Magnetic field dependence of proton spin-lattice relaxation times.

The magnetic field dependence of the water-proton spin-lattice relaxation rate (1/T(1)) in tissues results from magnetic coupling to the protons of the rotationally immobilized components of the tissue. As a consequence, the magnetic field dependence of the water-proton (1/T(1)) is a scaled report of the field dependence of the (1/T(1)) rate of the solid components of the tissue. The proton spin-lattice relaxation rate may be represented generally as a power law: 1/T(1)omega = A omega(-b), where b is usually found to be in the range of 0.5-0.8. We have shown that this power law may arise naturally from localized structural fluctuations along the backbone in biopolymers that modulate the proton dipole-dipole couplings. The protons in a protein form a spin communication network described by a fractal dimension that is less than the Euclidean dimension. The model proposed accounts quantitatively for the proton spin-lattice relaxation rates measured in immobilized protein systems at different water contents, and provides a fundamental basis for understanding the parametric dependence of proton spin-lattice relaxation rates in dynamically heterogeneous systems, such as tissues.     

T(1) relaxation in in vivo mouse brain at ultra-high field.

Accurate knowledge of relaxation times is imperative for adjustment of MRI parameters to obtain optimal signal-to-noise ratio (SNR) and contrast. As small animal MRI studies are extended to increasingly higher magnetic fields, these parameters must be assessed anew. The goal of this study was to obtain accurate spin-lattice (T(1)) relaxation times for the normal mouse brain at field strengths of 9.4 and 17.6 T. T(1) relaxation times were determined for cortex, corpus callosum, caudate putamen, hippocampus, periaqueductal gray, lateral ventricle, and cerebellum and varied from 1651 +/- 28 to 2449 +/- 150 ms at 9.4 T and 1824 +/- 101 to 2772 +/- 235 ms at 17.6 T. A field strength-dependent increase of T(1) relaxation times is shown. The SNR increase at 17.6 T is in good agreement with the expected SNR increase for a sample-dominated noise regime.     

High magnetic field water and metabolite proton T1 and T2 relaxation in rat brain in vivo.

Comprehensive and quantitative measurements of T1 and T2 relaxation times of water, metabolites, and macromolecules in rat brain under similar experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T) are presented. Water relaxation showed a highly significant increase (T1) and decrease (T2) with increasing field strength for all nine analyzed brain structures. Similar but less pronounced effects were observed for all metabolites. Macromolecules displayed field-independent T2 relaxation and a strong increase of T1 with field strength. Among other features, these data show that while spectral resolution continues to increase with field strength, the absolute signal-to-noise ratio (SNR) in T1/T2-based anatomical MRI quickly levels off beyond approximately 7 T and may actually decrease at higher magnetic fields.     

Characterization of bleomycin lung injury by nuclear magnetic resonance: correlation between NMR relaxation times and lung water and collagen content.

The response of the NMR relaxation times (T(1), CPMG T(2), and Hahn T(2)) to bleomycin-induced lung injury was studied in excised, unperfused rat lungs. NMR, histologic, and biochemical (collagen content measurement) analyses were performed 1, 2, 4, and 8 weeks after intratracheal instillation of saline (control lungs) or 10 U/kg bleomycin sulfate. The control lungs showed no important NMR, water content, histologic, or collagen content changes. The spin-spin relaxation times for the fast and intermediate components of the CPMG decay (T(2f) and T(2i), respectively) increased 1 week after bleomycin injury (acute inflammatory stage) and then progressively decreased during the following 2-8 weeks (i.e., with the development of the chronic, fibrotic stage of the injury). The slow component (T(2s)) showed no significant changes. The response of T(1) and the slow component of the Hahn T(2) was, on the whole, similar to that of CPMG T(2f) and T(2i). T(1) changes were very small. Lung water content increased 1 week after injury. Histologic and biochemical assessment of collagen showed that collagen content was close to control at 1 week, but markedly increased at 2, 4, and 8 weeks. T(1) and T(2) data were directly correlated with lung water content and inversely correlated with collagen content. Our results indicate that NMR relaxation time measurements (particularly T(2)) reflect the structural changes associated with bleomycin injury. The prolonged T(2) relaxation times observed in the acute stage are related to the presence of edema, whereas the subsequent decrease in these values marks the stage of the collagen deposition (fibrotic stage). CPMG-T(2) and Hahn-T(2) measurements can be valuable as a potentially noninvasive method for characterizing bleomycin-induced lung injury and pathologically related lung disorders.     

