A comparison of three SPRITE techniques for the quantitative 3D imaging of the 23Na spin density on a 4T whole-body machine

Comprehensive Account of Sodium Imaging and Spectroscopy for Brain Research

Sodium (²³Na) is a vital component of neuronal cells and plays a key role in various signal transmission processes. Hence, information on sodium distribution in the brain using magnetic resonance imaging (MRI) provides useful information on neuronal health. ²³Na MRI and MR spectroscopy (MRS) improve the diagnosis, prognosis, and clinical monitoring of neurological diseases but confront some inherent limitations that lead to low signal-to-noise ratio, longer scan time, and diminished partial volume effects. Recent advancements in multinuclear MR technology have helped in further exploration in this domain. We aim to provide a comprehensive description of ²³Na MRI and MRS for brain research including the following aspects: (a) theoretical background for understanding ²³Na MRI and MRS fundamentals; (b) technological advancements of ²³Na MRI with respect to pulse sequences, RF coils, and sodium compartmentalization; (c) applications of ²³Na MRI in the early diagnosis and prognosis of various neurological disorders; (d) structural-chronological evolution of sodium spectroscopy in terms of its numerous applications in human studies; (e) the data-processing tools utilized in the quantitation of sodium in the respective anatomical regions.

Frontiers of Sodium MRI Revisited: From Cartilage to Brain Imaging

Sodium magnetic resonance imaging (²³ Na-MRI) is a highly promising imaging modality that offers the possibility to noninvasively quantify sodium content in the tissue, one of the most relevant parameters for biochemical investigations. Despite its great potential, due to the intrinsically low signal-to-noise ratio (SNR) of sodium imaging generated by low in vivo sodium concentrations, low gyromagnetic ratio, and substantially shorter relaxation times than for proton (¹ H) imaging, ²³ Na-MRI is extremely challenging. In this article, we aim to provide a comprehensive overview of the literature that has been published in the last 10-15 years and which has demonstrated different technical designs for a range of ²³ Na-MRI methods applicable for disease diagnoses and treatment efficacy evaluations. Currently, a wider use of 3.0T and 7.0T systems provide imaging with the expected increase in SNR and, consequently, an increased image resolution and a reduced scanning time. A great interest in translational research has enlarged the field of sodium MRI applications to almost all parts of the body: articular cartilage tendons, spine, heart, breast, muscle, kidney, and brain, etc., and several pathological conditions, such as tumors, neurological and degenerative diseases, and others. The quantitative parameter, tissue sodium concentration, which reflects changes in intracellular sodium concentration, extracellular sodium concentration, and intra-/extracellular volume fractions is becoming acknowledged as a reliable biomarker. Although the great potential of this technique is evident, there must be steady technical development for ²³ Na-MRI to become a standard imaging tool. The future role of sodium imaging is not to be considered as an alternative to ¹ H MRI, but to provide early, diagnostically valuable information about altered metabolism or tissue function associated with disease genesis and progression. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 1.

Velocity mapping of fast flows using a linearly ramped gradient waveform

We report a new pure phase encoding measurement for velocity mapping. Velocity-sensitization is achieved using a repeating, linearly ramped gradient waveform instead of rectangular bipolar pulsed field gradients. This approach reduces eddy current effects and results in the sample experiencing a gradient waveform that more closely matches the ideal input. Errors in k-space mapping and calculated velocity values are reduced when contrasted with the previous measurement method. Velocity maps were acquired of high-speed (c. 6 m/s) water flow through a pipe constriction. The application of linearly ramped gradient waveforms to non-velocity-encoded imaging measurements is discussed.

Zero-gradient-excitation ramped hybrid encoding (zGRF -RHE) sodium MRI

PURPOSE

Fast bi-exponential transverse signal decay compounds sodium image quality. This work aims at enhancing image characteristics using a special case of ramped hybrid encoding (RHE). Zero-gradient-excitation (zG_RF )-RHE provides (1) gradient-free excitation for high flip angle, artifact-free excitation profiles and (2) gradient ramping during dead-time for the optimization of encoding time (t_enc ) to reduce T₂ ^* signal decay influence during acquisition.

