On-resonance and off-resonance continuous wave constant amplitude spin-lock and T1ρ quantification in the presence of B1 and B0 inhomogeneities

Predictive markers for head and neck cancer treatment response: T1rho imaging in nasopharyngeal carcinoma

OBJECTIVES

To investigate the potential of T1rho, a new quantitative imaging sequence for cancer, for pre and early intra-treatment prediction of treatment response in nasopharyngeal carcinoma (NPC) and compare the results with those of diffusion-weighted imaging (DWI).

MATERIALS AND METHODS

T1rho and DWI imaging of primary NPCs were performed pre- and early intra-treatment in 41 prospectively recruited patients. The mean preT1rho, preADC, intraT1rho, intraADC, and % changes in T1rho (ΔT1rho%) and ADC (ΔADC%) were compared between residual and non-residual groups based on biopsy in all patients after chemoradiotherapy (CRT) with (n = 29) or without (n = 12) induction chemotherapy (IC), and between responders and non-responders to IC in the subgroup who received IC, using Mann-Whitney U-test. A p-value of < 0.05 indicated statistical significance.

RESULTS

Significant early intra-treatment changes in mean T1rho (p = 0.049) and mean ADC (p < 0.01) were detected (using paired t-test), most showing a decrease in T1rho (63.4%) and an increase in ADC (95.1%). Responders to IC (n = 17), compared to non-responders (n = 12), showed higher preT1rho (64.0 ms vs 66.5 ms) and a greater decrease in ΔT1rho% (- 7.5% vs 1.3%) (p < 0.05). The non-residual group after CRT (n = 35), compared to the residual group (n = 6), showed higher intraADC (0.96 vs 1.09 × 10-3 mm2/s) and greater increase in ΔADC% (11.7% vs 27.0%) (p = 0.02).

CONCLUSION

Early intra-treatment changes are detectable on T1rho and show potential to predict tumour shrinkage after IC. T1rho may be complementary to DWI, which, unlike T1rho, did not predict response to IC but did predict non-residual disease after CRT.

CLINICAL RELEVANCE STATEMENT

T1rho has the potential to complement DWI in the prediction of treatment response. Unlike DWI, it predicted shrinkage of the primary NPC after IC but not residual disease after CRT.

KEY POINTS

Changes in T1rho were detected early during cancer treatment for NPC. Pre-treatment and early intra-treatment change in T1rho predicted response to IC, but not residual disease after CRT. T1rho can be used to complement DWI with DWI predicting residual disease after CRT.

Correction of errors in estimates of T1ρ at low spin-lock amplitudes in the presence of B0 and B1 inhomogeneities

Relaxation rates R1ρ in the rotating frame measured by spin-lock methods at very low locking amplitudes (≤ 100 Hz) are sensitive to the effects of water diffusion in intrinsic gradients and may provide information on tissue microvasculature, but accurate estimates are challenging in the presence of B0 and B1 inhomogeneities. Although composite pulse preparations have been developed to compensate for nonuniform fields, the transverse magnetization comprises different components and the spin-lock signals measured do not decay exponentially as a function of locking interval at low locking amplitudes. For example, during a typical preparation sequence, some of the magnetization in the transverse plane is nutated to the Z-axis and later tipped back, and so does not experience R1ρ relaxation. As a result, if the spin-lock signals are fit to a monoexponential decay with locking interval, there are residual errors in quantitative estimates of relaxation rates R1ρ and their dispersion with weak locking fields. We developed an approximate theoretical analysis to model the behaviors of the different components of the magnetization, which provides a means to correct these errors. The performance of this correction approach was evaluated both through numerical simulations and on human brain images at 3 T, and compared with a previous correction method using matrix multiplication. Our correction approach has better performance than the previous method at low locking amplitudes. Through careful shimming, the correction approach can be applied in studies using low spin-lock amplitudes to assess the contribution of diffusion to R1ρ dispersion and to derive estimates of microvascular sizes and spacings. The results of imaging eight healthy subjects suggest that R1ρ dispersion in human brain at low locking fields arises from diffusion among inhomogeneities that generate intrinsic gradients on a scale of capillaries (~7.4 ± 0.5 μm).

