Assessment of absolute metabolite concentrations in human tissue by 31P MRS in vivo. Part II: Muscle,liver, kidney

Absolute metabolite concentrations were assessed in the muscle, the liver, and the kidney of healthy human volunteers by ³¹P MRS. Fully relaxed in vivo spectra were acquired with a surface coil and were localized with an adiabatic lSlS pulse sequence. The spectra were quantified with a subsequent measurement of a calibration phantom and were processed iteratively in the time domain. The following mean metabolite concentrations (mmollliter) were measured in the resting male calf muscle (n = 9), in the fasting liver (n = 12), and in the orthiotopic kidney (n= 5): [PME] = 2.0 ± 0.6, 3.8 ± 0.7, and 2.6 ± 0.9, [Pi] = 2.9 ± 0.3, 1.8 ± 0.3, and 1.6 ± 0.4, [PDE] = 3.8 ± 0.8, 9.7 ± 1.5, and 4.9 ± 1.1, [PCr] = 22.0 ± 1.2, 0, and 0, [NTP] = 5.7 ± 0.4, 2.9 ± 0.4, and 2.0 ± 0.3, respectively. Several interesting findings are to be emphasized: The concentrations of Pi, PCr, and NTP were 20% lower in the muscle of women than of men. In addition, the pH, was significantly lower in female muscle (6.99 ± 0.03) than in male muscle (7.05 ± 0.03). The pH, in the liver (7.12 ± 0.09) and in the kidney (7.09 ± 0.08) were higher than in the muscle of both genders. The free magnesium concentration (mmollliter) was higher in the lliver (1.40 ± 0.64) than in the kidney (0.79 ± 0.39) and in the muscle (0.52 ± 0.10).     

Semi‐LASER localized dynamic 31P magnetic resonance spectroscopy in exercising muscle at ultra‐high magnetic field

Magnetic resonance spectroscopy (MRS) can benefit from increased signal‐to‐noise ratio (SNR) of high magnetic fields. In this work, the SNR gain of dynamic ³¹P MRS at 7 T was invested in temporal and spatial resolution. Using conventional slice selective excitation combined with localization by adiabatic selective refocusing (semi‐LASER) with short echo time (TE = 23 ms), phosphocreatine quantification in a 38 mL voxel inside a single exercising muscle becomes possible from single acquisitions, with SNR = 42 ± 4 in resting human medial gastrocnemius. The method was used to quantify the phosphocreatine time course during 5 min of plantar flexion exercise and recovery with a temporal resolution of 6 s (the chosen repetition time for moderate T₁ saturation). Quantification of inorganic phosphate and pH required accumulation of consecutively acquired spectra when (resting) Pi concentrations were low. The localization performance was excellent while keeping the chemical shift displacement acceptably small. The SNR and spectral line widths with and without localization were compared between 3 T and 7 T systems in phantoms and in vivo. The results demonstrate that increased sensitivity of ultra‐high field can be used to dynamically acquire metabolic information from a clearly defined region in a single exercising muscle while reaching a temporal resolution previously available with MRS in non‐localizing studies only. The method may improve the interpretation of dynamic muscle MRS data. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Regional metabolite T2 in the healthy rhesus macaque brain at 7T

Although the rhesus macaque brain is an excellent model system for the study of neurological diseases and their responses to treatment, its small size requires much higher spatial resolution, motivating use of ultra‐high‐field (B₀) imagers. Their weaker radio‐frequency fields, however, dictate longer pulses; hence longer TE localization sequences. Due to the shorter transverse relaxation time (T₂) at higher B₀s, these longer TEs subject metabolites to T₂‐weighting, that decrease their quantification accuracy. To address this we measured the T₂s of N‐acetylaspartate (NAA), choline (Cho), and creatine (Cr) in several gray matter (GM) and white matter (WM) regions of four healthy rhesus macaques at 7T using three‐dimensional (3D) proton MR spectroscopic imaging at (0.4 cm)³ = 64 μl spatial resolution. The results show that macaque T₂s are in good agreement with those reported in humans at 7T: 169 ± 2.3 ms for NAA (mean ± SEM), 114 ± 1.9 ms for Cr, and 128 ± 2.4 ms for Cho, with no significant differences between GM and WM. The T₂ histograms from 320 voxels in each animal for NAA, Cr, and Cho were similar in position and shape, indicating that they are potentially characteristic of “healthy” in this species. Magn Reson Med 59:1165–1169, 2008. © 2008 Wiley‐Liss, Inc.     

