Cit and does not suffer from low natural abundance, prostate was abandoned Recently, 13 13 C MRS of the C MRS of the prostate has regained interest with the sensitivity of 13 TE D new technical developments in the field of hyperpolarization The ability to increase the C-labeled substrates more than 10.000 fold using dynamic nuclear hyperpolarization (DNP) and to subsequently dissolve the sample quickly to create a solution EP that can be injected, makes it possible to study fast dynamic metabolic processes in vivo [107] After injection of the hyperpolarized 13 C-labeled substrate, the conversion of the AC C substrate into other metabolites can be observed with 13C MRS(I) Similar to the acquisition of 31P spectra, coils dedicated to the 13C frequency are necessary to acquire 13C signals in combination with 1H coils for anatomical reference images For the first in vivo study of the human prostate, a 13 C volume transmit coil was combined with an endorectal 1H/13C receive coil [108] 24 ACCEPTED MANUSCRIPT The acquisition of signals from labeled 13C-substrates is challenged by T1 relaxation by which the sample starts to lose polarization once dissolved and taken out of the polarizer As polarization also decreases due to RF excitation typically low flip angle approaches are used short imaging window, clever acquisition techniques are needed RI PT combined with short TRs To ensure that as much information as possible is obtained in the The substrates that can be measured with this technique are also limited by their T1- SC relaxation time [1-13C]-pyruvate is a popular substrate as the quaternary carbon has a relatively long T1 It also is a key metabolite at the crossroad of metabolic pathways (i.e M AN U glycolysis, citric acid cycle) The first human in vivo study using hyperpolarized 13 C-labeled pyruvate demonstrated the safety and feasibility of the method and also indicated clinical potential as the ratio of lactate to pyruvate was elevated in regions of prostate cancer AC C EP TE D (Figure 6) [108] 25 TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP Figure 6: Images were obtained from a patient with serum PSA of 9.5 ng/ml, who was diagnosed with bilateral 13 biopsy-proven Gleason grade + prostate cancer and received hyperpolarized [1- C] pyruvate (0.43 ml/kg) The axial T2-weighted image shows a unilateral region of reduced signal intensity (red arrows), which is consistent with a reduction in the corresponding apparent diffusion coefficient (ADC) The H spectral arrays supported these findings, with voxels with reduced citrate and elevated choline/citrate (highlighted in pink) on 13 the right side of the gland and voxels with normal metabolite ratios on the left side The C spectral arrays 13 13 show voxels with elevated levels of hyperpolarized [1- C]lactate/[1- C]pyruvate (highlighted in pink) on both the right and left sides of the prostate The location of colored regions in the metabolite image overlay had a 13 13 ratio of [1- C]lactate/[1- C]pyruvate greater than or equal to 0.6 Reprinted with permission from [107] The challenges of this technique include the way of polarization, substrate administration, dedicated acquisition methods, and data post-processing For instance in post-processing, the effects of T1 decay and metabolic conversion and supply on signal changes has to be taken into account, which requires complex modeling to understand the physiology that underlie these spectral changes [109] A detailed review of this promising technique is beyond the scope of this paper and more information can be found elsewhere [110, 111] 26 ACCEPTED MANUSCRIPT Outlook In this paper recent developments in hardware and software concerning prostate MRSI is reviewed A new hardware improvement involves the development of a dual channel RI PT endorectal receive coil with higher SNR in the dorsal part of the prostate compared to single channel receive coils Both inflatable and rigid coil designs have been equipped with this technology Whether further improvements can be made by adding more coil elements is SC questionable, as the penetration depth decreases for smaller coil elements However, further optimization of the external phased array coils towards MR without endorectal coil is M AN U probably more of clinical interest as it results in shorter patient preparation times, improved patient comfort and MRSI options for individual metabolite maps with uniform B1- over the prostate TE D Another hardware advance is the increased availability of 7T systems There is no information available yet whether the increased SNR