METHODS

Radial zG_RF -RHE and standard radial UTE were investigated over a range of receiver bandwidths in simulations, phantom and in vivo brain experiments. Central k-space in zG_RF -RHE was acquired through single point measurements at the minimum achievable TE. T₂ ^* blurring artifacts and image SNR and CNR were assessed.

RESULTS

zG_RF -RHE enabled 90° flip angle artifact-free excitation, whereas gradient pre-ramping provided greater t_enc efficiency for any readout bandwidths. Experiments confirmed simulation results, revealing sharper edge characteristics particularly at short readout durations (T_RO ). Significant SNR improvements of up to 4.8% were observed for longer T_RO .

CONCLUSION

zG_RF -RHE allows for artifact-free high flip angle excitation with time-efficient encoding improving on image characteristics. This hybrid encoding concept with gradient pre-ramping is trajectory independent and can be introduced in any center-out UTE trajectory design.

Quantitative sodium MR imaging: A review of its evolving role in medicine

Sodium magnetic resonance (MR) imaging in humans has promised metabolic information that can improve medical management in important diseases. This technology has yet to find a role in clinical practice, lagging proton MR imaging by decades. This review covers the literature that demonstrates that this delay is explained by initial challenges of low sensitivity at low magnetic fields and the limited performance of gradients and electronics available in the 1980s. These constraints were removed by the introduction of 3T and now ultrahigh (≥7T) magnetic field scanners with superior gradients and electronics for proton MR imaging. New projection pulse sequence designs have greatly improved sodium acquisition efficiency. The increased field strength has provided the expected increased sensitivity to achieve resolutions acceptable for metabolic interpretation even in small target tissues. Consistency of quantification of the sodium MR image to provide metabolic parametric maps has been demonstrated by several different pulse sequences and calibration procedures. The vital roles of sodium ion in membrane transport and the extracellular matrix will be reviewed to indicate the broad opportunities that now exist for clinical sodium MR imaging. The final challenge is for the technology to be supplied on clinical ≥3T scanners.

Quantitative sodium magnetic resonance imaging of cartilage, muscle, and tendon

Sodium magnetic resonance imaging (MRI), or imaging of the 23Na nucleus, has been under exploration for several decades, and holds promise for potentially revealing additional biochemical information about the health of tissues that cannot currently be obtained from conventional hydrogen (or proton) MRI. This additional information could serve as an important complement to conventional MRI for many applications. However, despite these exciting possibilities, sodium MRI is not yet used routinely in clinical practice, and will likely remain strictly in the domain of exploratory research for the coming decade. This paper begins with a technical overview of sodium MRI, including the nuclear magnetic resonance (NMR) signal characteristics of the sodium nucleus, the challenges associated with sodium MRI, and the specialized pulse sequences, hardware, and reconstruction techniques required. Various applications of sodium MRI for quantitative analysis of the musculoskeletal system are then reviewed, including the non-invasive assessment of cartilage degeneration in vivo, imaging of tendinopathy, applications in the assessment of various muscular pathologies, and assessment of muscle response to exercise.

Imaging of sodium in the brain: a brief review

Sodium-based MRI plays a vital role in the study of metabolism and can unveil valuable information about emerging and existing pathology--in particular in the human brain. Sodium is the second most abundant MR active nucleus in living tissue and, due to its quadrupolar nature, has magnetic properties not common to conventional proton MRI, which can reveal further insights, such as information on the compartmental distribution of intra- and extracellular sodium. Nevertheless, the use of sodium nuclei for imaging comes at the expense of a lower sensitivity and significantly reduced relaxation times, making in vivo sodium studies feasible only at high magnetic field strength and by the use of dedicated pulse sequences. Hybrid imaging combining sodium MRI and positron emission tomography (PET) simultaneously is a novel and promising approach to access information on dynamic metabolism with much increased, PET-derived specificity. Application of this new methodology is demonstrated herein using examples from tumour imaging.