New methods for robust continuous wave T1ρ relaxation preparation

Measurement of the longitudinal relaxation time in the rotating frame of reference (T_1ρ ) is sensitive to the fidelity of the main imaging magnetic field (B₀ ) and that of the RF pulse (B₁ ). The purpose of this study was to introduce methods for producing continuous wave (CW) T_1ρ contrast with improved robustness against field inhomogeneities and to compare the sensitivities of several existing and the novel T_1ρ contrast generation methods with the B₀ and B₁ field inhomogeneities. Four hard-pulse and four adiabatic CW-T_1ρ magnetization preparations were investigated. Bloch simulations and experimental measurements at different spin-lock amplitudes under ideal and non-ideal conditions, as well as theoretical analysis of the hard-pulse preparations, were conducted to assess the sensitivity of the methods to field inhomogeneities, at low (ω₁  << ΔB₀ ) and high (ω₁ >> ΔB₀ ) spin-locking field strengths. In simulations, previously reported single-refocus and new triple-refocus hard-pulse and double-refocus adiabatic preparation schemes were found to be the most robust. The mean normalized absolute deviation between the experimentally measured relaxation times under ideal and non-ideal conditions was found to be smallest for the refocused preparation schemes and broadly in agreement with the sensitivities observed in simulations. Experimentally, all refocused preparations performed better than those that were non-refocused. The findings promote the use of the previously reported hard-pulse single-refocus ΔB₀ and B₁ insensitive T_1ρ as a robust method with minimal RF energy deposition. The double-refocus adiabatic B₁ insensitive rotation-4 CW-T_1ρ preparation offers further improved insensitivity to field variations, but because of the extra RF deposition, may be preferred for ex vivo applications.

A robust adiabatic constant amplitude spin-lock preparation module for myocardial T1ρ quantification at 3 T

T_1ρ quantification has the potential to assess myocardial fibrosis without contrast agent. However, its preparation spin-lock pulse is sensitive to B₁ and B₀ inhomogeneities, resulting in severe banding artifacts in the heart region, especially at high magnetic field such as 3 T. We aimed to design a robust spin-lock (SL) preparation module that can be used in myocardial T_1ρ quantification at 3 T. We used the tan/tanh pulse to tip up and tip down the magnetization in the spin-lock preparation module (tan/tanh-SL). Bloch simulation was used to optimize pulse shape parameters of the tan/tanh with a pulse duration (T_p ) of 8, 4, and 2 ms, respectively. The designed tan/tanh-SL modules were implemented on a 3-T MR scanner. They were evaluated in phantom studies under three different cases of B₀ and B₁ inhomogeneities, and tested in cardiac T_1ρ quantification of healthy volunteers. The performance of the tan/tanh-SL was compared with the composite SL preparation pulses and the commonly used hyperbolic secant pulse for spin-lock (HS-SL). Feasible pulse shape parameters were obtained using Bloch simulation. Compared with HS-SL, the quantification error of tan/tanh-SL was reduced by 27.7% for T_p  = 8 ms (mean ∆Q = 126.15 vs. 174.42) and 75.6% for T_p  = 4 ms (mean ∆Q = 136.65 vs. 559.53). In the phantom study, tan/tanh-SL was less sensitive to B₁ and B₀ inhomogeneity compared with composite SL pulses and HS-SL. In cardiac T_1ρ quantification, the T_1ρ maps using tan/tanh-SL showed fewer banding artifacts than using composite SL pulses and HS-SL. The proposed tan/tanh-SL preparation module greatly improves the robustness to B₀ and B₁ field inhomogeneities and can be used in cardiac T_1ρ quantification at 3 T.

Uncertainty-aware self-supervised neural network for liverT1ρmapping with relaxation constraint

Objective.T1ρmapping is a promising quantitative MRI technique for the non-invasive assessment of tissue properties. Learning-based approaches can mapT1ρfrom a reduced number ofT1ρweighted images but requires significant amounts of high-quality training data. Moreover, existing methods do not provide the confidence level of theT1ρestimation. We aim to develop a learning-based liverT1ρmapping approach that can mapT1ρwith a reduced number of images and provide uncertainty estimation.Approach. We proposed a self-supervised neural network that learns aT1ρmapping using the relaxation constraint in the learning process. Epistemic uncertainty and aleatoric uncertainty are modelled for theT1ρquantification network to provide a Bayesian confidence estimation of theT1ρmapping. The uncertainty estimation can also regularize the model to prevent it from learning imperfect data. Main results. We conducted experiments onT1ρdata collected from 52 patients with non-alcoholic fatty liver disease. The results showed that when only collecting twoT1ρ-weighted images, our method outperformed the existing methods forT1ρquantification of the liver. Our uncertainty estimation can further regularize the model to improve the performance of the model and it is consistent with the confidence level of liverT1ρvalues.Significance. Our method demonstrates the potential for accelerating theT1ρmapping of the liver by using a reduced number of images. It simultaneously provides uncertainty ofT1ρquantification which is desirable in clinical applications.

