Spin lock magnetic resonance imaging in the differentiation of hepatic haemangiomas and metastases

Paired self-compensated spin-lock preparation for improved T1ρ quantification

PURPOSE

Spin-lock (SL) imaging allows quantification of the spin-lattice relaxation time in the rotating frame (T1ρ). B0 and B1 inhomogeneities impact T1ρ quantification because the preparatory block in SL imaging is sensitive to the field heterogeneities. Here, a modified preparatory block (PSC-SL) is proposed that attempts to alleviate SL sensitivity to field inhomogeneities in scenarios where existing approaches fail, i.e. high SL frequencies.

METHODS

Computer simulations, phantom and in vivo experiments were used to determine the effect of field inhomogeneities on T1ρ quantification. Existing SL preparations were compared with PSC-SL in different conditions to assess the advantages and disadvantages of each method.

RESULTS

Phantom experiments and computer modeling demonstrate that PSC-SL provides superior T1ρ quantification at high SL frequencies in situations where the existing SL preparation methods fail. This result has been confirmed in pre-clinical neuro and body imaging at 7T.

CONCLUSION

PSC-SL complements existing methods by increasing the accuracy of T1ρ quantification at high spin-lock frequencies when large field inhomogeneities are present. A-priory information about the experimental conditions such, as field distribution and spinlock frequency are useful for selecting an appropriate spin-lock preparation for specific applications.

Quantitative rotating frame relaxometry methods in MRI

Macromolecular degeneration and biochemical changes in tissue can be quantified using rotating frame relaxometry in MRI. It has been shown in several studies that the rotating frame longitudinal relaxation rate constant (R1ρ ) and the rotating frame transverse relaxation rate constant (R2ρ ) are sensitive biomarkers of phenomena at the cellular level. In this comprehensive review, existing MRI methods for probing the biophysical mechanisms that affect the rotating frame relaxation rates of the tissue (i.e. R1ρ and R2ρ ) are presented. Long acquisition times and high radiofrequency (RF) energy deposition into tissue during the process of spin-locking in rotating frame relaxometry are the major barriers to the establishment of these relaxation contrasts at high magnetic fields. Therefore, clinical applications of R1ρ and R2ρ MRI using on- or off-resonance RF excitation methods remain challenging. Accordingly, this review describes the theoretical and experimental approaches to the design of hard RF pulse cluster- and adiabatic RF pulse-based excitation schemes for accurate and precise measurements of R1ρ and R2ρ . The merits and drawbacks of different MRI acquisition strategies for quantitative relaxation rate measurement in the rotating frame regime are reviewed. In addition, this review summarizes current clinical applications of rotating frame MRI sequences. Copyright © 2016 John Wiley & Sons, Ltd.

T1ρ Dispersion profile of rat tissues in vitro at very low locking fields

The purpose of this study was to show the T(1rho) dispersion profile in various rat tissues (liver, brain, spleen, kidney, heart and skeletal muscle) at low (0.1 T) B(0) field at very low locking field B1, starting from 10 microT. The T(1rho) dispersion profile showed a quite similar pattern in all tissues. The highest R(1rho) relaxation rates were seen in the liver and muscle followed by the heart, whereas the values for spleen, kidney and brain were rather similar. The greatest difference between R2 relaxation rate and R(1rho) relaxation rate at B1=10 microT was seen in the liver and muscle. The steepest slope for a dispersion curve was seen in the muscle. The value of T(1rho) approximately approached the value of T2 when the locking field B1 approached 0. Except for the liver, the calculated apparent relaxation rate R2' was slightly larger than the calculated one. The potential value of T(1rho) imaging is to combine high R1 contrast of low-field imaging with the high signal-to-noise ratio (SNR) of high static field imaging. T(1rho) relaxation and dispersion data presented in the current study help to optimize the rotating-frame MR imaging.

Acute myocardial infarction: tissue characterization with T1rho-weighted MR imaging--initial experience

Acute myocardial injury was evaluated in 21 patients by using a contrast material-enhanced T1rho-weighted cine turbo field-echo magnetic resonance (MR) imaging sequence and a delayed-enhancement sequence. In 12 of 21 patients, conventional T1-weighted contrast-enhanced cine turbo field-echo MR images were also collected for direct comparison with T1rho-weighted images. Delayed-enhancement technique distinctly characterized irreversible injury (percentage enhancement, 588% +/- 344). With T1rho weighting, percentage enhancement of irreversibly injured myocardium was 68% +/- 41, compared with 23% +/- 24 without T1rho weighting (P <.006). The addition of T1rho weighting to contrast-enhanced cine turbo field-echo MR sequences may offer a new contrast enhancement mechanism for characterization of acutely infarcted myocardium.