Residual quadrupole interaction in brain and its effect on quantitative sodium imaging

Sodium MRI is particularly interesting given the key role that sodium ions play in cellular metabolism. To measure concentration, images must be free from contrast unrelated to sodium density. However, spin 3/2 NMR is complicated by more than rapid biexponential signal decay. Residual quadrupole interactions (described by frequency ωQ) can reduce Mxy development during RF excitation. Three experiments, each performed on the same four healthy volunteers, demonstrate that residual quadrupole interactions are of concern in quantitative sodium imaging of the brain. The first experiment shows a reliable increase in the rate of excitation 'flipping' (1%-6%), particularly in white matter with tracts running superior-inferior (i.e. parallel to B0). Increased flip-rates imply an associated signal loss and are to be expected when ωQ ~ ω1. The second experiment shows that a prescribed flip-angle decrease from 90° to 20°, with concomitant decrease in TE from 0.25 ms to 0.10 ms and no T1 weighting, results in a 14%-26% saline calibration phantom normalized signal (SN) increase in the white matter regions. The third experiment shows that this (SN) increase is primarily due to a residual quadrupole effect, with a small contribution from T2 weighting. There is an observed deviation from the spin 3/2 biexponential curve, also suggesting ωQ dephasing. Using simulation to explain the results of all three experiments, a model of brain tissue is hypothesized. It includes one pool (50%) with ωQ = 0, and another (50%) in which ωQ has a Gaussian distribution with a standard deviation of 625 Hz. Given the result of the second experiment, it is suggested that the use of reduced flip-angles with large ω1 will provide more accurate measures of sodium concentration than 'standard' methods using 90° pulses. Alternatively, further study of sodium ωQmay provide a means to explore tissue structure and organization.

Skin sodium measured with ²³Na MRI at 7.0 T

Skin sodium (Na(+) ) storage, as a physiologically important regulatory mechanism for blood pressure, volume regulation and, indeed, survival, has recently been rediscovered. This has prompted the development of MRI methods to assess Na(+) storage in humans ((23) Na MRI) at 3.0 T. This work examines the feasibility of high in-plane spatial resolution (23) Na MRI in skin at 7.0 T. A two-channel transceiver radiofrequency (RF) coil array tailored for skin MRI at 7.0 T (f = 78.5 MHz) is proposed. Specific absorption rate (SAR) simulations and a thorough assessment of RF power deposition were performed to meet the safety requirements. Human skin was examined in an in vivo feasibility study using two-dimensional gradient echo imaging. Normal male adult volunteers (n = 17; mean ± standard deviation, 46 ± 18 years; range, 20-79 years) were investigated. Transverse slices of the calf were imaged with (23) Na MRI using a high in-plane resolution of 0.9 × 0.9 mm(2) . Skin Na(+) content was determined using external agarose standards covering a physiological range of Na(+) concentrations. To assess the intra-subject reproducibility, each volunteer was examined three to five times with each session including a 5-min walk and repositioning/preparation of the subject. The age dependence of skin Na(+) content was investigated. The (23) Na RF coil provides improved sensitivity within a range of 1 cm from its surface versus a volume RF coil which facilitates high in-plane spatial resolution imaging of human skin. Intra-subject variability of human skin Na(+) content in the volunteer population was <10.3%. An age-dependent increase in skin Na(+) content was observed (r = 0.78). The assignment of Na(+) stores with (23) Na MRI techniques could be improved at 7.0 T compared with current 3.0 T technology. The benefits of such improvements may have the potential to aid basic research and clinical applications designed to unlock questions regarding the Na(+) balance and Na(+) storage function of skin.

