Comparison of two ultrashort echo time sequences for the quantification of T(1) within phantom and human Achilles tendon at 3 T

Ultrashort echo time (UTE) techniques enable direct imaging of musculoskeletal tissues with short T₂ allowing measurement of T₁ relaxation times. This article presents comparison of optimized 3D variable flip angle UTE (VFA‐UTE) and 2D saturation recovery UTE (SR‐UTE) sequences to quantify T₁ in agar phantoms and human Achilles tendon. Achilles tendon T₁ values for asymptomatic volunteers were compared to Achilles tendon T₁ values calculated from patients with clinical diagnoses of spondyloarthritis (SpA) and Achilles tendinopathy using an optimized VFA‐UTE sequence. T₁ values from phantom data for VFA‐ and SR‐UTE compare well against calculated T₁ values from an assumed gold standard inversion recovery spin echo sequence. Mean T₁ values in asymptomatic Achilles tendon were found to be 725 ± 42 ms and 698 ± 54 ms for SR‐ and VFA‐UTE, respectively. The patient group mean T₁ value for Achilles tendon was found to be 957 ± 173 ms (P < 0.05) using an optimized VFA‐UTE sequence with pulse repetition time of 6 ms and flip angles 4, 19, and 24°, taking a total 9 min acquisition time. The VFA‐UTE technique appears clinically feasible for quantifying T₁ in Achilles tendon. T₁ measurements offer potential for detecting changes in Achilles tendon due to SpA without need for intravenous contrast agents. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

k‐space water‐fat decomposition with T2* estimation and multifrequency fat spectrum modeling for ultrashort echo time imaging

Purpose:

To demonstrate the feasibility of combining a chemical shift‐based water‐fat separation method (IDEAL) with a 2D ultrashort echo time (UTE) sequence for imaging and quantification of the short T₂ tissues with robust fat suppression.

Materials and Methods:

A 2D multislice UTE data acquisition scheme was combined with IDEAL processing, including T₂* estimation, chemical shift artifacts correction, and multifrequency modeling of the fat spectrum to image short T₂ tissues such as the Achilles tendon and meniscus both in vitro and in vivo. The integration of an advanced field map estimation technique into this combined method, such as region growing (RG), is also investigated.

Results:

The combination of IDEAL with UTE imaging is feasible and excellent water‐fat separation can be achieved for the Achilles tendon and meniscus with simultaneous T₂* estimation and chemical shift artifact correction. Multifrequency modeling of the fat spectrum yields more complete water‐fat separation with more accurate correction for chemical shift artifacts. The RG scheme helps to avoid water‐fat swapping.

Conclusion:

The combination of UTE data acquisition with IDEAL has potential applications in imaging and quantifying short T₂ tissues, eliminating the necessity for fat suppression pulses that may directly suppress the short T₂ signals. J. Magn. Reson. Imaging 2010;31:1027–1034. ©2010 Wiley‐Liss, Inc.     

Quantitative magnetization transfer ultrashort echo time imaging of the Achilles tendon

Ultrashort echo time imaging allows the short T₂ Achilles tendon to be directly visualized with MRI. Radiofrequency saturation 1 kHz or less off‐resonance has been used previously to improve image contrast. In this study, magnetization transfer was investigated in the Achilles tendon of eight normal volunteers and one patient with psoriatic arthritis. 2D Ultrashort echo time images were acquired using saturation pulses 2–100 kHz off‐resonance at 4 pulse powers. On‐resonance saturation recovery images were also obtained to estimate T₁. The results were fitted to a two compartment quantitative magnetization transfer model. The estimated bound proton fraction for the eight healthy volunteers was 21.0 ± 1.2% (mean ± standard deviation) compared to 16.4% in the patient with psoriatic arthritis (P < 0.05). The T₂ of the bound protons was measured as 6.1 ± 0.3 μsec in the healthy volunteers and 6.0 μsec in the patient. This technique appears clinically feasible and may be useful for assessing the collagen and water changes which occur in Achilles tendinopathy. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Dual inversion recovery,ultrashort echo time (DIR UTE) imaging: Creating high contrast for short‐T2 species