NOE Enhancements and T1 Relaxation Times of Phosphorylated Metabolites in Human Calf Muscle at 1.5 Tesla

Nuclear Overhauser effect (NOE) enhancements and relaxation times of ³¹P metabolites in human calf were measured in 12 volunteers (4 men and 8 women) at 1.5 T using a dual tuned four-ring birdcage. The NOE enhancements of inorganic phosphate (P₁), phosphocreatine (PCr), γ-, α-, and β-nucleoside triphosphate (NTP) from 19 measurements were 0.51 ± 0.10, 0.64 ± 0.03, 0.53 ± 0.03, 0.56 ± 0.08, and 0.47 ± 0.05, respectively. The relaxation times were independent of proton irradiation and from 23 measurements were 3.49 ± 0.35, 4.97 ± 0.58, 4.07 ± 0.36, 2.90 ± 0.25, and 3.61 ± 0.25 s for P₁, PCr, γ-, α-, and β-NTP, respectively. No significant differences between gender and age were observed for either NOE enhancements or relaxation times. Also, among nine volunteers, we observed no significant differences in T₁ between the coupled and decoupled cases.     

Age dependence of regional proton metabolites T2 relaxation times in the human brain at 3 T

Although recent studies indicate that use of a single global transverse relaxation time, T₂, per metabolite is sufficient for better than ±10% quantification precision at intermediate and short echo‐time spectroscopy in young adults, the age‐dependence of this finding is unknown. Consequently, the age effect on regional brain choline (Cho), creatine (Cr), and N‐acetylaspartate (NAA) T₂s was examined in four age groups using 3D (four slices, 80 voxels 1 cm³ each) proton MR spectroscopy in an optimized two‐point protocol. Metabolite T₂s were estimated in each voxel and in 10 gray and white matter (GM, WM) structures in 20 healthy subjects: four adolescents (13 ± 1 years old), eight young adults (26 ± 1); two middle‐aged (51 ± 6), and six elderly (74 ± 3). The results reveal that T₂s in GM (average ± standard error of the mean) of adolescents (NAA: 301 ± 30, Cr: 162 ± 7, Cho: 263 ± 7 ms), young adults (NAA: 269 ± 7, Cr: 156 ± 7, Cho: 226 ± 9 ms), and elderly (NAA: 259 ± 13, Cr: 154 ± 8, Cho: 229 ± 14 ms), were 30%, 16%, and 10% shorter than in WM, yielding mean global T₂s of NAA: 343, Cr: 172, and Cho: 248 ms. The elderly NAA, Cr, and Cho T₂s were 12%, 6%, and 10% shorter than the adolescents, a change of under 1 ms/year assuming a linear decline with age. Formulae for T₂ age‐correction for higher quantification precision are provided. Magn Reson Med 60:790–795, 2008. © 2008 Wiley‐Liss, Inc.     

