Multi-component T1 relaxation and magnetisation transfer in peripheral nerve

Compartment-specific q-space analysis of diffusion-weighted data from isolated rhesus optic and sciatic nerves

We investigated compartment-specific water diffusion properties in two widely structurally different isolated bovine nerves. Sciatic and optic nerves were immersed in saline containing Gd-DTPA(2+). Consequently, T(1) became non-monoexponential and fit well to a biexponential function. q-Space diffusion data were collected for each component. In the sciatic nerve, the slow-decaying component (T(1s)) was considerably more restricted and directional than the fast-decaying component (T(1f)). In the optic nerve, fractional anisotropy of both components was comparable and similar to that of the total H(2)O signal. The root mean square of the displacement distribution functions of T(1s) correlated well with the widely different axonal diameters of both nerves. Possibly, the source of T(1s) is the intra-axonal compartment and that of T(1f) is associated with the inter-axonal space. The compartment specificity of the method shown makes it useful for the investigation of the contribution of each nerve compartment to diffusion tensor imaging measurements and other diffusion-based methods.

Investigating exchange and multicomponent relaxation in fully-balanced steady-state free precession imaging

PURPOSE

To investigate the effect of chemical exchange and multicomponent relaxation on the rapid T(2) mapping method, DESPOT2 (driven equilibrium single pulse observation of T(2)) and the steady-state free precession (SSFP) sequence upon which it is based. Although capable of rapid T(2) determination, an assumption implicit of the method is single-component relaxation. In many biological tissues (such as white and gray matter), it is well established that the T(2) decay curve is more accurately described by the summation of more than one relaxation species.

MATERIALS AND METHODS

The effects of exchange were first incorporated into the general SSFP magnetization expressions and its effect on the measured SSFP signal investigated using Bloch-McConnell simulations. Corresponding imaging experiments were performed to support the presented theory.

RESULTS

Simulations show the measured multicomponent SSFP signal may be expressed as a linear summation of signal from each species under usual imaging conditions where the repetition time is much less than T(2). Imaging experiments performed using dairy cream demonstrate strong agreement with the presented theory. Finally, using a dairy cream model, we demonstrate quantification of multicomponent relaxation from multiangle SSFP data for the first time, showing good agreement with reference spin-echo values.

CONCLUSION

SSFP and DESPOT2 may provide a new method for investigating multicomponent systems, such as human brain, and disease processes, such as multiple sclerosis.

Magnetic resonance imaging of myelin

The ability to measure myelin in vivo has great consequences for furthering our knowledge of normal development, as well as for understanding a wide range of neurological disorders. The following review summarizes the current state of myelin imaging using MR. We consider five MR techniques that have been used to study myelin: 1) conventional MR, 2) MR spectroscopy, 3) diffusion, 4) magnetization transfer, and 5) T2 relaxation. Fundamental studies involving peripheral nerve and MR/histology comparisons have aided in the interpretation and validation of MR data. We highlight a number of important findings related to myelin development, damage, and repair, and we conclude with a critical summary of the current techniques available and their potential to image myelin in vivo.

Transverse relaxation in rat optic nerve

In vitro investigations were performed to study the proton T(2) relaxation spectrum of rat optic nerve. Studies in which the nerve was incubated in a D(2)O-based solution revealed that >98% of the spectrum originated from the water protons. The spectrum was found to consist of three components having relaxation times and sizes similar to those reported in the literature for peripheral nerve. Procedures were taken to confirm the existence of a third optic nerve component and that it was not an artifact of the long-lived water protons of the in vitro incubation solution. Evidence using paramagnetic agents in the incubation solution, which removed the two longest-lived nerve components from the spectrum, revealed the existence of a small fourth component (<10% of total) having a T(2) relaxation time similar to that of the intermediate-lived nerve component. Bathing the nerves in a 10 mM glutamate solution, glutamate known to result in cellular swelling in mammalian central nervous system (CNS), was found to increase the component size of the longest-lived nerve component, suggestive that this component may result from cellular water.

Investigating the effect of exchange and multicomponentT1 relaxation on the short repetition time spoiled steady-state signal and the DESPOT1T1 quantification method

PURPOSE

To examine the spoiled steady-state (spoiled gradient-recalled echo sequence [SPGR]) signal arising from two-compartment systems and the role of experimental parameters, in particular TR for resolving signal from each compartment.

MATERIALS AND METHODS

Using Bloch-McConnell simulations, we examined the SPGR signal from two-component systems in which T(1) is much greater than the mean residence time (tau(m)) of proton spins in each component. Specifically, we examined the role of TR on the ability to resolve each components signal, as well as the influence of experimental parameters on derived DESPOT1 T(1) values.