Imaging of short‐T₂ species requires not only a short echo time but also efficient suppression of long‐T₂ species in order to maximize the short‐T₂ contrast and dynamic range. This paper introduces a method of long‐T₂ suppression using two long adiabatic inversion pulses. The first adiabatic inversion pulse inverts the magnetization of long‐T₂ water and the second one inverts that of fat. Short‐T₂ species experience a significant transverse relaxation during the long adiabatic inversion process and are minimally affected by the inversion pulses. Data acquisition with a short echo time of 8 μs starts following a time delay of inversion time (TI1) for the inverted water magnetization to reach a null point and a time delay of TI2 for the inverted fat magnetization to reach a null point. The suppression of long‐T₂ species depends on proper combination of TI1, TI2, and pulse repetition time. It is insensitive to radiofrequency inhomogeneities because of the adiabatic inversion pulses. The feasibility of this dual inversion recovery ultrashort echo time technique was demonstrated on phantoms, cadaveric specimens, and healthy volunteers, using a clinical 3‐T scanner. High image contrast was achieved for the deep radial and calcified layers of articular cartilage, cortical bone, and the Achilles tendon. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

MR‐based in vivo follow‐up study of Achilles tendon volume and hydration state after ankle‐loading activity

The purpose of this study was to evaluate temporal alterations of the Achilles tendon volume and hydration state after cross‐country‐running. Achilles tendons of six untrained participants were examined on a 3T MR‐scanner before running, immediately afterwards, and in the following 24, 48, and 72 h. Using a 3D‐UTE sequence, caudal (CA) and cranial (CR) mid‐portion tendon areas were examined with off‐resonance saturation ratios (OSR) and T2* relaxation times. Tendon volume was measured with a self‐written Matlab‐based automated contour detection algorithm (AVAT) in submillimeter T2‐weighted MR images. A significant influence of running in caudal (P = 0.017) and cranial OSR values (P = 0.001), tendon volume (P = 0.024), and cranial T2* measurements (P = 0.046), but not in caudal T2* values (P = 0.298) were found. In detail, mean individual OSR and tendon volume measurements demonstrated a similar but inverted course in their values after exercise: initially, OSR values increased after running (and tendon volume decreased), while subsequently a decrease of OSR values (with an increase of tendon volume) could be observed. OSR and tendon volume measurements are able to detect a physiological response of tendons to a mechanical stimulus. After a transient decrease of free water in the Achilles tendon, an increase with a maximum free water content 48 h after ankle loading and a tendency toward normalization after 72 h was found.     

Assessment of anterior cruciate ligament reconstruction using 3D ultrashort echo‐time MR imaging

This work demonstrates the potential of ultrashort TE (UTE) imaging for visualizing graft material and fixation elements after surgical repair of soft tissue trauma such as ligament or meniscal injury. Three asymptomatic patients with anterior cruciate ligament (ACL) reconstruction using different graft fixation methods were imaged at 1.5T using a 3D UTE sequence. Conventional multislice turbo spin‐echo (TSE) measurements were performed for comparison. 3D UTE imaging yields high signal from tendon graft material at isotropic spatial resolution, thus facilitating direct positive contrast graft visualization. Furthermore, metal and biopolymer graft fixation elements are clearly depicted due to the high contrast between the signal‐void implants and the graft material. Thus, the ability of UTE MRI to visualize short‐T₂ tissues such as tendons, ligaments, or tendon grafts can provide additional information about the status of the graft and its fixation in the situation after cruciate ligament repair. UTE MRI can therefore potentially support diagnosis when problems occur or persist after surgical procedures involving short‐T₂ tissues and implants. J. Magn. Reson. Imaging 2009;29:443–448. © 2009 Wiley‐Liss, Inc.     