SQUID-detected MRI at 132 microT with T1-weighted contrast established at 10 microT--300 mT.

T(1)-weighted contrast MRI with prepolarization was detected with a superconducting quantum interference device (SQUID). A spin evolution period in a variable field between prepolarization and detection enabled the measurement of T(1) in fields between 1.7 microT and 300 mT; T(1) dispersion curves of agarose gel samples over five decades in frequency were obtained. SQUID detection at 5.6 kHz drastically reduces the field homogeneity requirements compared to conventional field-cycling methods using Faraday coil detection. This allows T(1) dispersion measurements to be easily combined with MRI, so that T(1) in a wide range of fields can be used for tissue contrast. Images of gel phantoms with T(1)-weighted contrast at four different fields between 10 microT and 300 mT demonstrated dramatic contrast enhancement in low fields. A modified inversion recovery technique further enhanced the contrast by selectively suppressing the signal contribution for a specific value of the low-field T(1).     

In vivo blood T1 measurements at 1.5 T, 3 T,and 7 T

The longitudinal relaxation time of blood is a crucial parameter for quantification of cerebral blood flow by arterial spin labeling and is one of the main determinants of the signal‐to‐noise ratio of the resulting perfusion maps. Whereas at low and medium magnetic field strengths (B₀), its in vivo value is well established; at ultra‐high field, this is still uncertain. In this study, longitudinal relaxation time of blood in the sagittal sinus was measured at 1.5 T, 3 T, and 7 T. A nonselective inversion pulse preceding a Look‐Locker echo planar imaging sequence was performed to obtain the inversion recovery curve of venous blood. The results showed that longitudinal relaxation time of blood at 7 T was ～ 2.1 s which translates to an anticipated 33% gain in the signal‐to‐noise ratio in arterial spin labeling experiments due to T₁ relaxation alone compared with 3 T. In addition, the linear relationship between longitudinal relaxation time of blood and B₀ was confirmed. Magn Reson Med, 70:1082–1086, 2013. © 2012 Wiley Periodicals, Inc.     

Quantitative T(1rho) and adiabatic Carr-Purcell T2 magnetic resonance imaging of human occipital lobe at 4 T.

The feasibility of performing quantitative T(1rho) MRI in human brain at 4 T is shown. T(1rho) values obtained from five volunteers were compared with T2 and adiabatic Carr-Purcell (CP) T2 values. Measured relaxation time constants increased in order from T2, CP-T2, T(1rho) both in white and gray matter, demonstrating differential sensitivities of these methods to dipolar interactions and/or proton exchange and diffusion in local microscopic field gradients, which are so-called dynamic averaging (DA) processes. In occipital lobe, all relaxation time constants were found to be higher in white matter than in gray matter, demonstrating contrast denoted as an "inverse transverse relaxation contrast." This contrast persisted despite changing the delay between refocusing pulses or changing the magnitude of the spin-lock field strength, which suggests that it does not originate from DA, as might be induced by the presence of Fe, but rather is related to dipolar interactions in the brain tissue.     

