In vivo multiecho T2 relaxation measurements using variable TR to decrease scan time

Multiecho T2 relaxation measurements to determine geometric mean T2 (GMT2) and myelin water fraction (MWF) are lengthy, resulting in increased motion artefacts from patient discomfort and reduced patient compliance. The goal of this study was to shorten the acquisition time for multiecho T2 measurements without affecting T1 weighting by varying TR across k-space. Six phantoms and 10 healthy volunteers were imaged with both a constant TR and a variable TR multiecho T2 sequence. T1 weighting was determined by TR at the center of k-space; for variable TR measurement, TR was shortened linearly from the center to the edges of k-space. Phantoms showed excellent agreement for proton density and GMT2 between constant and variable TR measurements. No significant differences were found in proton density or MWF for any of the brain structures between the two measurements. The average GMT2 over all structures between the two experiments was not significantly different. In summary, with the variable TR approach, scan time was reduced by >20%, with minimal loss of image resolution and no significant affect on proton density, MWF or GMT2.     

Quantification of superparamagnetic iron oxide using inversion recovery balanced steady-state free precession

Cellular and molecular MRI trafficking studies using superparamagnetic iron oxide (SPIO) have greatly improved non-invasive investigations of disease progression and drug efficacy, but thus far, these studies have largely been restricted to qualitative assessment of hypo- or hyperintense areas near SPIO. In this work, SPIO quantification using inversion recovery balanced steady-state free precession (IR-bSSFP) was demonstrated at 3 T by extracting R₂ values from a monoexponential model (P. Schmitt et al., 2004). A low flip angle was shown to reduce the apparent recovery rate of the IR-bSSFP time course, thus extending the dynamic range of quantification. However, low flip angle acquisitions preclude the use of traditional methods for combining RF phase-cycled images to reduce banding artifacts arising from off-resonance due to B₀ inhomogeneity. To achieve R₂ quantification of SPIO, we present a new algorithm applicable to low flip angle IR-bSSFP acquisitions that is specifically designed to identify on-resonance acquisitions. We demonstrate in this work, using both theoretical and empirical methods, that the smallest estimated R₂ from multiple RF phase-cycled acquisitions correspond well to the on-resonance time course. Using this novel minimum R₂ algorithm, homogeneous R₂ maps and linear R₂ calibration curves were created up to 100 μg(Fe)/mL with 20° flip angles, despite substantial B₀ inhomogeneity. In addition, we have shown this technique to be feasible for pre-clinical research: the minimum R₂ algorithm was resistant to off-resonance in a single slice mouse R₂ map, whereas maximum intensity projection resulted in banding artifacts and overestimated R₂ values. With the application of recent advances in accelerated acquisitions, IR-bSSFP has the potential to quantify SPIO in vivo, thus providing important information for oncology, immunology, and regenerative medicine MRI studies.     

T2* measurements in human brain at 1.5, 3 and 7 T

Measurements have been carried out in six subjects at magnetic fields of 1.5, 3 and 7 T, with the aim of characterizing the variation of T2* with field strength in human brain. Accurate measurement of T2* in the presence of macroscopic magnetic field inhomogeneity is problematic due to signal decay resulting from through-slice dephasing. The approach employed here allowed the signal decay due to through-slice dephasing to be characterized and removed from data, thus facilitating an accurate measurement of T2* even at ultrahigh field. Using double inversion recovery turbo spin-echo images for tissue classification, an analysis of T2* relaxation times in cortical grey matter and white matter was carried out, along with an evaluation of the variation of T2* with field strength in the caudate nucleus and putamen. The results show an approximately linear increase in relaxation rate R2* with field strength for all tissues, leading to a greater range of relaxation times across tissue types at 7 T that can be exploited in high-resolution T2*-weighted imaging.     