Brain metabolites B1‐corrected proton T1 mapping in the rhesus macaque at 3 T

The accuracy of metabolic quantification in MR spectroscopy is limited by the unknown radiofrequency field and T₁. To address both issues in proton (¹H) MR spectroscopy, we obtained radiofrequency field–corrected T₁ maps of N‐acetylaspartate, choline, and creatine in five healthy rhesus macaques at 3 T. For efficient use of the 4 hour experiment, we used a new three‐point protocol that optimizes the precision of T₁ in three‐dimensional ¹H‐MR spectroscopy localization for extensive, ～30%, brain coverage at 0.6 × 0.6 × 0.5 cm³ = 180‐μL spatial resolution. The resulting mean T₁s in 700 voxels were N‐acetylaspartate = 1232 ± 44, creatine = 1238 ± 23 and choline = 1107 ± 56 ms (mean ± standard error of the mean). Their histograms from all 140 voxels in each animal were similar in position and shape, characterized by standard errors of the mean of the full width at half maximum divided by their means of better than 8%. Regional gray matter N‐acetylaspartate, choline, and creatine T₁s (1333 ± 43, 1265 ± 52, and 1131 ± 28 ms) were 5–10% longer than white matter: 1188 ± 34, 1201 ± 24, and 1082 ± 50 ms (statistically significant for the N‐acetylaspartate only), all within 10% of the corresponding published values in the human brain. Magn Reson Med 63:865–871, 2010. © 2010 Wiley‐Liss, Inc.     

Quantification of T2 in the abdomen at 3.0 T using a T2‐prepared balanced turbo field echo sequence

The T₂‐prepared balanced turbo field echo sequence has been used to measure T₂ in phantoms and in vivo in the abdomen with low sensitivity to radiofrequency pulse errors. The effects of noise, errors in the pulse flip angles, and off resonance effects on the results have been simulated. It was found from simulations that for normal conditions, including the flip angle in the fit improved the systematic errors due to radiofrequency pulse errors and noise in the results to less than 1% without significantly increasing the random errors. For a 0.3% noise level, the standard deviation in the measured T₂ was approximately 0.003 ms. Off‐resonance effects had a minimal effect on the measured T₂ value. The T₂ at 3.0 T of various abdominal organs was measured, in particular the liver (31 ± 6 ms), spleen (54 ± 15 ms), kidney cortex (76 ± 6 ms), kidney medulla (61 ± 8 ms), and pancreas (42 ± 20 ms). Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Temporal and anatomical variations of brain water apparent diffusion coefficient in perinatal cerebral hypoxic-ischemic injury: Relationships to cerebral energy metabolism

Cerebral apparent diffusion coefficients {ADCs) were determined in nine newborn piglets before and for 48 h after transient hypoxia-ischemia. Phosphorus MRS revealed severely reduced cerebral energy metabolism during the insult and an apparently complete recovery 2 h after resuscitation commenced. At this time, mean ADC over the imaging slice (ADC_global) was 0.88 (0.04) × 10^{? 9} m² · s^{? 1} (mean (SD}), which was close to the baseline value of 0.92 (0.4) × 10^{? 9} m² · s^{? 1}. In seven of the animals, a “secondary” failure of energy metabolism then evolved, accompanied by a decline in ADC_global to 0.64 (0.17) × 10^{? 9} m² · s^{? 1} at 46 h postresuscitation (P < 0.001 versus baseline). For these seven animals, ADC_global correlated linearly with the concentration ratio [phosphocreatine (PCr)][inorganic phosphate (Pi)] (0.94 r < 0.99; P > 0.001). A nonlinear relationship was demonstrated between ADC_global, and the concentration ratio [nucleotide triphosphate (NTP)]/ [Pi + PCr + 3 NTP]. The ADC reduction commenced in the parasagittal cortex before spreading in a characteristic pattern throughout the brain. ADC seems to be closely related to cerebral energy status and shows considerable potential for the assessment of hypoxic-ischemic injury in the newborn brain.     