3D-T1rho quantitation of patellar cartilage at 3.0T

PURPOSE: To demonstrate the feasibility of three-dimensional (3D) T(1rho)-weighted imaging of human knee joint at 3.0T without exceeding the specific absorption rate (SAR) limits and the measurement of the baseline T(1rho) values of patellar cartilage and several muscles at the knee joint. MATERIALS AND METHODS: 3D gradient-echo sequence with a self-compensating spin-lock pulse cluster of 250 Hz power was used to acquire 3D-T(1rho)-weighted images of the knee joint of five healthy subjects. Global and regional analysis of patellar cartilage T(1rho) were performed. Furthermore, T(1rho) of several periarticular muscles were analyzed. RESULTS: The global average T(1rho) value of the patellar cartilage varied from 39 to 43 msec. The regional average T(1rho) values varied from 38 to 42 msec, and from 42 to 44 msec for medial and lateral facets, respectively. In vivo reproducibility of average T(1rho) of patellar cartilage was found to be 5% (coefficient of variation). Similarly, the global average T(1rho) values for biceps femoris, lateral gastrocnemius, medial gastrocnemius, and sartorius varied between 31-46, 29-49, 35-48, and 32-50 msec, respectively. CONCLUSION: We demonstrated the feasibility of 3D-T(1rho)-weighted imaging of the knee joint at 3.0T without exceeding SAR limits.     

T1rho relaxation mapping in human osteoarthritis (OA) cartilage: comparison of T1rho with T2

PURPOSE: To quantify the spin-lattice relaxation time in the rotating frame (T1rho) in various clinical grades of human osteoarthritis (OA) cartilage specimens obtained from total knee replacement surgery, and to correlate the T1rho with OA disease progression and compare it with the transverse relaxation time (T2). MATERIALS AND METHODS: Human cartilage specimens were obtained from consenting patients (N = 8) who underwent total replacement of the knee joint at the Pennsylvania Hospital, Philadelphia, PA, USA. T2- and T1rho-weighted images were obtained on a 4.0 Tesla whole-body GE Signa scanner (GEMS, Milwaukee, WI, USA). A 7-cm diameter transmit/receive quadrature birdcage coil tuned to 170 MHz was employed. RESULTS: All of the surgical knee replacement OA cartilage specimens showed elevated relaxation times (T2 and T1rho) compared to healthy cartilage tissue. In various grades of OA specimens, the T1rho relaxation times varied from 62 +/- 5 msec to 100 +/- 8 msec (mean +/- SEM) depending on the degree of cartilage degeneration. However, T2 relaxation times varied only from 32 +/- 2 msec to 45 +/- 4 msec (mean +/- SEM) on the same cartilage specimens. The increase in T2 and T1rho in various clinical grades of OA specimens were approximately 5-50% and 30-120%, respectively, compared to healthy specimens. The degenerative status of the cartilage specimens was also confirmed by histological evaluation. CONCLUSION: Preliminary results from a limited number of knee specimens (N = 8) suggest that T1rho relaxation mapping is a sensitive noninvasive marker for quantitatively predicting and monitoring the status of macromolecules in early OA. Furthermore, T1rho has a higher dynamic range (>100%) for detecting early pathology compared to T2. This higher dynamic range can be exploited to measure even small macromolecular changes with greater accuracy compared to T2. Because of these advantages, T1rho relaxation mapping may be useful for evaluating early OA therapy.     

Proton T2 relaxation study of water, N-acetylaspartate, and creatine in human brain using Hahn and Carr-Purcell spin echoes at 4T and 7T.

Carr-Purcell and Hahn spin-echo (SE) measurements were used to estimate the apparent transverse relaxation time constant (T2) of water and metabolites in human brain at 4T and 7T. A significant reduction in the T2 values of proton resonances (water, N-acetylaspartate, and creatine/phosphocreatine) was observed with increasing magnetic field strength and was attributed mainly to increased dynamic dephasing due to increased local susceptibility gradients. At high field, signal loss resulting from T2 decay can be substantially reduced using a Carr-Purcell-type SE sequence.     