Temperature mapping in bread dough using SE and GE two-point MRI methods: experimental and theoretical estimation of uncertainty

Two-dimensional (2D)-SE, 2D-GE and tri-dimensional (3D)-GE two-point T(1)-weighted MRI methods were evaluated in this study in order to maximize the accuracy of temperature mapping of bread dough during thermal processing. Uncertainties were propagated throughout each protocol of measurement, and comparisons demonstrated that all the methods with comparable acquisition times minimized the temperature uncertainty to similar extent. The experimental uncertainties obtained with low-field MRI were also compared to the theoretical estimations. Some discrepancies were reported between experimental and theoretical values of uncertainties of temperature; however, experimental and theoretical trends with varying parameters agreed to a large extent for both SE and GE methods. The 2D-SE method was chosen for further applications on prefermented dough because of its lower sensitivity to susceptibility differences in porous media. It was applied for temperature mapping in prefermented dough during chilling prior to freezing and compared locally to optical fiber measurements.     

The flipped longitudinal polarization sequence

By flipping the longitudinal magnetization with a chain of 180° pulses it is possible to effectively restore the effects of relaxation so that the same longitudinal magnetization is periodically recovered. The pulse sequence for achieving this, called Flipped LOngitudinal Polarization (FLOP), can be incorporated into any pulse sequence whenever it is desired to stop the attenuation in longitudinal magnetization caused by relaxation. We illustrate its use for fast, single-shot measurements of the longitudinal relaxation time and for three-dimensional T₁ mapping.     

Feasibility of using limited-population-based average R10 for pharmacokinetic modeling of osteosarcoma dynamic contrast-enhanced magnetic resonance imaging data

Retrospective analyses of clinical dynamic contrast-enhanced (DCE) MRI studies may be limited by failure to measure the longitudinal relaxation rate constant (R₁) initially, which is necessary for quantitative analysis. In addition, errors in R₁ estimation in each individual experiment can cause inconsistent results in derivations of pharmacokinetic parameters, K^trans and v_e, by kinetic modeling of the DCE-MRI time course data. A total of 18 patients with lower extremity osteosarcomas underwent multislice DCE-MRI prior to surgery. For the individual R₁ measurement approach, the R₁ time course was obtained using the two-point R₁ determination method. For the average R₁₀ (precontrast R₁) approach, the R₁ time course was derived using the DCE-MRI pulse sequence signal intensity equation and the average R₁₀ value of this population. The whole tumor and histogram median K^trans (0.57±0.37 and 0.45±0.32 min⁻¹) and v_e (0.59±0.20 and 0.56±0.17) obtained with the individual R₁ measurement approach are not significantly different (paired t test) from those (K^trans: 0.61±0.46 and 0.44±0.33 min⁻¹; v_e: 0.61±0.19 and 0.55±0.14) obtained with the average R₁₀ approach. The results suggest that it is feasible, as well as practical, to use a limited-population-based average R₁₀ for pharmacokinetic modeling of osteosarcoma DCE-MRI data.     

Depth and orientational dependencies of MRI T(2) and T(1ρ) sensitivities towards trypsin degradation and Gd-DTPA(2-) presence in articular cartilage at microscopic resolution

Depth and orientational dependencies of microscopic magnetic resonance imaging (MRI) T(2) and T(1ρ) sensitivities were studied in native and trypsin-degraded articular cartilage before and after being soaked in 1 mM Gd-DTPA(2-) solution. When the cartilage surface was perpendicular to B(0), a typical laminar appearance was visible in T(2)-weighted images but not in T(1ρ)-weighted images, especially when the spin-lock field was high (2 kHz). At the magic angle (55°) orientation, neither T(2)- nor T(1ρ)-weighted image had a laminar appearance. Trypsin degradation caused a depth- and orientational-dependent T(2) increase (4%-64%) and a more uniform T(1ρ) increase at a sufficiently high spin-lock field (55%-81%). The presence of the Gd ions caused both T(2) and T(1ρ) to decrease significantly in the degraded tissue (6%-38% and 44%-49%, respectively) but less notably in the native tissue (5%-10% and 16%-28%, respectively). A quantity Sensitivity was introduced that combined both the percentage change and the absolute change in the relaxation analysis. An MRI experimental protocol based on two T(1ρ) measurements (without and with the presence of the Gd ions) was proposed to be a new imaging marker for cartilage degradation.     