Accelerated 3D free-breathing high-resolution myocardial T1ρ mapping at 3 Tesla

PURPOSE

To develop a fast free-breathing whole-heart high-resolution myocardial T_1ρ mapping technique with robust spin-lock preparation that can be performed at 3 Tesla.

METHODS

An adiabatically excited continuous-wave spin-lock module, insensitive to field inhomogeneities, was implemented with an electrocardiogram-triggered low-flip angle spoiled gradient echo sequence with variable-density 3D Cartesian undersampling at a 3 Tesla whole-body scanner. A saturation pulse was performed at the beginning of each cardiac cycle to null the magnetization before T_1ρ preparation. Multiple T_1ρ -weighted images were acquired with T_1ρ preparations with different spin-lock times in an interleaved fashion. Respiratory self-gating approach was adopted along with localized autofocus to enable 3D translational motion correction of the data acquired in each heartbeat. After motion correction, multi-contrast locally low-rank reconstruction was performed to reduce undersampling artifacts. The accuracy and feasibility of the 3D T_1ρ mapping technique was investigated in phantoms and in vivo in 10 healthy subjects compared with the 2D T_1ρ mapping.

RESULTS

The 3D T_1ρ mapping technique provided similar phantom T_1ρ measurements in the range of 25-120 ms to the 2D T_1ρ mapping reference over a wide range of simulated heart rates. With the robust adiabatically excited continuous-wave spin-lock preparation, good quality 2D and 3D in vivo T_1ρ -weighted images and T_1ρ maps were obtained. Myocardial T_1ρ values with the 3D T_1ρ mapping were slightly longer than 2D breath-hold measurements (septal T_1ρ : 52.7 ± 1.4 ms vs. 50.2 ± 1.8 ms, P < 0.01).

CONCLUSION

A fast 3D free-breathing whole-heart T_1ρ mapping technique was proposed for T_1ρ quantification at 3 T with isotropic spatial resolution (2 mm³ ) and short scan time of ∼4.5 min.

Characterization and correction of the effects of hepatic iron on T1ρ relaxation in the liver at 3.0T

PURPOSE

Quantitative T_1ρ imaging is an emerging technique to assess the biochemical properties of tissues. In this paper, we report our observation that liver iron content (LIC) affects T_1ρ quantification of the liver at 3.0T field strength and develop a method to correct the effect of LIC.

THEORY AND METHODS

On-resonance R_1ρ (1/T_1ρ ) is mainly affected by the intrinsic R₂ (1/T₂ ), which is influenced by LIC. As on-resonance R_1ρ is closely related to the Carr-Purcell-Meiboom-Gill (CPMG) R₂ , and because the calibration between CPMG R₂ and LIC has been reported at 1.5T, a correction method was proposed to correct the R₂ contribution to the R_1ρ . The correction coefficient was obtained from the calibration results and related transformed factors. To compensate for the difference between CPMG R₂ and R_1ρ , a scaling factor was determined using the values of CPMG R₂ and R_1ρ , obtained simultaneously from a single breath-hold from volunteers. The livers of 110 subjects were scanned to validate the correction method.

RESULTS

LIC was significantly correlated with R_1ρ in the liver. However, when the proposed correction method was applied to R_1ρ , LIC and the iron-corrected R_1ρ were not significantly correlated.

CONCLUSION

LIC can affect T_1ρ in the liver. We developed an iron-correction method for the quantification of T_1ρ in the liver at 3.0T.

Quantification correction for free-breathing myocardial T1ρ mapping in mice using a recursively derived description of a T1ρ* relaxation pathway

BACKGROUND

Fast and accurate T1ρ mapping in myocardium is still a major challenge, particularly in small animal models. The complex sequence design owing to electrocardiogram and respiratory gating leads to quantification errors in in vivo experiments, due to variations of the T1ρ relaxation pathway. In this study, we present an improved quantification method for T1ρ using a newly derived formalism of a T1ρ* relaxation pathway.