Change in the proton T1 of fat and water in mixture

This work describes observed changes in the proton T₁ relaxation time of both water and lipid when they are in relatively homogeneous mixtures. Results obtained from vegetable oil–water emulsions, pork kidney and lard mixtures, and excised samples of white and brown adipose tissues are presented to demonstrate this change in T₁ as a function of mixture fat fraction. As an initial proof of concept, a simpler acetone‐water experiment was performed to take advantage of complete miscibility between acetone and water and both components' single chemical shift peaks. Single‐voxel MR spectroscopy was used to measure the T₁ of predominant methylene spins in fat and the T₁ of water spins in each setup. In the vegetable oil–water emulsions, the T₁ of fat varied by as much as 3‐fold when water was the dominant mixture component. The T₁ of pure lard increased by 170 msec (+37%) when it was blended with lean kidney tissue in a 16% fatty mixture. The fat T₁ of lipid‐rich white adipose tissue was 312 msec. In contrast, the fat T₁ of leaner brown adipose tissue (fat fraction 53%) was 460 msec. A change in the water T₁ from that of pure water was also observed in the experiments. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Correction of main and transmit magnetic field (B0 and B1) inhomogeneity effects in multicomponent‐driven equilibrium single‐pulse observation of T1 and T2

Multicomponent‐driven equilibrium single‐component observation of T₁ and T₂ offers a new approach to multiple component relaxation time and myelin water analysis. The method derives two‐component relaxation information from spoiled and fully balanced steady‐state (SPGR and bSSFP) imaging data acquired over multiple flip angles. Although these steady‐state imaging techniques afford rapid acquisition times and high signal‐to‐noise ratio efficiency, they are also sensitive to main (B₀) and transmit (B₁) magnetic field inhomogeneities. These effects alter the measured signal from their theoretical values and lead to substantive errors in the derived myelin volume fraction estimates. Here, we incorporate correction techniques to mitigate these effects. DESPOT1‐HIFI is used to first calibrate the transmitted flip angles; and B₀ affects are removed through the inclusion of an additional parameter in the multicomponent‐driven equilibrium single‐component observation of T₁ and T₂ fitting, coupled with the acquisition of multiple phase‐cycled bSSFP data. The performance of these correction techniques was evaluated using numerical simulations, demonstrating effective removal of B₀ and B₁‐induced errors in the derived myelin fraction relaxation parameters. The approach was also successfully demonstrated in vivo, with near artifact‐free whole‐brain, high spatial resolution (1.7 mm × 1.7 mm × 1.7 mm isotropic voxels) myelin water fraction maps acquired in a clinically feasible 16 min. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

In vivo 3T spiral imaging based multi-slice T(1rho) mapping of knee cartilage in osteoarthritis.

T(1rho) describes the spin-lattice relaxation in the rotating frame and has been proposed for detecting damage to the cartilage collagen-proteoglycan matrix in osteoarthritis. In this study, a multi-slice T(1rho) imaging method for knee cartilage was developed using spin-lock techniques and a spiral imaging sequence. The adverse effect of T(1) regrowth during the multi-slice acquisition was eliminated by RF cycling. Agarose phantoms with different concentrations, 10 healthy volunteers, and 9 osteoarthritis patients were scanned at 3T. T(1rho) values decreased as agarose concentration increased. T(1rho) values obtained with imaging methods were compared with those obtained with spectroscopic methods. T(1rho) values obtained during multi-slice acquisition were validated with those obtained in a single slice acquisition. Reproducibility was assessed using the average coefficient of variation of median T(1rho), which was 0.68% in phantoms and 4.8% in healthy volunteers. There was a significant difference (P = 0.002) in the average T(1rho) within patellar and femoral cartilage between controls (45.04 +/- 2.59 ms) and osteoarthritis patients (53.06 +/- 4.60 ms). A significant correlation was found between T(1rho) and T(2); however, the difference of T(2) was not significant between controls and osteoarthritis patients. The results suggest that T(1rho) relaxation times may be a promising clinical tool for osteoarthritis detection and treatment monitoring.     