Equilibrium signal intensity mapping, an MRI method for fast mapping of longitudinal relaxation rates and for image enhancement

INTRODUCTION: Inhomogeneity of magnetic fields, both B(0) and B(1), has been a major challenge in magnetic resonance imaging (MRI). Field inhomogeneity leads to image artifacts and unreliability of signal intensity (SI) measurements. This work proposes and shows the feasibility of generating equilibrium signal intensity (SI(Eq)) maps that can be utilized either to speed up relaxation-rate measurement or to enhance image quality and relaxation-rate-based weighting in various applications. METHODS: A 1.5-T MRI scanner was used. In canines (n=4), myocardial infarction was induced, and 48 h after the administration of 0.05 mmol kg(-1) Gd(ABE-DTTA), a contrast agent with slow tissue kinetics, in vivo R(1) mapping was carried out using an inversion recovery (IR)-prepared, fast gradient-echo sequence with varying inversion times (TIs). To test the SI(Eq) mapping method without the confounding effects of motion and blood flow, we carried out ex vivo R(1) mapping after the administration of 0.2 mmol kg(-1) Gd(DTPA) using an IR-prepared, fast spin-echo sequence in another group of dogs (n=2). R(1,full) maps and SI(Eq) maps were generated from the data from both sequences by three-parameter nonlinear curve fitting of the SI versus TI dependence. R(1,full) maps served as the reference standard. Raw IR images were then divided by the SI(Eq) maps, yielding corrected SI maps (COSIMs). Additionally, R(1) values were calculated from each single-TI image separately, using the SI(Eq) value and a one-parameter curve-fitting procedure (R(1,single)). Voxelwise correlation analysis was carried out for the COSIMs and the R(1,single) maps, both versus the standard R(1,full) maps. Deviations of R(1,single) from R(1,full) were statistically evaluated. RESULTS: In vivo, COSIM versus R(1,full) showed significantly (P<.05) better correlation [correlation coefficient (CC)=0.95] than SI versus R(1,full) with a TI=700-800 ms, which is 200-300 ms longer than the tau(null) (500 ms) of viable myocardium. With such TIs, SI versus R(1,full) yielded CCs of 0.86-0.88. R(1,single) versus R(1,full) yielded a peak CC of 0.96 at TI=700-900 ms. Mean deviations of R(1,single) from R(1,full) were below 5% for TIs between 500 and 1000 ms. Ex vivo, where tau(null) was 300 ms, the advantage of correction with SI(Eq) was not in the improvement of linear correlation but more in the reduction of scatter. Peak CCs for SI versus R(1,full) and COSIM versus R(1,full) at TI=500 ms were 0.96 for both. The ex vivo CC for R(1,single) versus R(1,full) at TI=500 ms was 0.98. Mean deviations of R(1,single) from R(1,full) were below 5% for TIs between 400 and 700 ms. CONCLUSIONS: Once the corresponding SI(Eq) map is obtained from a control stack, R(1) can be obtained accurately, using only a single IR image and without the need for a stack of TI-varied images. This approach could be applied in various dynamic MRI studies where short measurement time, once the dynamics has started, is of essence. When using this method with IR-prepared T(1)-weighted images, it is essential that the single TI be chosen such that the longitudinal relaxation in all voxels of interest would have passed tau(null). SI(Eq) maps are also useful in eliminating confounders from MR images to allow obtaining SI values that reflect more faithfully the relaxation parameter (R(1)) sought.     