METHODS

The new signal equation was derived by solving a recursion problem for spin-lock prepared fast gradient echo readouts. Based on Bloch simulations, we compared quantification errors using the common monoexponential model and our corrected model. The method was validated in phantom experiments and tested in vivo for myocardial T1ρ mapping in mice. Here, the impact of the breath dependent spin recovery time Trec on the quantification results was examined in detail.

RESULTS

Simulations indicate that a correction is necessary, since systematically underestimated values are measured under in vivo conditions. In the phantom study, the mean quantification error could be reduced from - 7.4% to - 0.97%. In vivo, a correlation of uncorrected T1ρ with the respiratory cycle was observed. Using the newly derived correction method, this correlation was significantly reduced from r = 0.708 (p < 0.001) to r = 0.204 and the standard deviation of left ventricular T1ρ values in different animals was reduced by at least 39%.

CONCLUSION

The suggested quantification formalism enables fast and precise myocardial T1ρ quantification for small animals during free breathing and can improve the comparability of study results. Our new technique offers a reasonable tool for assessing myocardial diseases, since pathologies that cause a change in heart or breathing rates do not lead to systematic misinterpretations. Besides, the derived signal equation can be used for sequence optimization or for subsequent correction of prior study results.

Simultaneous T1 , T2 , and T1ρ relaxation mapping of the lower leg muscle with MR fingerprinting

PURPOSE

To develop a novel MR-fingerprinting (MRF) pulse sequence that is insensitive to B 1 + and B₀ imperfections for simultaneous T₁ , T₂ , and T_1ρ relaxation mapping.

METHODS

We implemented a totally balanced spin-lock (TB-SL) module to encode T_1ρ relaxation into an existing MRF framework that encoded T₁ and T₂ . The spin-lock module used two 180° pulses with compensatory phases to reduce T_1ρ sensitivity to B₁ and B₀ inhomogeneities. We compared T_1ρ measured using TB-SL MRF in Bloch simulations, model agar phantoms, and in vivo experiments to those with a self-compensated spin-lock preparation module (SC-SL). The TB-SL MRF repeatability was evaluated in maps acquired in the lower leg skeletal muscle of 12 diabetic peripheral neuropathy patients, scanned two times each during visits separated by about 30 days.

RESULTS

The phantom relaxation times measured with TB-SL and SC-SL MRF were in good agreement with reference values in regions with low B₁ inhomogeneities. Compared with SC-SL, TB-SL MRF showed in experiments greater robustness against severe B₁ inhomogeneities and in Bloch simulations greater robustness against B₁ and B₀ . We measured with TB-SL MRF an average T₁ = 950.1 ± 28.7 ms, T₂ = 26.0 ± 1.2 ms, and T_1ρ = 31.7 ± 3.2 ms in skeletal muscle across patients. Bland-Altman analysis demonstrated low bias between TB-SL and SC-SL MRF and between TB-SL MRF maps acquired in two visits. The coefficient of variation was less than 3% for all measurements.

CONCLUSION

The proposed TB-SL MRF sequence is fast and insensitive to B 1 + and B₀ imperfections. It can simultaneously map T₁ , T₂ , T_1ρ , and B 1 + in a single scan and can potentially be used to study muscle composition.

Test-retest repeatability of T1rho (T1ρ) MR imaging in the head and neck

PURPOSE

T1rho imaging is a new quantitative MRI sequence for head and neck cancer and the repeatability for this region is unknown. This study aimed to evaluate the repeatability of quantitative T1rho imaging in the head and neck.

MATERIALS AND METHODS

T1rho imaging of the head and neck was prospectively performed in 15 healthy participants on three occasions. Scan 1 and 2 were performed with a time interval of 30 minutes (intra-session) and scan 3 was performed 14 days later (inter-session). T1rho values for normal tissues (parotid glands, palatine tonsils, pterygoid muscles, and tongue) were obtained on each scan. Intra-class coefficients (ICCs), within-subject coefficient of variances (wCoVs), and repeatability coefficient (RCs) of the intra-session scan (scan 1 vs 2) and inter-session scan (scan 1 vs 3) for the normal tissues were calculated.