The effect of hyperoxygenation on T1 relaxation time in vitro

RATIONALE AND OBJECTIVES: Ventilation with high oxygen (O2) concentrations has been shown to decrease T1 in blood and tissues of patients. This study aims to assess the effect of hyperoxygenation on the T1 relaxation time of blood and other physiologic solutions. MATERIALS AND METHODS: Varied gaseous mixtures of O2 and air between 21% and 100% O2 were created using an experimental circuit at room temperature, and used to saturate human blood, plasma, or normal saline. The samples were studied using an 8.45-Tesla magnetic resonance (MR) system and a 1.5-Tesla clinical MR scanner. RESULTS: MR spectroscopy at 8.45 Tesla showed that the percentage of O2 chosen for saturation correlated negatively with T1 (R2 = 1.00 for blood, 0.99 for plasma, and 1.00 for normal saline). The reduction in T1 between solutions saturated with 21% and 100% O2 was 487 milliseconds (22% of the baseline T1 value) for blood, 391 milliseconds (15%) for plasma and 622 milliseconds (19%) for saline. Similarly, MR measurements at 1.5 Tesla showed T1 reduction with increasing O2 concentration. Conclusion. The decreasing T1 in blood depends strongly on the fraction of dissolved O2 in solution and is largely independent of the hemoglobin content.     

T1rho contrast in functional magnetic resonance imaging.

The application of T1 in the rotating frame (T1rho) to functional MRI in humans was studied at 3 T. Increases in neural activity increased parenchymal T1rho. Modeling suggested that cerebral blood volume mediated this increase. A pulse sequence named spin-locked echo planar imaging (SLEPI) that produces both T1rho and T2* contrast was developed and used in a visual functional MRI (fMRI)experiment. Spin-locked contrast significantly augments the T2* blood oxygen level-dependent (BOLD) contrast in this sequence. The total functional contrast generated by the SLEPI sequence (1.31%) was 54% larger than the contrast (0.85%) obtained from a conventional gradient-echo EPI sequence using echo times of 30 ms. Analysis of image SNR revealed that the spin-locked preparation period of the sequence produced negligible signal loss from static dephasing effects. The SLEPI sequence appears to be an attractive alternative to conventional BOLD fMRI, particularly when long echo times are undesirable, such as when studying prefrontal cortex or ventral regions, where static susceptibility gradients often degrade T2*-weighted images.     

Rapid 3D-T1rho mapping of the knee joint at 3.0T with parallel imaging.

Three-dimensional spin-lattice relaxation time in the rotating frame (3D-T1rho) with parallel imaging at 3.0T was implemented on a whole-body clinical scanner. A 3D gradient-echo sequence with a self-compensating spin-lock pulse cluster was combined with generalized autocalibrating partially parallel acquisitions (GRAPPA) to acquire T1rho-weighted images. 3D-T1rho maps of an agarose phantom and three healthy subjects were constructed using an eight-channel phased-array coil without parallel imaging and with parallel imaging acceleration factors of 2 and 3, in order to assess the reproducibility of the method. The coefficient of variation (CV) of the median T1rho of the agarose phantom was 0.44%, which shows excellent reproducibility. The reproducibility of in vivo 3D-T1rho maps was also investigated in three healthy subjects. The CV of the median T1rho of the patellar cartilage varied between approximately 1.1% and 4.3%. Similarly, the CV varied between approximately 2.1-5.8%, approximately 1.4-8.7%, and approximately 1.5-4.1% for the biceps femoris and lateral and medial gastrocnemius muscles, respectively. The preliminary results demonstrate that 3D-T1rho maps can be constructed with good reproducibility using parallel imaging. 3D-T1rho with parallel imaging capability is an important clinical tool for reducing both the total acquisition time and RF energy deposition at 3T.     

On- and off-resonance T(1rho) MRI in acute cerebral ischemia of the rat.