Assessment of interstitial water content of articular cartilage with T1 relaxation

The interstitial water content typically increases in the early degeneration of articular cartilage. Previously, T₂ relaxation has been related to water content, yet it is known to be strongly affected by the collagen orientation. Articular cartilage plugs from the bovine patella, femur and tibia (N=20) were mapped for T₁ and T₂ at 9.4 T to test the ability of T₁ relaxation to reflect cartilage water content. As a reference, water and proteoglycan (PG) contents were determined. Significant (P<.01) linear associations were demonstrated between the relaxation rates and tissue water content (R₁: r=−.81, R₂: r=−.60) and PG content (R₁: r=.75). After adjustment for the tissue water content, partial correlation analysis did not show significant associations between the relaxation rates and tissue PG content. After the effect of PGs was removed, significant (P<.05) linear correlation between the relaxation rates and tissue water content (R₁: r=−.48, R₂: r=−.50) was observed. Thus, the spin-lattice relaxation rate is proposed to provide a biomarker for water content in articular cartilage.     

Evaluation of diethylenetriaminepentaacetic acid-manganese(II) complexes modified by narrow molecular weight distribution of chitosan oligosaccharides as potential magnetic resonance imaging contrast agents

Novel conjugates of narrow molecular weight distribution of chitosan oligosaccharides (CSn; n=6, 8, 11) with manganese-diethylenetriaminepentaacetic acid (Mn-DTPA) as potential magnetic resonance imaging (MRI) contrast agents were synthesized. The structures were characterized by means of Fourier transform infrared spectra, ¹³C nuclear magnetic resonance, size exclusion chromatography and inductively coupled plasma atomic emission spectrometry. The characterization results showed that Mn-DTPA was successfully linked to aminated CSn by an amide function. The magnetic properties were characterized by in vitro and T₁-weighted FLASH image experiments. Relaxivities studies indicated that Mn-DTPA-CSn (n=8, 11) provided higher relaxivity, either in aqueous or bovine serum albumin solution (0.725 mM), than commercial contrast agent Gd-DTPA. The stability results showed that Mn-DTPA-CSn in aqueous were stable enough to prevent Mn^II ions from releasing. The preliminary in vitro and T₁-weighted FLASH image studies suggested that Mn-DTPA-CSn had the advantage of becoming promising MRI contrast agents.     

Novel S-Gal analogs as H MRI reporters for in vivo detection of β-galactosidase

The quantitative assessment of gene expression and related enzyme activity in vivo could be important for the characterization of gene altering diseases and therapy. The development of imaging techniques, based on specific reporter molecules may enable routine non-invasive assessment of enzyme activity and gene expression in vivo. We recently reported the use of commercially available S-Gal^® as a β-galactosidase reporter for ¹H MRI, and the synthesis of several S-Gal^® analogs with enhanced response to β-galactosidase activity. We have now compared these analogs in vitro and have identified the optimal analog, C3-GD, based on strong T₁ and T₂ response to enzyme presence (ΔR₁ and ΔR₂ ~ 1.8 times S-Gal^®). Moreover, application is demonstrated in vivo in human breast tumor xenografts. MRI studies in MCF7-lacZ tumors implanted subcutaneously in athymic nude mice (n = 6), showed significant reduction in T₁ and T₂ values (each ~ 13%) 2 h after intra-tumoral injection of C3-GD, whereas the MCF7 (wild type) tumors showed slight increase. Thus, C3-GD successfully detects β-galactosidase activity in vivo and shows promise as a lacZ gene ¹H MR reporter molecule.     

Two-dimensional correlation relaxation studies of cement pastes

Two-dimensional nuclear magnetic resonance relaxation correlation studies of cement pastes have been performed on a unilateral magnet, the Surface GARField. Through these measurements, the hydration process can be observed by monitoring the evolution of porosity. Characteristic relaxation time distributions have been observed in different cement pastes: fresh white cement, prehydrated white cement and ordinary Portland cement. The observed T(1)/T(2) ratio in these cements has been shown to agree with expectations based on high field values.     