Metabolite proton T2 mapping in the healthy rhesus macaque brain at 3 T

The structure and metabolism of the rhesus macaque brain, an advanced model for neurologic diseases and their treatment response, is often studied noninvasively with MRI and ¹H‐MR spectroscopy. Due to the shorter transverse relaxation time (T₂) at the higher magnetic fields these studies favor, the echo times used in ¹H‐MR spectroscopy subject the metabolites to unknown T₂ weighting, decreasing the accuracy of quantification which is key for inter‐ and intra‐animal comparisons. To establish the “baseline” (healthy animal) T₂ values, we mapped them for the three main metabolites' T₂s at 3 T in four healthy rhesus macaques and tested the hypotheses that their mean values are similar (i) among animals; and (ii) to analogs regions in the human brain. This was done with three‐dimensional multivoxel ¹H‐MR spectroscopy at (0.6 × 0.6 × 0.5 cm)³ = 180 μL spatial resolution over a 4.2 × 3.0 × 2.0 = 25 cm³ (～30%) of the macaque brain in a two‐point protocol that optimizes T₂ precision per unit time. The estimated T₂s in several gray and white matter regions are all within 10% of those reported in the human brain (mean ± standard error of the mean): N‐acetylaspartate = 316 ± 7, creatine = 177 ± 3, and choline = 264 ± 9 ms, with no statistically significant gray versus white matter differences. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Measurement of spin-lattice relaxation times and kinetic rate constants in rat muscle using progressive partial saturation and steady-state saturation transfer

³¹P spin-lattice relaxation times (T₁) of metabolites in rat calf muscle at 1.9 Tesla and the forward rate through the creatine kinase (CK) reaction have been measured using a new method based on modeling progressive saturation explicitly incorporating the effect of chemical exchange. In a separate series of experiments, we compared our method with inversion recovery both in vitro and in vivo, finding agreement between the techniques. We found that the T₁ values of phosphocreatine (PCr) (6.6 ± 0.3 s), γ-ATP (2.6 ± 0.6 s), α-ATP (2.4 ± 0.4 s) and β-ATP (2.2 ± 0.2 s) are unchanged by stimulation of sufficient intensity to induce a 32% drop in PCr level. The errors in T₁ values which arise when chemical exchange is neglected are calculated. These are found to be on the order of 20% for PCr and 30–50% for γ-ATP under typical conditions. Use of longer repetition times results in larger errors in measured values of T₁. This source of error can be effectively eliminated by use of sufficiently short repetition times. We found that the rate constant of the forward CK reaction was increased 60% by stimulation, from 0.20 ± 0.03 s⁻¹ to 0.32 ± 0.03 s⁻¹, but that the phosphorus flux did not change.     

Metabolic characterization of primary and metastatic ovarian cancer by 1H‐MRS in vivo at 3T

¹H‐MRS was performed on 12 women (age range 45–72) with ovarian cancer of FIGO stage 3 or above using a 3T MRI system with an 8‐channel cardiac receive coil. Respiratory‐triggered PRESS‐localized spectra (TE = 144 ms) were obtained separately from an ovarian mass and from metastatic disease. Peak areas were quantified relative to unsuppressed water using LCModel and spectra were discarded if LCModel reported signal‐to‐noise ratio (SNR) < 3 or if no metabolites were reported with standard deviation (SD) < 30%. The cystic fraction of each voxel was estimated by thresholding T₂‐weighted images, and this was used both to correct the reported metabolite concentrations and to calculate an expected SNR of choline using the measured SNR of water. Choline was detected in 10/12 primary tumors and 5/11 metastatic lesions (range 2.0–16.6 mM). Of the 8/23 failures, 7 had a predicted choline SNR < 2, confirming that the failure to detect choline could be explained by technical problems. Glycine was observed in one benign lesion. ¹H‐MRS can be used to quantify choline in primary and metastatic masses in ovarian cancer, but the moderately high rate of failure to detect choline necessitates careful recording of data quality parameters to discriminate true from false negatives. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Measurement of transverse relaxation times of J‐coupled metabolites in the human visual cortex at 4 T

Accurate quantification of ¹H NMR spectra often requires knowledge of the relaxation times to correct for signal losses due to relaxation and saturation. In human brain, T₂ values for singlets such as N‐acetylaspartate, creatine, and choline have been reported, but few T₂ values are available for J‐coupled spin systems. The purpose of this study was to measure the T₂ relaxation times of J‐coupled metabolites in the human occipital lobe using the LASER sequence. Spectra were acquired at multiple echo times and were analyzed with an LCModel using basis sets simulated at each echo time. Separate basis spectra were used for resonances of protons belonging to the same molecule but having very different T₂ values (e.g., two separate basis spectra were used for the singlet and multiplet signal in N‐acetylaspartate). The T₂ values for the N‐acetylaspartate multiplet (149 ± 12 ms), glutamate (125 ± 10 ms), myo‐inositol (139 ± 20 ms), and taurine (196 ± 28 ms) were successfully measured in the human visual cortex at 4 T. These measured T₂ relaxation times have enabled the accurate and absolute quantification of cerebral metabolites at longer echo times. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Evaluation of cerebral gray and white matter metabolite differences by spectroscopic imaging at 4.1T