and spectral resolution of this field strength will have added value for prostate cancer management However, it can be EP expected that the increased SNR can be used for either higher spatial resolution or shorter AC C measurement times As discussed, these measurements are not straightforward yet; there is a lot of room for improvement and clever acquisition techniques with minimal SARdeposition are required Ultra-high field MR systems open the possibility to perform 3D 31P MRSI of the entire prostate in a reasonable measurement time There are some indications that 31P MR spectra might contain interesting information for prostate cancer management, but more patients need to be evaluated, preferably with more aggressive and larger tumours 27 ACCEPTED MANUSCRIPT Besides progress in hardware, the recent improvements in 1H MRSI acquisition are of interest, in particular for clinical applications The sLASER sequence enables accurate volume selection of the prostate and has a minimal chemical shift displacement artefact resulting in a more robust acquisition with little lipid signal contamination The sLASER sequence RI PT combined with a spiral readout, offers a flexible approach to acquire 3D 1H MRSI data of the prostate in 5-8 without an endorectal coil at 3T This measurement time approaches the acquisition time of techniques like DWI and DCE-MRI Still, for a more widespread use of SC MRSI in a mpMRI examination, not only fast and robust acquisition is desired, but also M AN U display of the results in a reliable and easy digestible way To this end, full automation in post-processing is needed Further refinement of MRSI acquisition is possible, such as movement correction With fully automated post-processing and intuitive display in place, MRSI might be a good TE D alternative for DCE-MRI in an mpMRI prostate exam The use of gadolinium as contrast agent is under discussion, because of complications such as accumulation of residual gadolinium in the brain and bone, and nephrogenic systemic fibrosis [111-113] Especially for EP clinical applications where MR-examinations are repeated over time, MRSI might be AC C preferable over DCE-MRI Several studies have shown the ability of prostate MRSI (in particular combined with DWI) to identify aggressive prostate cancers, suggesting an important role for MRSI in selecting patients for active surveillance programs [114, 115] Lastly, the promising development in hyperpolarized 13C MRSI are discussed So far, only [113 C]-pyruvate has been studied in the human prostate, but more substrates can be polarized to investigate metabolic processes In contrast to 1H and 31P MRSI providing static levels of 28 ACCEPTED MANUSCRIPT metabolites, hyperpolarized 13C MRSI monitors dynamic processes of metabolism The true value of [1-13C]-pyruvate and other substrates in cancer diagnosis needs more investigation A prostate MR examination is multi-parametric and at this moment, MRSI is often not RI PT included in this approach However, with all recent advancements, a new evaluation of the complementary role of MRSI for prostate cancer management is in place SC Acknowledgment This work was funded by the EU Marie Curie FP&-PEOPLE-2012-ITN project TRANSACT (PITN- M AN U GA-2012-316679) and the European Research Council: FP7/2007-2013 – ERC Grant agreement 243115 Furthermore, the authors thank Sjaak J van Asten, Miriam W Lagemaat AC C EP TE D and Isabell K Steinseifer for their contributions to the manuscript 29 ACCEPTED MANUSCRIPT 1.5T Choline Heerschap et al., 1997[28] T1 (sec) T2 (sec) 0.84 ± 0.09 0.23 ± 0.06 3.1 0.21 ± 0.1 4.4 ± 0.8 Citrate 0.34 ± 0.04 Creatine 0.86 ± 0.1 Citrate Heerschap et al.[116] Citrate Lowry et al., 1996 [56] in vivo MRS 0.18± 0.1 0.84 ± 0.08 0.17 ± 0.02 (PZ) 0.12 ± 0.03 (TZ) 3T 0.22 ± 0.09 0.17 ± 0.05 0.987 ± 0.071 0.239 ± 0.024 2.6 ± 0.3 Citrate 0.476 ± 0.07 0.11 ± 0.018 26.9 ± 5.5 Creatine 1.128 ± 0.149 0.188 ± 0.020 5.8 ± 1.3 Choline Weis et al., 2013 [117] Basharat et al., 2014 [73] Choline Spermine myo-inositol Creatine Citrate Choline Spermine myo-inositol Creatine Citrate TE D Basharat et al., 2015 [72] M AN U Citrate SC 1.1 ± 0.4 0.47± 0.14 Choline Scheenen et al., 2005 [61] concentration (mM) RI PT Metabolite Reference Normal CG Normal PZ 0.062 ± 0.017 5±4 ±3 0.053 ± 0.016 7±4 10 ± 0.090 ± 0.048  15 ± 12 10 ± 8±7 ±5 32 ± 17 64 ± 22 Normal tissue Tumor 0.073 ±0.027 4±3 4±2 0.096 ± 0.056 9±3 6±3 18 ± 14 ± 9±7 10 ± 19 ± 11 Liney et al., 1997 [55] 50 ± 34 40 Spermine Kavanagh et al., 1985 [118] 12 AC C EP Citrate Table 1: 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