Regional variations in MR relaxation of hip joint cartilage in subjects with and without femoralacetabular impingement

The objective of this study was to analyze regional variations of magnetic resonance (MR) relaxation times (T_1ρ and T₂) in hip joint cartilage of healthy volunteers and subjects with femoral acetabular impingement (FAI). Morphological and quantitative images of the hip joints of 12 healthy volunteers and 9 FAI patients were obtained using a 3 T MR scanner. Both femoral and acetabular cartilage layers in each joint were semi-automatically segmented on sagittal 3D high-resolution spoiled gradient echo (SPGR) images. These segmented regions of interest (ROIs) were automatically divided radially into twelve equal sub-regions (30⁰ intervals) based on the fitted center of the femur head. The mean value of T_1ρ/T₂ was calculated in each sub-region after superimposing the divided cartilage contours on the MR relaxation (T_1ρ/T₂) maps to quantify the relaxation times. T_1ρ and T₂ relaxation times of the femoral cartilage were significantly higher in FAI subjects compared to healthy controls (39.9 ± 3.3 msec in FAI vs. 35.4 ± 2.3 msec in controls for T_1ρ (P = 0.0020); 33.9 ± 3.1 msec in FAI vs. 31.1 ± 1.7 msec in controls for T₂ (P = 0.0160)). Sub-regional analysis showed significantly different T_1ρ and T₂ relaxation times in the anterior-superior region (R9) of the hip joint cartilage between subjects with FAI and healthy subjects, suggesting possible regional differences in cartilage matrix composition between these two groups. Receiver operating characteristic (ROC) analysis showed that sub-regional analysis in femoral cartilage was more sensitive in discriminating FAI joint cartilage from that of healthy joints than global analysis of the whole region (T_1ρ: area under the curve (AUC) = 0.981, P = 0.0001 for R9 sub-region; AUC = 0.901, P = 0.002 for whole region; T₂: AUC = 0.976, P = 0.0005 for R9 sub-region; AUC = 0.808, P = 0.0124 for whole region). The results of this study demonstrated regional variations in hip cartilage composition using MR relaxation times (T_1ρ and T₂) and suggested that analysis based on local regions was more sensitive than global measures in subjects with and without FAI.     

Sensitive and automated detection of iron-oxide-labeled cells using phase image cross-correlation analysis

Superparamagnetic iron oxide (SPIO) nanoparticles are increasingly being used to noninvasively track cells, target specific molecules and monitor gene expression in vivo. Contrast changes that are subtle relative to intrinsic sources of contrast present a significant detection challenge. Here, we describe a postprocessing algorithm, called Phase map cross-correlation Detection and Quantification (PDQ), with the purpose of automating identification and quantification of localized accumulations of SPIO agents. The method is designed to sacrifice little flexibility - it works on previously acquired data and allows the use of conventional high-SNR pulse sequences with no extra scan time. We first investigated the theoretical detection limits of PDQ using a simulated dipole field. This method was then applied to three-dimensional (3D) MRI data sets of agarose gel containing isolated dipoles and ex vivo transplanted allogenic rat hearts infiltrated by numerous iron-oxide-labeled macrophages as a result of organ rejection. A simulated dipole field showed this method to be robust in very low signal-to-noise ratio images. Analysis of agarose gel and allogenic rat heart shows that this method can automatically identify and count dipoles while visualizing their biodistribution in 3D renderings. In the heart, this information was used to calculate a quantitative index that may indicate its degree of cellular infiltration.     