RESULTS

The ICCs of T1rho values for normal tissues were almost perfect (0.83-0.97) for intra-session scans and were substantial (0.71-0.80) for inter-session scans. The wCoVs showed a small range (2.46%-3.30%) for intra-session scans, and slightly greater range (3.27%-6.51%) for inter-session scan. The greatest and lowest wCoVs of T1rho were found in the parotid gland and muscles, respectively. The T1rho RCs varied for all tissues between intra- and inter- sessions, and the greatest RC of 10.07 msec was observed for parotid gland on inter-session scan.

CONCLUSION

T1rho imaging is a repeatable quantitative MRI sequence in the head and neck but variances of T1rho values among tissues should be take into account during analysis.

Balanced spin-lock preparation for B1 -insensitive and B0 -insensitive quantification of the rotating frame relaxation time T1ρ

PURPOSE

Accurate and artifact-free T_1ρ quantification is still a major challenge due to a susceptibility of the spin-locking module to B₀ and/or B₁ field inhomogeneities. In this study, we present a novel spin-lock preparation module (B-SL) that enables an almost full compensation of both types of inhomogeneities.

METHODS

The new B-SL module contains a second 180° refocusing pulse to compensate each pulse in the preparation block by a corresponding pulse with opposite phase. For evaluation and validation of B-SL, extensive simulations as well as phantom measurements were performed. Furthermore, the new module was compared to three common established compensation methods.

RESULTS

Both simulations and measurements demonstrate a much lower susceptibility to artifacts for the B-SL module, therefore providing an improved accuracy in T_1ρ quantification. In the presence of field inhomogeneities, measurements revealed an increased banding compensation by 79% compared with the frequently used composite module. The goodness of the mono-exponential T_1ρ fitting procedure was improved by 58%.

CONCLUSION

The B-SL preparation enables the generation of accurate relaxation maps with significantly reduced artifacts, even in the case of large field imperfections. Therefore, the B-SL module is suggested to be highly beneficial for in vivo T_1ρ quantification.

Quantitative T1ρ MRI of the Head and Neck Discriminates Carcinoma and Benign Hyperplasia in the Nasopharynx

BACKGROUND AND PURPOSE

T1ρ imaging is a new quantitative MR imaging pulse sequence with the potential to discriminate between malignant and benign tissue. In this study, we evaluated the capability of T1ρ imaging to characterize tissue by applying T1ρ imaging to malignant and benign tissue in the nasopharynx and to normal tissue in the head and neck.

MATERIALS AND METHODS

Participants with undifferentiated nasopharyngeal carcinoma and benign hyperplasia of the nasopharynx prospectively underwent T1ρ imaging. T1ρ measurements obtained from the histogram analysis for nasopharyngeal carcinoma in 43 participants were compared with those for benign hyperplasia and for normal tissue (brain, muscle, and parotid glands) in 41 participants using the Mann-Whitney U test. The area under the curve of significant T1ρ measurements was calculated and compared using receiver operating characteristic analysis and the Delong test, respectively. A P < . 05 indicated statistical significance.

RESULTS

There were significant differences in T1ρ measurements between nasopharyngeal carcinoma and benign hyperplasia and between nasopharyngeal carcinoma and normal tissue (all, P < . 05). Compared with benign hyperplasia, nasopharyngeal carcinoma showed a lower T1ρ mean (62.14 versus 65.45 × ms), SD (12.60 versus 17.73 × ms), and skewness (0.61 versus 0.76) (all P < .05), but no difference in kurtosis (P = . 18). The T1ρ SD showed the highest area under the curve of 0.95 compared with the T1ρ mean (area under the curve  = 0.72) and T1ρ skewness (area under the curve  = 0.72) for discriminating nasopharyngeal carcinoma and benign hyperplasia (all, P < .05).

CONCLUSIONS

Quantitative T1ρ imaging has the potential to discriminate malignant from benign and normal tissue in the head and neck.

Macromolecular proton fraction mapping based on spin-lock magnetic resonance imaging

PURPOSE

In MRI, the macromolecular proton fraction (MPF) is a key parameter of magnetization transfer (MT). It represents the relative amount of immobile protons associated with semi-solid macromolecules involved in MT with free water protons. We aim to quantify MPF based on spin-lock MRI and explore its advantages over the existing MPF-mapping methods.