The ability of on-resonance T(1rho) (T(1rho)) and off-resonance T(1rho) (T(1rho)(off)) measurements to indicate acute cerebral ischemia in a rat model of transient middle cerebral artery (MCA) occlusion was investigated at 4.7 T. T(1rho) was determined with B(1) fields of 0.4, 0.8, and 1.6 G, and T(1rho)(off) with five offset frequencies ((Delta)omega) ranging from 0-7.5 kHz at B(1) of 0.4 G, yielding effective B(1) (B(eff)) from 0.4 to 1.8 G. Diffusion, T(1), and T(2) were also quantified. Both T(1rho) and T(1rho)(off) acquired with (Delta)(o)< 2.5 kHz showed positive contrast during the first hours of MCA occlusion in the ischemic tissue delineated by low diffusion. Interestingly, T(1rho)(off) contrast acquired with (Delta)omega > 2.5 kHz was clearly less sensitive to ischemic alterations, and developed with a delayed time course. This discrepancy is thought to be a consequence of the frequency dependency of cross-relaxation during irradiation with spin-lock pulses.     

Age dependence of T1 perfusion MRI‐based hemodynamic parameters in human kidneys

Purpose

To determine the association between renal cortical perfusion parameters from T1‐DCE magnetic resonance imaging (MRI) and age in human kidney.

Materials and Methods

Thirty‐five patients (mean age: 53 years, SD = 15 years) were imaged using inversion recovery (IR)‐prepared FLASH (pulse repitition time [TR] = 4.4 msec, echo time [TE] 2.2 msec, inversion time [TI] = 180 msec, FA 50°, matrix 128 × 256, 0.3 sec/slice) during the injection of Gadolinium‐DTPA. Tissue concentration–time courses were deconvolved. Renal blood flow (RBF), volume of distribution (RVD), and mean transit time (MTT) were derived from the resulting impulse response function.

Results

Mean RBF, RVD, and MTT were 127 mL/min/100 mL (SD = 81 mL/min/100 mL), 40 mL/100 mL (SD 23 mL/100 mL), and 22 sec (SD = 9 sec). A significant moderately negative correlation was found between RBF and age (R = ?0.447, P = 0.007), RVD and age (R = ?0.420, P = 0.012). MTT and age did not show a significant correlation (R = 0.017, P = 0.924). Repeating this analysis for each gender revealed a moderate age dependence of RBF (R = ?0.600 with P = 0.009) and RVD (R = ?0.540 with P = 0.021) in the male group only.

Conclusion

T1‐DCE quantitative perfusion MRI was sufficiently sensitive to demonstrate a significant negative correlation of RBF and RVD with patient age. This was due to a moderate age dependence of these quantities in males that seems to be absent in females. J. Magn. Reson. Imaging 2009;29:398–403. © 2009 Wiley‐Liss, Inc.

     

B1 transmission‐field inhomogeneity and enhancement ratio errors in dynamic contrast‐enhanced MRI (DCE‐MRI) of the breast at 3T

Purpose:

To quantify B₁ transmission‐field inhomogeneity in breast imaging of normal volunteers at 3T using 3D T₁‐weighted spoiled gradient echo and to assess the resulting errors in enhancement ratio (ER) measured in dynamic contrast‐enhanced MRI (DCE‐MRI) studies of the breast.

Materials and Methods:

A total of 25 volunteers underwent breast imaging at 3T and the B₁ transmission‐fields were mapped. Gel phantoms that simulate pre‐ and postcontrast breast tissue T₁ were developed. The effects of B₁‐field inhomogeneity on ER, as measured using a 3D spoiled gradient echo sequence, were investigated by computer simulation and experiments on gel phantoms.

Results:

It was observed that by using the patient orientation and MR scanner employed in this study, the B₁ transmission‐field field is always reduced toward the volunteer's right side. The median B₁‐field in the right breast is reduced around 40% of the expected B₁‐field. For some volunteers the amplitude was reduced by more than 50%. Computer simulation and experiment showed that a reduction in B₁‐field decreases ER. This reduction increases with both B₁‐field error and contrast agent uptake.

Conclusion:

B₁ transmission‐field inhomogeneity is a critical issue in breast imaging at 3T and causes errors in quantifying ER. These errors would be sufficient to reduce the conspicuity of a malignant lesion and could result in reduced sensitivity for cancer detection. J. Magn. Reson. Imaging 2010;31:234–239. © 2009 Wiley‐Liss, Inc.