UTE imaging in the musculoskeletal system

Tissues, such as bone, tendon, and ligaments, contain a high fraction of components with "short" and "ultrashort" transverse relaxation times and therefore have short mean transverse relaxation times. With conventional magnetic resonance imaging (MRI) sequences that employ relatively long echo times (TEs), there is no opportunity to encode the decaying signal of short and ultrashort T₂/T₂* tissues before it has reached zero or near zero. The clinically compatible ultrashort TE (UTE) sequence has been increasingly used to study the musculoskeletal system. This article reviews the UTE sequence as well as various modifications that have been implemented since its introduction. These modifications have been used to improve efficiency or contrast as well as provide quantitative analysis. This article reviews several clinical musculoskeletal applications of UTE. J. Magn. Reson. Imaging 2015;41:870–883 . © 2014 Wiley Periodicals, Inc .     

Assessment of cortical bone with clinical and ultrashort echo time sequences

We describe the use of ultrashort echo time (UTE) sequences and fast spin echo sequences to assess cortical bone using a clinical 3T scanner. Regular two‐ and three‐dimensional UTE sequences were used to image both bound and free water in cortical bone. Adiabatic inversion recovery prepared UTE sequences were used to image water bound to the organic matrix. Two‐dimensional fast spin echo sequences were used to image free water. Regular UTE sequences were used together with bicomponent analysis to measure T*₂s and relative fractions of bound and free water components in cortical bone. Inversion recovery prepared UTE sequences were used to measure the T*₂ of bound water. Saturation recovery UTE sequences were used to measure the T₁ of bone water. Eight cadaveric human cortical bone samples and a lower leg specimen were studied. Preliminary results show two distinct components in UTE detected signal decay, a single component in inversion recovery prepared UTE detected signal decay, and a single component in saturation recovery UTE detected signal recovery. Regular UTE sequences appear to depict both bound and free water in cortical bone. Inversion recovery prepared UTE sequences appear to depict water bound to the organic matrix. Two‐dimensional fast spin echo sequences appear to depict bone structure corresponding to free water in large pores. Magn Reson Med 70:697–704, 2013. © 2012 Wiley Periodicals, Inc.     

Quantitative MRI measurements of the Achilles tendon in spondyloarthritis using ultrashort echo times

Objectives

Tendon involvement is common in spondyloarthritis. The MRI signal from the Achilles tendon has been used to quantify mechanical tendinopathy; however, conventional MRI is limited by the short T₂ of normal tendon. Short and ultrashort echo time (UTE) MRI have the potential to better measure signal intensity reflecting changes in T₂ or gadolinium enhancement. Furthermore, UTE images could be used for normalisation to reduce variability. The aim of this work was to investigate such techniques in patients with spondyloarthritis (SpA).

Methods

The Achilles tendons of 14 healthy volunteers and 24 patients with symptomatic spondyloarthritis were studied. Combined UTE (TE=0.07 ms) and gradient echo (TE=4.9 ms) images were acquired before and after intravenous gadolinium together with pre-contrast gradient echo images (TE=2 ms). The signal intensity from a region of interest in the Achilles tendon above the calcaneus was measured. The relative enhancement at echo times of 0.07 ms (RE_0.1) and 4.9 ms (RE₅) were calculated. The ratios of the signal intensities from both 4.9 ms and 2 ms gradient echo images to the signal intensity from the UTE image were calculated (RTE₅ and RTE₂ respectively).

Results

Interobserver intraclass correlation coefficients were excellent (≥0.97). The contrast-to-noise ratio was higher for enhancement on UTE images than on gradient echo images. RE_0.1, RTE₅ and RTE₂ were significantly higher in SpA patients than controls.