RESULTS

Results revealed that when TR < or = 0.01 tau(m), the measured SPGR signal may be modeled as a summation of signal from each species using a no-exchange approximation. Additionally, under this short TR condition, the driven equilibrium single pulse observation of T(1) (DESPOT1) mapping approach provides T(1) values preferentially biased toward the short or long T(1) species, depending on the choice of flip angles.

CONCLUSION

The ability to model the SPGR signal using a no-exchange approximation may permit the quantification multicomponent T(1) relaxation in vivo. Additionally, the ability to preferentially weight the DESPOT1 T(1) value toward the short or long T(1) may provide a useful window into these components.

Diffusion of myelin water

We studied compartmentally specific characteristics of water diffusion in excised frog sciatic nerve by combining T1 or T2 selective acquisitions with pulse-gradient spin-echo (PGSE) diffusion weighting, with the specific objective of characterizing myelin water diffusion. Combining a PGSE with a Carr-Purcell-Meiboom-Gill (CPMG) acquisition provided apparent diffusion coefficients (ADCs) for each of the three T2 components found in nerve, including the short-lived component believed to be derived from myelin water. Double-inversion-recovery (DIR) preparation provided an alternate means of discriminating myelin water, and in combination with PGSE provided somewhat different measures of ADC. The DIR measures yielded myelin water ADCs of 0.37 microm2/ms (parallel to nerve) and 0.13 microm2/ms (perpendicular to nerve). These ADC estimates were postulated to be more accurate than those based on T2 discrimination, although the difference between the two findings is not clear.

Selective excitation of myelin water using inversion-recovery-based preparations

T1 and T2 relaxation of excised frog sciatic nerve water was characterized at 7 T. Based on these findings, optimal timings for multiple inversion-recovery magnetization preparations were determined to selectively excite the so-called myelin-water T2 component. Subsequent double inversion-recovery and triple inversion-recovery preparations were used in combination with CPMG acquisitions to experimentally determine optimal timings and effect of the preparation. Using double inversion-recovery, optimal timings were found to excite magnetization that is predominantly (approximately 93%) derived from the myelin-water component. Greater selectivity (approximately 96%) was found by extending the preparation to triple inversion-recovery, at the price of decreasing SNR by a factor of approximately 2.

MR properties of excised neural tissue following experimentally induced demyelination

Changes in the magnetic resonance (MR) parameters of demyelinated neural tissue were measured in vitro using an experimental animal model. A tellurium (Te) diet was applied to weanling rats to induce the demyelination process in the sciatic nerve. The quantitative MR parameters, such as T(1), T(2) relaxation time constants and magnetization transfer (MT) were measured each day after applying the Te diet (up to 7 days) and were found to be substantially different from those of normal nerves. An increase in the average T(1) and T(2) was observed along with a decrease in the MT ratio (MTR) and the quantitative MT parameter M(0B), which describes the semisolid pool of protons. Most of the MR parameters correlated very well with the myelin fraction of neural tissue evaluated by quantitative histopathology. The T(2) relaxation spectrum provided the most efficient quantitative assessment of changes in neural tissue microstructure and its analysis resulted in a powerful tool to distinguish the processes of demyelination and inflammation. In comparison, the MT measurements were less successful.

A continuous-flow perfusion system for the maintenance and NMR study of small tissue samples in vitro

To describe and evaluate a novel perfusion system developed to maintain excised tissue in a flowing, oxygenated bathing solution during acquisition of nuclear magnetic resonance (NMR) data, and in addition allow precise data to be acquired continuously while altering the composition of the bathing solution surrounding the tissue. A chamber to house the tissue sample was constructed of interlocking sections of polyethylene tubing, and had approximate internal dimensions of 4 mm in diameter and 4 mm in height. Temperature-controlled, physiologically appropriate buffer solution was pumped via an infusion pump through the chamber, entering and exiting by way of small openings on either end. Immediately surrounding the polyethylene chamber was a tight-fitting four-loop solenoid RF coil. Measured proton NMR parameters were found to be fairly insensitive to the flow rate of the buffer if this coil was used only for reception and a larger-volume transmit-only coil was used for excitation. Temperature control of the sample was successfully implemented between 25 and 40 degrees C. The perfusion system was found to be resistant to the effects of flow rate, as well as a useful tool for the administration of drugs or agents to the tissue. Changes in buffer composition could be performed on the fly without the need to reposition the sample each time a change was made. This avoidance of repositioning was found to yield a fivefold improvement in the precision of T(2) spectral parameters (using frog sciatic nerve as a sample).