Using a 4.1 T whole body system, we have acquired ¹H spectroscopic imaging (SI) data of N-acetyl (NA) compounds, creatine (CR), and choline (CH) with nominal voxel sizes of 0.5 cc (1.15 cc after filtering). We have used the SI data to estimate differences in cerebral metabolites of human gray and white matter. To evaluate the origin of an increased CWNA and CWNA ratios in gray matter relative to white matter, we measured the T₁ and T₂ of CR, NA, and CH in gray and white matter using moderate resolution SI imaging. In white matter the T₂s of NA, CR, and CH were 233 ± 27,141 ± 18, and 167 ± 20 ms, respectively, and 227 ± 27,140 ± 16, and 189 ± 25 ms in gray matter. The T, values for NA, CR, and CH were 1267 ±141, 1487 ± 146, and 1111 ± 136 ms in gray matter and 1260 ± 154, 1429 & 233, and 1074 ± 146 ms in white matter. After correcting for T₁ and T₂ losses, creatine content was significantly lower in white matter than gray (P < e 0.01, t-test), with a white/gray content ratio of 0.8, in agreement with biopsy and in vivo measurements at 1.5 and 2.0T.     

Phosphorus‐31 magnetic resonance spectroscopy of skeletal muscle in maternally inherited diabetes and deafness A3243G mitochondrial mutation carriers

Purpose

To investigate high‐energy phosphate metabolism in striated skeletal muscle of patients with Maternally Inherited Diabetes and Deafness (MIDD) syndrome.

Materials and Methods

In 11 patients with the MIDD mutation (six with diabetes mellitus [DM] and five non‐DM) and eight healthy subjects, phosphocreatine (PCr) and inorganic phosphate (Pi) in the vastus medialis muscle was measured immediately after exercise using ³¹P‐magnetic resonance spectroscopy (MRS). The half‐time of recovery (t1/2) of monoexponentially fitted (PCr+Pi)/PCr was calculated from spectra obtained every 4 seconds after cessation of exercise. A multiple linear regression model was used for statistical analysis.

Results

Patients with the MIDD mutation showed a significantly prolonged t1/2 (PCr+Pi)/PCr after exercise as compared to controls (13.6±3.0 vs. 8.7±1.3 sec, P = 0.01). No association between the presence of DM and t1/2 (PCr + Pi)/PCr was found (P = 0.382).

Conclusion

MIDD patients showed impaired mitochondrial oxidative phosphorylation in skeletal muscle shortly after exercise, irrespective of the presence of DM. J. Magn. Reson. Imaging 2009;29:127–131. © 2008 Wiley‐Liss, Inc.     

Myocardial fat quantification in humans: Evaluation by two‐point water‐fat imaging and localized proton spectroscopy

Proton MR spectroscopy (¹H‐MRS) has been used for in vivo quantification of intracellular triglycerides within the sarcolemma. The purpose of this study was to assess whether breath‐hold dual‐echo in‐ and out‐of‐phase MRI at 3.0 T can quantify the fat content of the myocardium. Biases, including T₁, T*₂, and noise, that confound the calculation of the fat fraction were carefully corrected. Thirty‐four of 46 participants had both MRI and MRS data. The fat fractions from MRI showed a strong correlation with fat fractions from MRS (r = 0.78; P < 0.05). The mean myocardial fat fraction for all 34 subjects was 0.7 ± 0.5% (range: 0.11–3%) assessed with MRS and 1.04 ± 0.4% (range: 0.32–2.44%) assessed with in‐ and out‐of‐phase MRI (P < 0.05). Scanning times were less than 15 sec for Dixon imaging, plus an additional minute for the acquisition used for T*₂ calculation, and 15‐20 min for MRS. The average postprocessing time for MRS was 3 min and 5 min for MRI including T*₂ measurement. We conclude that the dual echo method provides a rapid means to detect and quantifying myocardial fat content in vivo. Correction/adjustment for field inhomogeneity using three or more echoes seems crucial for the dual echo approach. Magn Reson Med 63:892–901, 2010. © 2010 Wiley‐Liss, Inc.     