Conclusion

Signal intensity ratios using UTE images allow quantitative measurements to be made which are sensitive to tendon T₂ or contrast enhancement and which are increased in spondyloarthritis. They therefore have the potential for use as measures of tendon disease in spondyloarthritis.A cardinal manifestation of seronegative spondyloarthritis (SpA) is inflammation at tendons or ligaments near their insertions, which is well described at many disease sites [1]; the Achilles tendon is the largest such structure in the body. SpA is associated with various MRI findings, including changes within and around the tendon near its insertion, and erosion and oedema of the adjacent bone [2].Conventional MRI has been used to study the Achilles tendon in patients with mechanical tendinopathy and scoring systems have been devised that correlate with surgical outcome [3]. Techniques for quantifying Achilles tendinopathy in these patients based on signal intensity have been shown to correlate with pain and functional impairment [4]. However, measurements of signal intensity from conventional MRI are limited by the short T₂ of the normal Achilles tendon (1–2 ms) [5-7], which limits the detection of subtle increases in T₂ or contrast enhancement [8]. Short echo time (STE) gradient echo images can detect small increases in T₂ which may occur in mild or early tendinopathy [9]. Ultrashort echo time (UTE) techniques reduce the time between excitation and acquisition still further, to under 100 μs, directly visualising the tendon and enabling the demonstration of contrast enhancement despite the short T₂ relaxation times of the normal tendon.Single measurements of signal intensity are prone to variation (e.g. due to radiofrequency coil inhomogeneities, coil loading and variation between patients). Unenhanced UTE images are insensitive to changes in T₂ due to their short effective echo times and can therefore be used to normalise contrast-enhanced or STE images to give a ratio that is strongly dependent on only contrast enhancement or T₂*.The aim of this work was to investigate such quantitative measurements from the Achilles tendon and to apply them in patients with symptomatic SpA.     

Radiofrequency pulses for simultaneous short T2 excitation and long T2 suppression

Magnetic resonance imaging of short T₂ musculoskeletal tissues such as ligaments, tendon, and cortical bone often requires specialized pulse sequences to detect sufficiently high levels of signal, as well as additional techniques to suppress unwanted long T₂ signals. We describe a specialized radiofrequency technique for imaging short T₂ tissues based on applying hard 180° radiofrequency excitation pulses to achieve simultaneous short T₂ tissue excitation and long T₂ tissue signal suppression for three‐dimensional ultrashort echo time applications. A criterion for the pulse duration of the 180° radiofrequency pulses is derived that allows simultaneous water and fat suppression. This opens up possibilities for direct imaging of short T₂ tissues, without the need for additional suppression techniques. Bloch simulations and experimental studies on short T₂ phantoms and specimen were used to test the sequence performance. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Ultrashort echo time MR imaging with off‐resonance saturation for characterization of pathologically altered Achilles tendons at 3 T

Off‐resonance radiofrequency saturation pulses applied prior to regular excitation in MR sequences can be used to modify signal contrast based on magnetization transfer and direct saturation effects. Clinical applicability and value of ultrashort echo time sequences combined with off‐resonance saturation pulses was tested in 16 healthy and 14 tendinopathic as well as paratendinopathic Achilles tendons in vivo at 3 T. A 3D ultrashort echo time sequence in combination with a gaussian off‐resonance saturation pulse (frequency offset: 1000–5000 Hz) was used to modify the detectable MR signal intensity from the Achilles tendon. Off‐resonance saturation ratio was calculated as the relative reduction in signal intensity under selective off‐resonance saturation in relation to a reference measurement without any saturation pulse. Off‐resonance saturation ratio in tendons of healthy volunteers ranged from 0.52 ± 0.06 (1000 Hz) to 0.24 ± 0.02 (5000 Hz), whereas symptomatic tendinopathic tendons (0.35 ± 0.04 to 0.17 ± 0.02) and asymptomatic tendinopathic tendons (0.41 ± 0.06 to 0.21 ± 0.02) showed significantly lower mean off‐resonance saturation ratio values. Off‐resonance saturation ratio values might provide a sensitive and quantitative marker for assessment of pathological microstructure alterations of the Achilles tendon. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

On the dual contrast enhancement mechanism in frequency‐selective inversion‐recovery magnetic resonance angiography (IRON‐MRA)

The susceptibility of blood changes after administration of a paramagnetic contrast agent that shortens T₁. Concomitantly, the resonance frequency of the blood vessels shifts in a geometry‐dependent way. This frequency change may be exploited for incremental contrast generation by applying a frequency‐selective saturation prepulse prior to the imaging sequence. The dual origin of vascular enhancement depending first on off‐resonance and second on T₁ lowering was investigated in vitro, together with the geometry dependence of the signal at 3T. First results obtained in an in vivo rabbit model are presented. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Ultrashort TE T1rho (UTE T1rho) imaging of the Achilles tendon and meniscus