In vivo quantification ofT1? using a multislice spin-lock pulse sequence

A multislice spin-lock (MS-SL) pulse sequence is implemented on a clinical scanner to acquire multiple images with spin-lock-generated contrast of the knee joints of six healthy human subjects. The MS-SL sequence produces images with T1rho contrast with an additional factor of intrinsic T2rho weighting, which hinders direct measurement of T1rho. A method is presented to compensate the MS-SL-generated data with regard to T2rho in an effort to accurately calculate multislice T1rho maps in a feasible experimental time. The T2rho-compensated multislice T1rho maps produced errors in the measurement of T1rho in healthy patellar cartilage of approximately 5% compared to the gold standard measurement of T1rho acquired with single-slice spin-lock pulse sequence. The MS-SL sequence has potential as an important clinical tool for the acquisition of multislice T1rho-weighted images and/or quantitative multislice T1rho maps.

Blood NMR relaxation in the rotating frame: mechanistic implications

The rotating frame nuclear magnetic resonance relaxation rate R(1rho) in the blood and cell lysate was studied at 4.7T to provide reference values for in vivo modeling and to address the mechanisms contributing to net relaxation. A strong dependence on oxygenation, hematocrit, and spin lock field strength B(1) (0.2-1.6G) was observed in whole blood, whereas in lysate the effects were severely attenuated. The results were further compared to transverse relaxation rate R(2). A good agreement in low-field asymptotes of these two relaxation rates was found. R(1rho) field dispersion was fitted to Lorenzian line shape and resulted in correlation times around 40 micros. The dispersion behavior was related to motional properties of intracellular hemoglobin and effects of susceptibility shift interface across the cell membrane induced by compartmentalization of Hb into cells in blood.

Multiple-slice spin lock imaging of head and neck tumors at 0.1 Tesla: exploring appropriate imaging parameters with reference to T2-weighted spin-echo technique

RATIONALE AND OBJECTIVES

Spin lock imaging has been shown to be useful in characterizing head and neck tumors. The purposes of this study were to explore and develop multiple-slice spin lock gradient-echo (SL-GRE) sequences for head and neck imaging and to compare the tumor contrast on SL images to spin-echo (SE) T2-weighted images at 0.1 T.

METHODS

On the basis of measured relaxation times of tumors and head and neck tissues, the authors evaluated with signal equations the effect of imaging parameters on tissue contrast produced by the SL-GRE sequence. In the clinical study, 34 patients with pathologically verified head and neck tumors were imaged with multiple-slice SL-GRE (repetition time 1500 ms/echo time 30 ms) out-of-phase fat/water sequences and compared with T2-weighted SE (repetition time 1500 ms/echo time 120 ms) sequences. The conspicuity of tumors was evaluated by calculating the contrast-to-noise ratios (CNRs).

RESULTS

The combination of a short echo time of 30 ms and the length of locking pulses in the range of 10 to 35 ms produced optimal CNRs for head and neck tumor imaging. The measured CNRs and subjective evaluation for tumor detection were satisfactory with both imaging sequences. However, the CNRs between tumors and salivary gland tissues were significantly greater with the SL sequence than with the T2-weighted sequence.

CONCLUSIONS

The multiple-slice SL-GRE technique provides image contrast comparable to that of SE T2-weighted imaging for head and neck tumors at 0.1 T. With short locking pulse lengths and echo times, wide anatomic coverage and reduced motion and susceptibility artifacts can be achieved. The out-of-phase SL technique is useful in imaging salivary gland tumors.

Magnetic resonance imaging (MRI) and diseases of the liver and biliary tract. Part 1. Basic principles, MRI in the assessment of diffuse and focal hepatic disease

Magnetic resonance imaging (MRI) relies on the physical properties of unpaired protons in tissues to generate images. Unpaired protons behave like tiny bar magnets and will align themselves in a magnetic field. Radiofrequency pulses will excite these aligned protons to higher energy states. As they return to their original state, they will release this energy as radio waves. The frequency of the radio waves depends on the local magnetic field and by varying this over a subject, it is possible to build the images we are familiar with. In general, MRI has not been sufficiently sensitive or specific in the assessment of diffuse liver disease for clinical use. However, because of the specific characteristics of fat and iron, it may be useful in the assessment of hepatic steatosis and iron overload. Magnetic resonance imaging is useful in the assessment of focal liver disease, particularly in conjunction with contrast agents. Haemangiomas have a characteristic bright appearance on T2 weighted images because of the slow flowing blood in dilated sinusoids. Focal nodular hyperplasia (FNH) has a homogenous appearance, and enhances early in the arterial phase after gadolinium injection, while the central scar typically enhances late. Hepatic adenomas have a more heterogenous appearance and also enhance in the arterial phase, but less briskly than FNH. Hepatocellular carcinoma is similar to an adenoma, but typically occurs in a cirrhotic liver and has earlier washout of contrast. The appearance of metastases depends on the underlying primary malignancy. Overall, MRI appears more sensitive and specific than computed tomography with contrast for the detection and evaluation of malignant lesions.