New opportunities for quantitative and time efficient 3D MRI of liquid and solid electrochemical cell components: Sectoral Fast Spin Echo and SPRITE

The ability to image electrochemical processes in situ using nuclear magnetic resonance imaging (MRI) offers exciting possibilities for understanding and optimizing materials in batteries, fuel cells and supercapacitors. In these applications, however, the quality of the MRI measurement is inherently limited by the presence of conductive elements in the cell or device. To overcome related difficulties, optimal methodologies have to be employed. We show that time-efficient three dimensional (3D) imaging of liquid and solid lithium battery components can be performed by Sectoral Fast Spin Echo and Single Point Imaging with T1 Enhancement (SPRITE), respectively. The former method is based on the generalized phase encoding concept employed in clinical MRI, which we have adapted and optimized for materials science and electrochemistry applications. Hard radio frequency pulses, short echo spacing and centrically ordered sectoral phase encoding ensure accurate and time-efficient full volume imaging. Mapping of density, diffusivity and relaxation time constants in metal-containing liquid electrolytes is demonstrated. 1, 2 and 3D SPRITE approaches show strong potential for rapid high resolution (7)Li MRI of lithium electrode components.

Sodium MRI: methods and applications

Sodium NMR spectroscopy and MRI have become popular in recent years through the increased availability of high-field MRI scanners, advanced scanner hardware and improved methodology. Sodium MRI is being evaluated for stroke and tumor detection, for breast cancer studies, and for the assessment of osteoarthritis and muscle and kidney functions, to name just a few. In this article, we aim to present an up-to-date review of the theoretical background, the methodology, the challenges, limitations, and current and potential new applications of sodium MRI.

High-resolution quantitative sodium imaging at 9.4 Tesla

PURPOSE

Investigation of the feasibility to perform high-resolution quantitative sodium imaging at 9.4 Tesla (T).

METHODS

A proton patch antenna was combined with a sodium birdcage coil to provide a proton signal without compromising the efficiency of the X-nucleus coil. Sodium density weighted images with a nominal resolution of 1 × 1 × 5 mm(3) were acquired within 30 min with an ultrashort echo time sequence. The methods used for signal calibration as well as for B0, B1, and off-resonance correction were verified on a phantom and five healthy volunteers.

RESULTS

An actual voxel volume of roughly 40 μL could be achieved at 9.4T, while maintaining an acceptable signal-to-noise ratio (8 for brain tissue and 35 for cerebrospinal fluid). The measured mean sodium concentrations for gray and white matter were 36 ± 2 and 31 ± 1 mmol/L of wet tissue, which are comparable to values previously reported in the literature.

CONCLUSION

The reduction of partial volume effects is essential for accurate measurement of the sodium concentration in the human brain. Ultrahigh field imaging is a viable tool to achieve this goal due to its increased sensitivity.

Purely phase-encoded MRI of turbulent flow through a dysfunctional bileaflet mechanical heart valve

OBJECT

We have used a purely phase-encoded magnetic resonance imaging (MRI) technique, single-point ramped imaging with T1 enhancement (SPRITE), to investigate the steady, turbulent flow dynamics through a bileaflet mechanical heart valve (BMHV).

MATERIALS AND METHODS

We have measured in vitro the turbulent diffusivity and velocity downstream of the valve in two configurations (fully opened and partially opened), which mimic normal and dysfunctional operation. Our constant-time implementation of the MRI measurement is unusually robust to fast turbulent flows, and to artefacts caused by the pyrolytic carbon construction of the valve.

RESULTS

Turbulent diffusivity downstream of the normally functioning valve peaks at 1.05 × 10(-6)m(2)/s, while the turbulent diffusivity is higher downstream of the dysfunctional valve (peaking at 3.15 × 10(-6) m(2)/s) and is accompanied by a high-velocity fluid jet and re-circulating flow. The fluid jet is not along the centreline of the valve, as might be anticipated in conventional Doppler echocardiography measurements.