Changes of relaxation times (T1, T2) and apparent diffusion coefficient after permanent middle cerebral artery occlusion in the rat: temporal evolution,regional extent,and comparison with histology

The quantitative NMR parameters T₁, T₂, p, and apparent diffusion coefficient (ADC) were determined during the 7 h after middle cerebral artery occlusion in rats. In the normal caudate-putamen (CP), 869 ± 145 ms and 72 ± 2ms for T₁ and for T₂, respectively, were found; the corresponding values for cortex were 928 ± 117 ms and 73 ± 2 ms. The ADC showed significant dependence on gradient direction: diffusion along x resulted in 534 ± 53 μm²/s (CP) and 554 ± 62 μm²/s (cortex), and along y in 697 ± 58 μm²/s (CP) and 675 ± 53 μm²/s (cortex). In the ischemic territory, a continuous increase over time of both relaxation times was observed in the CP, leading to an increase of 29 ± 20% (T₁) and 51 ± 41% (T₂ above control level. ADC dropped to 63 ± 15% of control in the CP and to 74 ± 4% of control in the temporal cortex. No significant change was noted in proton density during the observation period. Strongest ADC reduction was in the center of the ischemic territory (≤ 60% of control) surrounded by a region of lesser reduction (≤ 80% of control). During the early part of the study, the area of reduced ADC was larger than that of elevated relaxation times. Toward the end of the experiment, the area of increased relaxation times approached that of decreased ADC at ≤ 80% of control. Good agreement of histological presentation of infarct with the total area of decreased ADC (≤ 80%) was demonstrated.     

Anomalous Transverse Relaxation in 1H Spectroscopy in Human Brain at 4 Tesla

Longitudinal (T₁) and apparent transverse relaxation times (T₂) of choline-containing compounds (Cho), creatine/phospho-creatine (Cr/PCr), and N-acetyl aspartate (NAA) were measured in vivo in human brain at 4 Tesla. Measurements were performed using a water suppressed stimulated echo pulse sequence with complete outside volume presaturation to improve volume localization at short echo times. T₁-values of Cho (1.2 ± 0.1 s), Cr (1.6 ± 0.3 s), and NAA (1.6 ± 0.2 s) at 4 Tesla in occipital brain were only slightly larger than those reported in the literature at 1.5 Tesla. Thus, TR will not adversely affect the expected enhancement of signal-to-noise at 4 Tesla. Surprisingly, apparent T₂-values of Cho (142 ± 34 ms), Cr (140 ± 13 ms), and NAA (185 ± 24 ms) at 4 Tesla were significantly smaller than those at 1.5 Tesla and further decreased when increasing the mixing interval TM. Potential contributing factors, such as diffusion in local susceptibility related gradients, dipolar relaxation due to intracellular paramagnetic substances and motion effects are discussed. The results suggest that short echo time spectroscopy is advantageous to maintain signal to noise at 4 Tesla.     