T1rho dispersion of rat tissues in vitro

The purpose of this study was to demonstrate T1rho dispersion in different rat tissues (liver, brain, spleen, kidney, heart, and skeletal muscle), and to compare the 1/T1rho data to previous 1/T1 data and magnetization transfer of rat tissues at low (0.1 T) B0 field. The 1/T1rho dispersion showed a fairly similar pattern in all tissues. The highest 1/T1rho relaxation rates were seen in liver and muscle followed by heart, whereas the values for spleen, kidney, and brain were quite similar. Compared to 1/T2 relaxation rate, the greatest difference was seen in liver and muscle. The rank order 1/T1rho value at each locking field B1 was the same as the transfer rate of magnetization from the water to the macromolecular pool (Rwm) for liver, muscle, heart, and brain. The potential value T1rho imaging is to combine high T1 contrast of low field imaging with the high signal to noise ratio of high static field imaging. When the T1rho value for a given tissue is known, the contrast between different tissues can be optimized by adjusting the locking time TL. Further studies are encouraged to fully exploit this. Targets for more detailed research include brain infarct, brain and liver tumors.

3D spin-lock imaging of human gliomas

We investigated whether the simultaneous use of paramagnetic contrast medium and 3D on-resonance spin lock (SL) imaging could improve the contrast of enhancing brain tumors at 0.1 T. A phantom containing serial concentrations of gadopentetate dimeglumine (Gd-DTPA) in cross-linked bovine serum albumin (BSA) was imaged. Eleven patients with histologically verified glioma were also studied. T1-weighted 3D gradient echo images with and without SL pulse were acquired before and after a Gd-DTPA injection. SL effect, contrast, and contrast-to-noise ratio (CNR) were calculated for each patient. In the glioma patients, the SL effect was significantly smaller in the tumor than in the white and gray matter both before (p = 0.001, p = 0.025, respectively), and after contrast medium injection (p < 0.001, p < 0.001, respectively). On post-contrast images, SL imaging significantly improved tumor contrast (p = 0.001) whereas tumor CNR decreased slightly (p = 0.024). The combined use of SL imaging and paramagnetic Gd-DTPA contrast agent offers a modality for improving tumor contrast in magnetic resonance imaging (MRI) of enhancing brain tumors. 3D gradient echo SL imaging has also shown potential to increase tissue characterization properties of MR imaging of human gliomas.

On- and off-resonance spin-lock MR imaging of normal human brain at 0.1 T: possibilities to modify image contrast

The aim of the present investigation was to determine spin lock (SL) relaxation parameters for the normal brain tissues and thus, to provide basis for optimizing the imaging contrast at 0.1 T. 68 healthy volunteers were included. On-resonance spin lock relaxation time (T1rho) and off-resonance spin lock relaxation parameters (T1rho(off), Me/Mo), MT parameters (T1sat, Ms/Mo), and T1, T2 were determined for the cortical gray matter, and for the frontal and parietal white matters. The T1rho for the frontal and parietal white matters ranged from 110 to 133 ms and from 122 to 155 ms with locking field strengths from 50 microT to 250 microT, respectively. Accordingly, the values for the gray matter ranged from 127 to 155 ms. With a locking field strength of 50 microT, T1rho(off) for the frontal and parietal white matters were from 114 to 217 ms and from 126 to 219 ms, and for the gray matter from 136 to 267 ms with the angle between the effective magnetic field (B(eff)) and the z-axis (theta) ranging from 60 degrees to 15 degrees, respectively. The T1rho of the white and gray matters increased significantly with increasing locking field amplitude (p < 0.001). The T1rho(off) decreased significantly with increasing theta (p < 0.001). T1rho and T1rho(off) with theta > or = 30 degrees were statistically significantly shorter in the frontal than in the parietal white matters (p < 0.05). The duration, amplitude and theta of the locking pulse provide additional parameters to optimize contrast in brain SL imaging.

Determination of T1rho values for head and neck tissues at 0.1 T: a comparison to T1 and T2 relaxation times

In order to optimize head and neck magnetic resonance (MR) imaging with the spin-lock (SL) technique, the T1rho relaxation times for normal tissues were determined. Furthermore, T1rho was compared to T1 and T2 relaxation times. Ten healthy volunteers were studied with a 0.1 T clinical MR imager. T1rho values were determined by first measuring the tissue signal intensities with different locking pulse durations (TL), and then by fitting the signal intensity values to the equation with the least-squares method. The T1rho relaxation times were shortest for the muscle and tongue, intermediate for lymphatic and parotid gland tissue and longest for fat. T1rho demonstrated statistically significant differences (p < 0.05) between all tissues, except between muscle and tongue. T1rho values measured at locking field strength (B1L) of 35 microT were close to T2 values, the only exception being fat tissue, which showed T1rho values much longer than T2 values. Determination of tissue relaxation times may be utilized to optimize image contrast, and also to achieve better tissue discrimination potential, by choosing appropriate imaging parameters for the head and neck spin-lock sequences.