CONCLUSION

The nature of motion-sensitized SPRITE makes it unusually capable in turbulent flows and near to boundaries between different magnetic susceptibilities. These qualities have allowed us to compare the three-dimensional flow fields through normal and dysfunctional BMHVs.

Biomedical applications of sodium MRI in vivo

In this article we present an up-to-date overview of the potential biomedical applications of sodium magnetic resonance imaging (MRI) in vivo. Sodium MRI is a subject of increasing interest in translational imaging research as it can give some direct and quantitative biochemical information on the tissue viability, cell integrity and function, and therefore not only help the diagnosis but also the prognosis of diseases and treatment outcomes. It has already been applied in vivo in most human tissues, such as brain for stroke or tumor detection and therapeutic response, in breast cancer, in articular cartilage, in muscle, and in kidney, and it was shown in some studies that it could provide very useful new information not available through standard proton MRI. However, this technique is still very challenging due to the low detectable sodium signal in biological tissue with MRI and hardware/software limitations of the clinical scanners. The article is divided in three parts: 1) the role of sodium in biological tissues, 2) a short review on sodium magnetic resonance, and 3) a review of some studies on sodium MRI on different organs/diseases to date.

Multi-Frame SPRITE: a method for resolution enhancement of multiple-point SPRITE data

The Single Point Ramped Imaging with T1 Enhancement (SPRITE) sequence is well suited for the acquisition of magnetic resonance signals from fast relaxing nuclei and from heterogeneous materials. However, it is time inefficient compared to sequences that are based on frequency encoding because only one single point is acquired per excitation. Multiple-point SPRITE (mSPRITE) mitigates this problem with the acquisition of multiple FID points. mSPRITE images reconstructed from early FID samples suffer from reduced spatial resolution due to the limited extent of its corresponding k-space. In this work we present a new reconstruction algorithm for spatial resolution enhancement that solves this problem without changes to the mSPRITE sequence. The method, called Multi-Frame mSPRITE, substitutes high spatial frequencies from late FID points into k-spaces of limited extent constructed from early FID points. In this way, images of high quality and resolution can be obtained despite a large range of zoom factors used to reconstruct images with the same FOV and resolution.

Improvements in MR imaging of solids through gradient waveform optimization

Magnetic resonance imaging (MRI) is known to provide a useful approach for the exploration of the chemistry and dynamics of a wide range of soft condensed materials. However, its application to solids has been limited to those materials with relatively narrow resonances. The time needed to obtain an image of a solid with a given resolution and signal-to-noise ratio (SNR) is directly proportional to the line width of the resonance. For MRI to become practical for the imaging of solids it will have to rely on the development and use of MR sequences that avoid the issues raised by line broadening of the resonance. In this paper we review the latest contributions towards MR imaging of solids from our laboratory, in particular, applications using optimized gradient waveforms. Acoustic noise reduction and SNR improvement obtained with modifications of the standard single-point imaging sequence are presented and discussed using examples.

Sodium MRI

Sodium ((23)Na) imaging has a place somewhere between (1)H-MRI and MR spectroscopy (MRS). Like MRS it potentially provides information on metabolic processes, but only one single resonance of ionic (23)Na is observed. Therefore pulse sequences do not need to code for a chemical shift dimension, allowing (23)Na images to be obtained at high resolutions as compared to MRS. In this chapter the biological significance of sodium in the brain will be discussed, as well as methods for observing it with (23)Na-MRI. Many vital cellular processes and interactions in excitable tissues depend on the maintenance of a low intracellular and high extracellular sodium concentration. Healthy cells maintain this concentration gradient at the cost of energy. Leaky cell membranes or an impaired energy metabolism immediately leads to an increase in cytosolic total tissue sodium. This makes sodium a biomarker for ischemia, cancer, excessive tissue activation, or tissue damage as might be caused by ablation therapy. Special techniques allow quantification of tissue sodium for the monitoring of disease or therapy in longitudinal studies or preferential observation of the intracellular component of the tissue sodium. New methods and high-field magnet technology provide new opportunities for (23)Na-MRI in clinical and biomedical research.