TriTone: a radiofrequency field (B1)-insensitive T1 estimator for MRI at high magnetic fields

Fast, high-resolution, longitudinal relaxation time (T1) mapping is invaluable in clinical and research applications. It has been shown that two spoiled gradient recalled echo (SPGR) images acquired in steady state with variable flip angles is an attractive alternative to the multi-image sets previously acquired with inversion or saturation recovery. The known sensitivity of the two-point method to transmit radiofrequency field (B1) inhomogeneity exacerbated at 3 T and above, however, mandates its combination with an additional, time-consuming and possibly specific-absorption-rate-intensive B1 measurement, preventing direct migration of the method to these fields. To address this, we introduce a method designed to be free of systematic errors caused by B1 inhomogeneity in which the value of T1 is extracted from three SPGR images acquired with echo planar imaging (EPI) readout. The precision of the T1 maps produced is found to be comparable to the two-point method, while the accuracy is greatly improved in the same time and spatial resolution. A welcome byproduct of the method is a map of B1 that can be used to correct other acquisitions in the same session. Tables of the optimal acquisition protocols are provided for several total imaging times.     

Quantitative MRI using T1ρ and T2 in human osteoarthritic cartilage specimens: correlation with biochemical measurements and histology

Purpose

A direct correlation between T_1ρ, T₂ and quantified proteoglycan and collagen contents in human osteoarthritic cartilage has yet to be documented. We aimed to investigate the orientation effect on T_1ρ and T₂ values in human osteoarthritic cartilage and to quantify the correlation between T_1ρ, T₂ vs. biochemical composition and histology in human osteoarthritic cartilage.

Materials and methods

Thirty-three cartilage specimens were collected from patients who underwent total knee arthroplasty due to severe osteoarthritis and scanned with a 3T MR scanner for T_1ρ and T₂ quantification. Nine specimens were scanned at three different orientations with respect to the B₀: 0°, 90° and 54.7°. Core punches were taken after MRI. Collagen and proteoglycan contents were quantified using biochemical assays. Histology sections were graded using Mankin scores. The correlation between imaging parameters, biochemical contents and histological scores were studied.

Results

Both mean T_1ρ and T₂ at 54.7° were significantly higher than those measured at 90° and 0°, with T_1ρ showing less increase compared to T₂. R_1ρ (1/T_1ρ) values had a significant but moderate correlation with proteoglycan contents (R=.45, P=.002), while R₂ (1/T₂) was not correlated with proteoglycan. No significant correlation was found between relaxation times (T_1ρ or T₂) and collagen contents. The T_1ρ values of specimen sections with high Mankin scores were significantly higher than those with low Mankin scores (P<.05).

Conclusions

Quantitative MRI has a great potential to provide noninvasive imaging biomarkers for cartilage degeneration in osteoarthritis.     

Consideration of slice profiles in inversion recovery Look–Locker relaxation parameter mapping

Purpose

To include the flip angle distribution caused by the slice profile into the model used for describing the relaxation curves observed in inversion recovery Look–Locker FLASH T₁ mapping for a more accurate determination of the relaxation parameters.

Materials and methods

For each inversion time, the flip angle dependent signal of the mono-exponential relaxation model is integrated across the slice profile. The resulting Consideration of Slice Profiles (CSP) relaxation curves are compared to the mono-exponential signal model in numerical simulations as well as in phantom and in-vivo experiments.

Results

All measured relaxation curves showed systematic deviations from a mono-exponential curve increasing with flip angle and T₁ but decreasing with repetition time. Additionally, the accuracy of T₁ was found to be largely dependent on the temporal coverage of the relaxation curve. All these systematic errors were largely reduced by the CSP model.

Conclusion

The proposed CSP model represents a useful extension of the conventionally used mono-exponential relaxation model. Despite inherent model inaccuracies, the mono-exponential model was found to be sufficient for many T₁ mapping situations. However, if only a poor temporal coverage of the relaxation process is achievable or a very precise modeling of the relaxation course is needed as in model-based techniques, the mono-exponential model leads to systematic errors and the CSP model should be used instead.     