Low field T1rho imaging of myositis

The purpose of this study was to evaluate 1/T1rho in relation to 1/T1 and 1/T2 in characterizing normal and diseased muscle. We measured the muscle relaxation rates 1/T1 and 1/T2 at 0.1 T and 1/T1rho at on-resonance locking fields B1 between 10 and 160 microT in myositis patients and normal volunteers. 1/T2 and 1/T1rho of muscle were lower in the patients than in the volunteers, whereas there was no difference in the 1/T1 values. The lower relaxation rates 1/T2 and 1/T1rho in the diseased muscle may be due to fat and connective tissue infiltrations and edema. 1/T1rho contrast between muscle and subcutaneous fat was higher than 1/T2 and 1/T1 contrast. This may be explained by the different B1 dispersion behavior of these two tissue types. 1/T1rho of fat is B1 field independent, whereas 1/T1rho of muscle decreases clearly with increasing B1 field. In conclusion, 1/T1rho provides a useful tool in manipulating contrast in magnetic resonance imaging of diseased muscle.

T1 rho MR imaging characteristics of human anterior tibial and gastrocnemius muscles

RATIONALE AND OBJECTIVES

The authors evaluated the value of T1 rho in relation to T1 and T2 in the characterization of human muscles.

MATERIALS AND METHODS

The authors studied the effect of muscle type (anterior tibial [AT] and gastrocnemius [GC]), sex, and age on 1/T1 and 1/T2 at 0.1 T and on 1/ T1 rho at locking-field B1s (spin-locking radio-frequency magnetic induction field) of 10-160 microT in 38 healthy volunteers. The contrast-to-noise ratio (CNR) between muscle and fat was evaluated with different T1-, T2-, and T1 rho-weighted magnetic resonance (MR) sequences.

RESULTS

The 1/T1, 1/T2, and 1/T1 rho were slightly higher in AT than in GC muscles. The 1/T2 and 1/T1 rho of AT muscles showed a sex dependence, whereas no correlation with age was found. The CNR of the T1 rho-weighted images did not markedly differ from that of the T1- and T2-weighted images.

CONCLUSION

T1 rho is as sensitive as T2 to the composition of muscle, whereas T1 is less sensitive. In MR imaging of normal muscle, T1 rho and T2 provide a relatively similar tissue contrast.

T1 rho dispersion imaging of head and neck tumors: a comparison to spin lock and magnetization transfer techniques

The potential of T1 rho dispersion, spin lock (SL), and magnetization transfer (MT) techniques to differentiate benign and malignant head and neck tumors was evaluated. Twenty-four patients with pathologically verified head and neck tumors were studied with a .1-T MR imager. T1 rho dispersion effect was defined as 1 -(intensity with lower locking field amplitude/intensity with higher locking field amplitude). T1 rho dispersion effects were higher for malignant than benign tumors (P = .001). With T1 rho dispersion effect .14 as the threshold, sensitivity for detecting a malignant tumor was 91%, specificity was 77%, and accuracy was 83%. A strong correlation between T1 rho dispersion effects and SL effects and between T1 rho dispersion effects and MT effects in the head and neck tumors was found (r = .87, P < .001 and r = .90, P < .001, respectively). High T1 rho dispersion effects are not specific indicators of malignancy, because chronic infections, some benign tumors, and malignancies may overlap. Low T1 rho dispersion effect values are characteristic of a benign tumor.

T1rho of protein solutions at very low fields: dependence on molecular weight, concentration, and structure

The effect of molecular weight, concentration, and structure on 1/T1rho, the rotating frame relaxation rate, was investigated for several proteins using the on-resonance spin-lock technique, for locking fields B1 < 200 microT. The measured values of 1/T1rho were fitted to a simple theoretical model to obtain the dispersion curves 1/T1rho(omega1) and the relaxation rate at zero B1 field, 1/T1rho(0). 1/T1rho was highly sensitive to the molecular weight, concentration, and structure of the protein. The amount of intra- and intermolecular hydrogen and disulfide bonds especially contributed to 1/T1rho. In all samples, 1/T1rho(0) was equal to 1/T2 measured at the main magnetic field Bo = 0.1 T, but at higher locking fields the dispersion curves monotonically decreased. The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for T1rho relaxation in protein solutions is not valid at very low B1 fields. The underlying mechanism for the relaxation rate 1/T1rho at B1 fields below 200 microT is discussed.