Increased anisotropy in acute stroke: a possible explanation

BACKGROUND AND PURPOSE

The increase in fractional anisotropy (FA) in acute stroke has yet to be explained. Using an engineering methodology known as pq diagrams, we sought to explain the increase in FA by describing changes in the total magnitude of the diffusion tensor (L) as well as the isotropic (p) and anisotropic (q) components.

METHODS

Diffusion tensor imaging was performed in 10 patients with stroke <27 hours old. The diffusion tensor was decomposed into the p and q components and plotted to describe the diffusion trajectories. FA was also calculated and compared.

RESULTS

There was significant and consistent reduction in p, q, and L (p: mean, -50.0%; range, -36.6% to -64.5%; q: mean, -50.8%; range, -30.8% to -72.8%; L: mean, -50.3%; range, -37.0% to -65.1%). There were inconsistent changes in FA (mean, -0.5%; range, -44.9% to +45.0%). Five patients had elevated FA due to proportionately higher loss of L than q.

CONCLUSIONS

Changes in FA only occur when there is a change in the ratio of q/L. Acute elevation of FA occurred in the context of a larger reduction in L than q. The elevation in FA occurs in the context of a reduction in the anisotropic tensor and therefore is a consequence of ratio-metric measurement. This appears to clarify the reported increase in FA in terms of alterations in the shape of the apparent diffusion tensor. pq diagrams appear to offer improved resolution of acute diffusion changes in ischemia.

Compartmental study of T(1) and T(2) in rat brain and trigeminal nerve in vivo

The integrated T(1)-T(2) characteristics of rat brain and trigeminal nerve water were studied in vivo using a rapid method for acquiring a series of images that depend on T(1) and T(2) relaxation times. Gray matter regions showed only one signal component in both the T(1) and T(2) domains. Trigeminal nerve, however, which has been shown previously to exhibit three T(2) components, was found to also exhibit three T(1) components. The correlations between these T(1) and T(2) components were demonstrated by uniquely filtering out each of the three T(2) components using an inversion-recovery preparation, as determined by the component T(1) values. Based on previous works, it is postulated that each of these three signal components is derived from a unique microanatomical region of the nerve. Knowledge of these T(1) components may thus prove valuable in devising novel methods of identifying the presence and quantifying the volume of tissue subtypes such as myelin.

MR properties of rat sciatic nerve following trauma

T(1) and T(2) relaxation times, magnetization transfer (MT), and diffusion anisotropy of rat sciatic nerve were measured at different time intervals following trauma. The nerve injury was induced by either cutting (irreversible nerve degeneration) or crushing (degeneration followed by regeneration). The MR properties were measured for proximal and distal portions of the injured nerve. The portions of the nerve proximal to the induced injury exhibited MR characteristics similar to those of normal nerves, whereas the distal portions showed significant differences in all MR parameters. These differences diminished in the regenerating nerves within approximately 4 weeks post injury. In the case of irreversible nerve damage, the differences in the distal nerves were slightly larger and did not resolve even 6 weeks after induced trauma. The MR measurements were correlated with histopathology exams. Observed changes in tissue microstructure, such as demyelination, inflammation, and axonal loss, can result in a significant increase in the average T(1) and T(2) relaxation times, reduction in the MT effect, and decrease in diffusion anisotropy. MR parameters, therefore, are very good indicators of nerve damage and may be useful in monitoring therapies that assist nerve regeneration. Magn Reson Med 45:415-420, 2001.

Different magnetization transfer effects exhibited by the short and long T(2) components in human brain

Magnetization transfer ratios (MTRs) were measured separately for the two T(2) components in white matter. For both binomial and off-resonance sinc MT pulses, the MTR was larger for the short T(2) component than for the long T(2) component. This differential MT effect disappeared for delays between the MT pulse and the multi-echo pulse sequence longer than 200 msec, indicating exchange between the two components. When using the sinc MT pulse, the MTR for the short T(2) component was similar for different white matter structures, whereas it varied for different white matter structures when using the binomial pulse-a phenomenon attributed to direct saturation. When the sinc pulse frequency was brought closer to resonance, MTRs in white matter and doped water phantoms increased for both components but more so for the shorter T(2) component. This behavior was consistent with a Bloch equation model of direct saturation.

Rapid acquisition transverse relaxometric imaging

Segmented echo-planar acquisitions have been incorporated into a multiecho imaging sequence to produce a MRI method for rapid transverse relaxometry. The method is demonstrated on gel phantoms and rat brain and found to produce unbiased estimates of T(2). Gradient performance can be a limiting factor for the implementation of this technique and there is a cost in signal-to-noise ratio resulting from the higher bandwidth required, as is typical for echo-planar acquisitions.