In this study, we report the use of a novel ultrashort echo time T₁rhoT₁ sequence that combines a spin‐lock preparation pulse with a two‐dimensional ultrashort echo time sequence of a nominal echo time 8 μsec. The ultrashort echo time‐T₁rho sequence was employed to quantify T₁rho in short T₂ tissues including the Achilles tendon and the meniscus. T₁rho dispersion was investigated by varying the spin‐lock field strength. Preliminary results on six cadaveric ankle specimens and five healthy volunteers show that the ultrashort echo time‐T₁rho sequence provides high signal and contrast for both the Achilles tendon and the meniscus. The mean T₁rho of the Achilles tendon ranged from 3.06 ± 0.51 msec for healthy volunteers to 5.22 ± 0.58 msec for cadaveric specimens. T₁rho increased to 8.99 ± 0.24 msec in one specimen with tendon degeneration. A mean T₁rho of 7.98 ± 1.43 msec was observed in the meniscus of the healthy volunteers. There was significant T₁rho dispersion in both the Achilles tendon and the meniscus. Mean T₁rho increased from 2.06 ± 0.23 to 7.85 ± 0.74 msec in normal Achilles tendon and from 7.08 ± 0.64 to 13.42 ± 0.93 msec in normal meniscus when the spin‐lock field was increased from 250 to 1,000 Hz. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

MR imaging near metal with undersampled 3D radial UTE‐MAVRIC sequences

Recently developed techniques such as the multiple acquisition with variable resonance image combination and slice encoding for metal artifact correction techniques have improved the ability of clinical magnetic resonance scanners to image near metal implants. These sequences are based on fast spin echo sequences which preclude detection of short T₂ tissues such as tendons, ligaments, and cortical bone. Ultrashort echo time sequences have the potential to detect signals from these tissues. In this study, we investigate the potential of combining ultrashort echo time with multiple acquisition with variable resonance image combination to image short T₂ musculoskeletal tissues adjacent to metallic implants. Different radio frequency excitation pulse types and spectral binning strategies were studied. We found that ultrashort echo time‐multiple acquisition with variable resonance image combination sequences were able to significantly reduce typical artifacts near metal, as well as detect very short T₂ signals that are usually not visualized using clinical pulse sequences. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

T₁-weighted ultrashort echo time method for positive contrast imaging of magnetic nanoparticles and cancer cells bound with the targeted nanoparticles

Purpose

To obtain positive contrast based on T₁ weighting from magnetic iron oxide nanoparticle (IONP) using ultrashort echo time (UTE) imaging and investigate quantitative relationship between positive contrast and the core size and concentration of IONPs.

Materials and Methods

Solutions of IONPs with different core sizes and concentrations were prepared. T₁ and T₂ relaxation times of IONPs were measured using the inversion recovery turbo spin echo (TSE) and multi‐echo spin echo sequences at 3 Tesla. T₁‐weighted UTE gradient echo and T₂‐weighted TSE sequences were used to image IONP samples. U87MG glioblastoma cells bound with arginine‐glycine‐aspartic acid (RGD) peptide and IONP conjugates were scanned using UTE, T₁ and T₂‐weighted sequences.

Results

Positive contrast was obtained by UTE imaging from IONPs with different core sizes and concentrations. The relative‐contrast‐to‐water ratio of UTE images was three to four times higher than those of T₂‐weighted TSE images. The signal intensity increases as the function of the core size and concentration. Positive contrast was also evident in cell samples bound with RGD‐IONPs.

Conclusion

UTE imaging allows for imaging of IONPs and IONP bound tumor cells with positive contrast and provides contrast enhancement and potential quantification of IONPs in molecular imaging applications. J. Magn. Reson. Imaging 2011;33:194–202. © 2010 Wiley‐Liss, Inc.     