Quantitative T(1)(ρ) imaging using phase cycling for B0 and B1 field inhomogeneity compensation

T_1ρ imaging is useful in a number of clinical applications. T_1ρ preparation methods, however, are sensitive to non-uniformities of the B₀ magnetic field and the B₁ RF field. These common system imperfections can result in image artifacts and quantification errors in T_1ρ imaging. We report on a phase-cycling method which can eliminate B₁ RF inhomogeneity effects in T_1ρ imaging. This method does not only correct for image artifacts but also for T_2ρ contamination caused by B₁ RF inhomogeneity. The presence of B₀ magnetic field inhomogeneity can compromise the effectiveness of this method for B₁ RF inhomogeneity correction. We demonstrate that, by combining the spin-locking scheme reported by Dixon et al. (Myocardial suppression in vivo by spin locking with composite pulses. Magn Reson Med 1996; 36:90-94) with phase cycling, we can simultaneously correct B₀ magnetic field inhomogeneity effects and B₁ RF inhomogeneity effects in T_1ρ imaging. Phantom and in vivo data sets are used to demonstrate the proposed methods and to compare them with other existing T_1ρ preparation methods.     

The effect of varying echo spacing within a multiecho acquisition: better characterization of long T2 components

A 48-echo pulse sequence with five different echo-spacing combinations was examined to determine how one can most effectively measure the T2 relaxation characteristics of cerebral tissue containing a long T2 component. For each scan, the first 32 echoes had an echo spacing of 10 ms, while the spacing for Echoes 33-48 (DeltaTE2) was 10, 20, 30, 40 or 50 ms. In an in vivo study using 10 normal volunteers, it was found that the resolution of T2 distribution peaks for both myelin water (approximately 20 ms) and intracellular/extracellular (IE) water (approximately 80 ms) improved as DeltaTE2 increased. The geometric mean T2 values of the main peak agreed within the error for all DeltaTE2 values. A phantom study simulated T2 relaxation distributions that are expected in the brains of patients with demyelinating diseases. For phantoms in which the T2 values of the IE and lesion (200-500 ms) water compartments were separated by at least a factor of 3, each compartment in the distribution was better resolved when DeltaTE2=40 or 50 ms. On the basis of these results, we recommend the use of extended DeltaTE2 values for imaging patients with lesions, without the risk of losing valuable short T2 information.     

Use of fat suppression in R₂ relaxometry with MRI for the quantification of tissue iron overload in beta-thalassemic patients

Purpose

To assess the performance and results of R₂ relaxometry using a fat-suppressed (FS) multiecho sequence and compare these to conventional R₂ relaxometry in estimating tissue iron overload.

Materials and Methods

Relaxation rate values (R₂=1/T2) of the liver, spleen, pancreas and vertebral bone marrow (VBM) were estimated in 21 patients with β-thalassemia major, using a respiratory-triggered 16-echo Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence before (R₂) and after (R₂ FS) the application of chemically selective fat suppression.

Results

Hepatic and splenic R₂ FS values correlated with respective R₂ values (r=0.98 and r=0.96, P<.001), whereas correlations between R₂ FS and R₂ values for pancreas and VBM were not statistically significant. Bland–Altman plots show disagreement between R₂ and R₂ FS values, particularly for pancreas and VBM. Hepatic, pancreatic and VBM R₂ FS values correlated with serum ferritin (r=0.88, P<.001; r=0.51, P<.003; and r=0.75, P<.002, respectively). Hepatic R₂ FS values correlated with splenic R₂ FS (r=0.77, P<.03), pancreatic R₂ FS (r=0.61, P<.006) and VBM R₂ FS values (r=0.70, P<.001), whereas pancreatic R₂ FS values correlated also with VMB R₂ FS values. On the contrary, among the R₂ values of the above tissues, obtained without fat suppression, only hepatic R₂ values correlated with serum ferritin, whereas no correlation was documented between hepatic and pancreatic or VBM R₂ values. The application of fat suppression did not improve breathing or flow artifacts.

Conclusion

Application of fat suppression in the standard CPMG sequence improved the capability of MRI in noninvasive quantification of iron, particularly in lipid-rich tissues, such as vertebral bone marrow (VBM) and pancreas.