Compartmental study of diffusion and relaxation measured in vivo in normal and ischemic rat brain and trigeminal nerve

The correlation between the apparent diffusion coefficient (ADC) and T(2) of water in rat brain and trigeminal nerve was investigated using a hybrid diffusion-weighted-CPMG imaging sequence. Little dependence of ADC on T(2) was found in brain regions of interest, which is postulated to be due to rapid exchange between intra- and extracellular water. Conversely, the ADC of water in trigeminal nerve was found to change significantly with echo time (TE). Parallel to the nerve and with a constant diffusion time (t(diff) = 10.8 ms), the ADC increased by approximately 30% between TEs of 25 ms and 185 ms; perpendicular to the nerve, the ADC decreased by a similar amount over the same range of TE. Measurements made following the onset of global ischemia yielded lower ADCs, with similar dependence on TE. Observations that transverse relaxation of water in nerves is multiexponential have previously been interpreted in terms of microanatomical compartments in slow exchange. In the context of this interpretation, our data suggest that diffusional anisotropy is greater outside than within the myelinated axons. Further, data following the onset of global ischemia suggest that the mechanism(s) by which ADC is reduced affect most or all microanatomical environments of nerve, at least insofar as they are represented over the TE domain investigated. Magn Reson Med 43:837-844, 2000.

Characterizing white matter with magnetization transfer and T(2)

A magnetization-transfer (MT) CPMG hybrid experiment was performed to analyze T(2) relaxation and MT characteristics in bovine optic nerve. Two exchanging liquid pools with their own, independent MT characteristics were necessary to model both the T(2) relaxation and the MT data. The model agrees well with the experimental data and yields physically realistic parameters. The MT effect for myelin water is approximately nine time larger than that for intra/intercellular water, indicating that the MT characteristics observed for white matter are mainly related to myelin. The model can be used to probe parameters that would be difficult to achieve experimentally. The exchange process between the two tissue compartments does not drastically affect the amplitudes and relaxation rates of the T(2) components, but is fast enough to significantly influence their MT characteristics. Although, both the MT and T(2) experiments described in this paper are too time consuming to be applied in routine clinical work, presented results can be useful in interpreting clinical pulse sequences that are sensitive to myelin. Magn Reson Med 42:1128-1136, 1999.

Water diffusion, T(2), and compartmentation in frog sciatic nerve

A potential relationship between structural compartments in neural tissue and NMR parameters may increase the specificity of MRI in diagnosing diseases. Nevertheless, our understanding of MR of nerves and white matter is limited, particularly the influence of various water compartments on the MR signal is not known. In this study, components of the (1)H transverse relaxation decay curve in frog peripheral nerve were correlated with the diffusion characteristics of the water in the nerve. Three T(2) values were identified with nerve. Water mobility was found to be unrestricted on the timescale of 100 msec in the component of the signal with the intermediate T(2) time, suggesting some contribution from the interstitial space to this T(2) component. Restricted diffusion was observed in the component with the longest T(2) time, supporting the assignment of at least part of the spins contributing to this component to an intracellular compartment. The observed nonexponential behavior of the diffusion attenuation curves was investigated and shown to be potentially caused by the wide range of axon sizes in the nerve. Magn Reson Med 42:911-918, 1999.

Intimate combination of low- and high-resolution image data: I. Real-space PET and (1)H(2)O MRI, PETAMRI

Two different types of (co-registered) images of the same slice of tissue will generally have different spatial resolutions. The judicious pixel-by-pixel combination of their data can be accomplished to yield a single image exhibiting properties of both. Here, axial (18)FDG PET and (1)H(2)O MR images of the human brain are used as the low- and high-resolution members of the pair. A color scale is necessary in order to provide for separate intensity parameters from the two image types. However, not all color scales can accommodate this separability. The HSV color model allows one to choose a color scale in which the intensity of the low-resolution image type is coded as hue, while that of the high-resolution type is coded as value, a reasonably independent parameter. Furthermore, the high-resolution image must have high contrast and be quantitative in the same sense as the low-resolution image almost always is. Here, relaxographic MR images (naturally segmented quantitative (1)H(2)O spin-density components) are used. Their essentially complete contrast serves to effect an apparent editing function when encoded as the value of the color scale. Thus, the combination of (18)FDG PET images with gray-matter (GM) relaxographic (1)H(2)O images produces visually "GM-edited" (18)FDG PETAMR (positron emission tomography and magnetic resonance) images. These exhibit the high sensitivity to tracer amounts characteristic of PET along with the high spatial resolution of (1)H(2)O MRI. At the same time, however, they retain the complete quantitative measures of each of their basis images. Magn Reson Med 42:345-360, 1999. Published 1999 Wiley-Liss, Inc.