Quadrupolar-coupling-specific binomial pulse sequences for in vivo 23Na NMR and MRI

Aimed at selective detection of (23)Na with specific quadrupolar couplings for in vitro NMR and MRI, we present a series of quadrupolar binomial pulse sequences offering high specificity with respect to the quadrupolar couplings of the excited species. It is demonstrated that pulse sequences with an increasing number of elements, e.g., 11, 121, 1331, 14641, and 15101051, with the units representing flip angles smaller than the 90 degrees pulses typically encountered in binomial spin-1/2 solvent suppression experiments, and different phase combinations may provide a high degree of flexibility with respect to quadrupolar coupling selectivity and robustness towards rf inhomogeneity. This may facilitate efficient separation of, for example, intra and extracellular (23)Na in tissues with efficient control of the excitation (or suppression) of central as well as satellite transitions through on- and off-resonance irradiation. The pulse sequences are described in terms of their analogy to binomial liquid-state NMR solvent suppression experiments and demonstrated numerically and experimentally through NMR and MRI experiments on a 7 T horizontal small-bore animal magnet system.

Imaging of multiphase fluid saturation within a porous material via sodium NMR

We present in this paper a method to monitor multiphase fluid core saturation through measurement of the sodium NMR signal. In a rock core saturated with water and oil, sodium will be present only in the water phase, and therefore can be used to separate the two fluids. Two dimensional sodium images were taken to monitor the movement of brine into oil saturated rock cores. The measured fluid exchange agrees well with expected behavior from traditional core analysis methods. Indications of damage to the rock structure can be seen from the patterns of fluid imbibition.

Repetition time and flip angle variation in SPRITE imaging for acquisition time and SAR reduction

Single point imaging methods such as SPRITE are often the technique of choice for imaging fast-relaxing nuclei in solids. Single point imaging sequences based on SPRITE in their conventional form are ill-suited for in vivo applications since the acquisition time is long and the SAR is high. A new sequence design is presented employing variable repetition times and variable flip angles in order to improve the characteristics of SPRITE for in vivo applications. The achievable acquisition time savings as well as SAR reductions and/or SNR increases afforded by this approach were investigated using a resolution phantom as well as PSF simulations. Imaging results in phantoms indicate that acquisition times may be reduced by up to 70% and the SAR may be reduced by 40% without an appreciable loss of image quality.

Quantitative 23Na magnetic resonance imaging of model foods

Partial (23)Na MRI invisibility in muscle foods is often referred to as an inherent drawback of the MRI technique, impairing quantitative sodium analysis. Several model samples were designed to simulate muscle foods with a broad variation in protein, fat, moisture, and salt content. (23)Na spin-echo MRI and a recently developed (23)Na SPRITE MRI approach were compared for quantitative sodium imaging, demonstrating the possibility of accurate quantitative (23)Na MRI by the latter method. Good correlations with chemically determined standards were also obtained from bulk (23)Na free induction decay (FID) and CPMG relaxation experiments on the same sample set, indicating their potential use for rapid bulk NaCl measurements. Thus, the sodium MRI invisibility is a methodological problem that can easily be circumvented by using the SPRITE MRI technique.

Fast quantitative mapping of absolute water content with full brain coverage

Quantitative mapping of water content, especially in the human brain, has the potential to provide important information for the study and diagnosis of diseases associated with a focal or global change in tissue water homeostasis. In the current work, an imaging method for the precise and accurate quantification of tissue water content is presented. The method allows the acquisition of water content maps with voxel dimensions of 1x1x2 mm(3) and full brain coverage in less than 10 min on a standard clinical 1.5 T scanner. The precision was optimised for human brain imaging and possible sources of systematic error were carefully investigated, demonstrating the ability of the method to quantify water content with high accuracy and precision. The approach was validated in phantom experiments and quantitative cerebral water content maps of a group of 10 healthy volunteers were obtained.