METHODS

In the proposed method, termed MPF quantification based on spin-lock (MPF-SL), off-resonance spin-lock is used to sensitively measure the MT effect. MPF-SL is designed to measure a relaxation rate (R_mpfsl ) that is specific to the MT effect by removing the R_1ρ relaxation due to the mobile water and chemical exchange pools. A theory is derived to quantify MPF from the measured R_mpfsl . No prior knowledge of tissue relaxation parameters, including T₁ or T₂ , is needed to quantify MPF using MPF-SL. The proposed approach is validated with Bloch-McConnell simulations, phantom, and in vivo liver studies at 3.0T.

RESULTS

Both Bloch-McConnell simulations and phantom experiments show that MPF-SL is insensitive to variations of the mobile water pool and the chemical exchange pool. MPF-SL is specific to the MT effect and can measure MPF reliably. In vivo liver studies show that MPF-SL can be used to detect collagen deposition in patients with liver fibrosis.

CONCLUSION

A novel MPF imaging method based on spin-lock MRI is proposed. The confounding factors are removed, and the measurement is specific to the MT effect. It holds promise for MPF-sensitive diagnostic imaging in clinical settings.

Chemical-shift encoding-based water-fat separation with multifrequency fat spectrum modeling in spin-lock MRI

PURPOSE

Chemical exchange saturation transfer is used commonly to generate MRI contrast based on the chemical exchange effect. The spin-lock techniques can also be used to probe the chemical exchange and other molecular motion processes in tissues. The presence of fat can cause errors in spin-lock MRI. Signals from fat are typically suppressed based on spectral selectivity or T₁ nulling approaches in spin-lock imaging. However, these methods cannot be used to suppress fat signals from multiple fat peaks. To address this problem, we report chemical-shift encoding-based water-fat separation approaches with multifrequency fat spectrum modeling.

METHODS

Both the conventional spin-lock and the adiabatic continuous-wave constant-amplitude spin lock (ACCSL) with multi-echo acquisitions are investigated for chemical-shift encoding-based water-fat separation in spin-lock imaging. A comparison is made of reconstructions based on 3 models: a single-peak fat spectrum model, a standard precalibrated proton density 6-peak fat spectrum model, and the self-calibrated relaxation-dependent 3-peak fat spectrum model. Comparisons were performed using Bloch simulations, phantom, and in vivo experiments at 3 T.

RESULTS

Conventional spin-lock acquisitions cannot be used for reliable water-fat separation with a multipeak fat spectrum model. Water-fat separation based on ACCSL acquisitions achieves superior performance compared with the use of conventional spin-lock acquisitions. The best result is achieved from ACCSL acquisition with self-calibrated relaxation-dependent multipeak fat spectrum modeling.

CONCLUSION

The ACCSL acquisition can be used for chemical-shift encoding-based water-fat separation with multipeak fat spectrum modeling. This approach has the potential to improve quantitative analysis using spin-lock MRI for assessing the biochemical properties of tissues.

Probing chemical exchange using quantitative spin-lock R1ρ asymmetry imaging with adiabatic RF pulses

PURPOSE

CEST is commonly used to probe the effects of chemical exchange. Although R_1ρ asymmetry quantification has also been described as a promising option for detecting the effects of chemical exchanges, the existing acquisition approaches are highly susceptible to B₁ RF and B₀ field inhomogeneities. To address this problem, we report a new R_1ρ asymmetry imaging approach, AC-iTIP, which is based on the previously reported techniques of irradiation with toggling inversion preparation (iTIP) and adiabatic continuous wave constant amplitude spin-lock RF pulses (ACCSL). We also derived the optimal spin-lock RF pulse B₁ amplitude that yielded the greatest R_1ρ asymmetry.

METHODS

Bloch-McConnell simulations were used to verify the analytical formula derived for the optimal spin-lock RF pulse B₁ amplitude. The performance of the AC-iTIP approach was compared to that of the iTIP approach based on hard RF pulses and the R_1ρ -spectrum acquired using adiabatic RF pulses with the conventional fitting method. Comparisons were performed using Bloch-McConnell simulations, phantom, and in vivo experiments at 3.0T.

RESULTS

The analytical prediction of the optimal B₁ was validated. Compared to the other 2 approaches, the AC-iTIP approach was more robust under the influences of B₁ RF and B₀ field inhomogeneities. A linear relationship was observed between the measured R_1ρ asymmetry and the metabolite concentration.

CONCLUSION

The AC-iTIP approach could probe the chemical exchange effect more robustly than the existing R_1ρ asymmetry acquisition approaches. Therefore, AC-iTIP is a promising technique for metabolite imaging based on the chemical exchange effect.