Altered energy metabolism after myocardial infarction assessed by 31P-MR-spectroscopy in humans

The value of ³¹P-magnetic resonance spectroscopy (MRS) as a possible tool to distinguish viable from non-viable tissue after myocardial infarction was analysed in humans. Fifteen patients 3 weeks after anterior myocardial infarction were studied with breath-hold cine MRI and 3D-CSI MRS (1.5 T system). ³¹P-spectra were obtained from infarcted as well as non-infarcted myocardium (voxel size 25 cm³ each). Gold standard for viability was recovery of regional function, as determined by a control MRI 6 months after revascularization. Ten age-matched healthy volunteers served as control group. No significant difference was found between the phosphocreatine to adenosinetriphosphate (PCr/ATP) ratio of volunteers (SD 1.72 ± 0.31) and non-infarcted septal myocardium of patients. Cine MRI demonstrated recovery of regional function in 10 patients, i. e. 10 patients showed viable and 5 non-viable myocardium. In viable myocardium, the PCr/ATP ratio was 1.47 ± 0.38 (non-significant vs volunteers; p > 0.05). In the 5 patients with akinetic myocardium, PCr peaks could not be detected. Therefore, calculation of PCr/ATP ratios was not possible. However, a significant reduction of the ATP signal-to-noise ratio (SNR) was observed (2.92 ± 0.73 vs 6.68 ± 0.80; patients vs volunteers; p <0.05). The SNR of ATP of akinetic regions may predict recovery of function after revascularization in patients with myocardial infarction. Received: 9 September 1999; Revised: 30 November 1999; Accepted: 24 December 1999     

Prostate T(1) quantification using a magnetization-prepared spiral technique

Purpose

To adapt a magnetization‐prepared spiral imaging technique, termed T1prep, for time‐efficient radiofrequency (RF)‐insensitive prostate T₁ quantification at 1.5 T and evaluate signal‐to‐noise ratio (SNR) limits to voxel‐based versus subregion analysis.

Materials and Methods

A magnetization‐prepared spiral imaging technique was adapted for robust T₁ contrast development, multislice imaging within 5 minutes, and data regression to a monoexponential decay. In vitro testing evaluated RF insensitivity of the multislice acquisition plus method accuracy. A pilot study was performed in 15 patients with low or intermediate risk localized prostate cancer.

Results

The multislice design displayed excellent RF insensitivity (<1% error for RF mistunings to ± 20%) and accuracy (within 3% of gold standard for T₁ values between 140 and 2100 msec). A clinical pilot study reported significantly reduced T₁ from PZ to CG to tumor subregions (PZ: 1421 ± 168 msec, n = 11; CG: 1314 ± 49 msec, n = 13; 1246 ± 68 msec, n = 8). SNR measurements identified an inappropriateness of voxel‐based analysis.

Conclusion

T1prep can quantify prostate T₁ as an adjunct measure for quantitative perfusion measurements and longitudinal treatment response monitoring. Intrapatient heterogeneities support T₁ assessment within individual patients. SNR calculations will support a transition to voxel‐based analysis in future trials. J. Magn. Reson. Imaging 2011. © 2011 Wiley‐Liss, Inc.     

Pliosphocreatine T1 measurements with and without exchange in the heart

The intrinsic phosphocreatine (PCr) T₁ values measured by time-dependent magnetization transfer in isolated perfused rat, hamster, and turkey hearts were indistinguishable. The value of 3.5 ± 0.3 s for the rat heart is similar to values measured by other magnetization transfer methods. Irreversibly inhibiting the phosphoryl exchange between PCr and ATP in the rat heart using iodoacetamide changed the apparent T₁ values of the two exchanging species when measured by inversion recovery: The apparent T₁ of PCr increased from 1.92 ± 0.06 s to 3.55 ± 0.06 s, in excellent agreement with the intrinsic T₁, measured by magnetization transfer. The apparent T₁ of [γ-P]ATP decreased from 0.92 ± 0.07 s to 0.44 ± 0.03 s. The value for the T₁ of [γ-P]ATP in hearts with inhibited phosphoryl exchange was similar to T₁ values for [α-P]ATP and [β-P]ATP, which remained unchanged. This illustrates that apparent T₁ values for PCr and [γ-P]ATP measured by inversion recovery in the presence of exchange are average T₁ values in between the intrinsic values. The large differences between the intrinsic T₁ measured by magnetization transfer and the T₁ measured by inversion recovery makes the use of the appropriate value in different applications quantitatively important.