Sodium renal imaging in mice at high magnetic fields

This work presents the first sodium MRI functional renal study on a mouse model. The tissue sodium concentration was monitored during induced diuresis with furosemide. By using density-weighted chemical shift imaging (DWCSI) at high field strength a temporal resolution of less than 5 min for three dimensional (3D) data sets with high spatial resolution was achieved. A maximum increase of 20% in the cortex and a decrease of 45% of the original signal strength in the medulla were observed. These findings correspond well with experiments conducted on much larger rodent models.

(1)H and (23)Na MRI studies of Atlantic salmon (Salmo salar) and Atlantic cod (Gadus morhua) fillet pieces salted in different brine concentrations

BACKGROUND

Applicability of magnetic resonance imaging (MRI) to quantitative analysis of sodium in salted fish products is usually impaired by the partial (23)Na MRI 'invisibility' phenomena as well as high investment costs of the MRI equipment.

RESULTS

Salmon and cod fillet pieces, unsalted and brine salted (50, 100, 150, 200 and 250 g kg(-1) NaCl) for 48 h, were studied using (1)H and (23)Na MRI. Based on MRI results, T(1) and T(2) relaxation times were calculated for (1)H and the T(2) time for (23)Na nuclei. In addition, water diffusion images for all fillet samples and reference brine solutions were obtained. Variation of the nuclear magnetic resonance relaxation times and water diffusion constants with brine concentration is discussed in terms of the muscle structural changes. Sodium MRI visibility factors for the MRI method used were determined for all fish samples.

CONCLUSION

Observed changes in proton and sodium NMR relaxation times with the salt content reflect complex counteraction of several factors related to the muscle structural changes. Sodium MRI visibility factors appear to be dependent on a number of experimental factors in a complex matter, making quantitative sodium analysis by the MRI technique used non-trivial. Copyright © 2007 Society of Chemical Industry.

Single-point imaging with a variable phase encoding interval

A modified single-point imaging (SPI) technique using a variable phase encoding interval is proposed. This method is based on the minimization of the phase encoding interval for further signal-to-noise ratio (SNR) optimization. This is particularly beneficial when the maximum gradient amplitude limits an optimal phase encoding interval, and the resulting SNR suffers from T(2)-related signal attenuation. Theoretical calculation of the SNR and simulation of the point spread function (PSF) for the different experimental parameters are presented. Experiments using a rubber sample (T(2)* approximately 73 micros) and a tooth (bi-exponential relaxation: T(2,1)*=111 micros and T(2,1)*=872 micros) showed a significant increase in SNR (>3 and >2, respectively) when compared with images acquired with conventional SPI.

Clinical ultrashort echo time imaging of bone and other connective tissues

The background underpinning the clinical use of ultrashort echo time, SPRITE and other pulse sequences for imaging bone and other connective tissues with short T2 is reviewed. Features of the basic physics relevant to UTE imaging are described, including the consequences when the radiofrequency pulse duration is of the order of T2 so that rotation of tissue magnetization into the transverse plane is incomplete. Consequences of the broad linewidth of short T2 components are also discussed, including partial saturation by off-resonance fat suppression pulses as well as those used in multislice and multiecho imaging. The need for rapid data acquisition of the order of T2 is explained. The basic two-dimensional UTE pulse sequence with its half excitation pulse and radial imaging from the centre of k-space is described, together with options that suppress fat and/or reduce the signal from long T2 components. The basic features of SPRITE and other sequences with very short TE are described. Image interpretation is discussed. Clinical features of the imaging of cortical bone, tendons, ligaments, menisci, periosteum and the spine are illustrated. The source of the short T2 signal in these tissues is predominantly collagen and water tightly bound to collagen. Short T2 components in all of these tissues are detectible and may show high signals. Possible future developments are outlined, as are technical limitations of clinical magnetic resonance systems.