Small‐tip fast recovery imaging using non‐slice‐selective tailored tip‐up pulses and radiofrequency‐spoiling

Small‐tip fast recovery (STFR) imaging is a new steady‐state imaging sequence that is a potential alternative to balanced steady‐state free precession. Under ideal imaging conditions, STFR may provide comparable signal‐to‐noise ratio and image contrast as balanced steady‐state free precession, but without signal variations due to resonance offset. STFR relies on a tailored “tip‐up,” or “fast recovery,” radiofrequency pulse to align the spins with the longitudinal axis after each data readout segment. The design of the tip‐up pulse is based on the acquisition of a separate off‐resonance (B0) map. Unfortunately, the design of fast (a few ms) slice‐ or slab‐selective radiofrequency pulses that accurately tailor the excitation pattern to the local B0 inhomogeneity over the entire imaging volume remains a challenging and unsolved problem. We introduce a novel implementation of STFR imaging based on “non‐slice‐selective” tip‐up pulses, which simplifies the radiofrequency pulse design problem significantly. Out‐of‐slice magnetization pathways are suppressed using radiofrequency‐spoiling. Brain images obtained with this technique show excellent gray/white matter contrast, and point to the possibility of rapid steady‐state T₂/T₁‐weighted imaging with intrinsic suppression of cerebrospinal fluid, through‐plane vessel signal, and off‐resonance artifacts. In the future, we expect STFR imaging to benefit significantly from parallel excitation hardware and high‐order gradient shim systems. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Spin‐locked balanced steady‐state free‐precession (slSSFP)

A spin‐locked balanced steady‐state free‐precession (slSSFP) pulse sequence is described that combines a balanced gradient‐echo acquisition with an off‐resonance spin‐lock pulse for fast MRI. The transient and steady‐state magnetization trajectory was solved numerically using the Bloch equations and was shown to be similar to balanced steady‐state free‐precession (bSSFP) for a range of T₂/T₁ and flip angles, although the slSSFP steady‐state could be maintained with considerably lower radio frequency (RF) power. In both simulations and brain scans performed at 7T, slSSFP was shown to exhibit similar contrast and signal‐to‐noise ratio (SNR) efficiency to bSSFP, but with significantly lower power. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

SWIFT‐CEST: A new MRI method to overcome T2 shortening caused by PARACEST contrast agents

The exchange of water molecules between the inner sphere of a paramagnetic chemical exchange saturation transfer (PARACEST) contrast agent and bulk water can shorten the bulk water T₂ through the T₂‐exchange (T_2ex) mechanism. The line‐broadening T_2ex effect is proportional to the agent concentration, the chemical shift of the exchanging water molecule, and is highly dependent on the water molecule exchange rate. A significant T_2ex contribution to the bulk water linewidth can make the regions of agent uptake appear dark when imaging with conventional sequences like gradient‐echo and fast spin‐echo. The minimum echo times for these sequences (1–10 ms) are not fast enough to capture signal from the regions of shortened T₂. This makes “Off” (saturation at ?Δω) minus “On” (saturation at +Δω) imaging of PARACEST agents difficult, because the regions of uptake are dark in both images. It is shown here that the loss of bulk water signal due to T_2ex can be reclaimed using the ultrashort echo times (<10 μs) achieved with the sweep imaging with Fourier transform pulse sequence. Modification of the sweep imaging with Fourier transform sequence for PARACEST imaging is first discussed, followed by parameter optimization using in vitro experiments. In vivo PARACEST studies comparing fast spin‐echo to sweep imaging with Fourier transform were performed using EuDOTA‐(gly) uptake in healthy mouse kidneys. The results show that the negative contrast caused by T_2ex can be overcome using the ultrashort echo time achieved with sweep imaging with Fourier transform, thereby enabling fast and sensitive in vivo PARACEST imaging. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Transverse relaxation time (T2) mapping in the brain with off‐resonance correction using phase‐cycled steady‐state free precession imaging

Purpose

To investigate a new approach for more completely accounting for off‐resonance affects in the DESPOT2 (driven equilibrium single pulse observation of T₂) mapping technique.

Materials and Methods

The DESPOT2 method derives T₂ information from fully balanced steady‐state free precession (bSSFP) images acquired over multiple flip angles. Off‐resonance affects, which present as bands of altered signal intensity throughout the bSSFP images, results in erroneous T₂ values in the corresponding calculated maps. Radiofrequency (RF) phase‐cycling, in which the phase of the RF pulse is incremented along the pulse train, offers a potential method for eliminating these artifacts. In this work we present a general method, referred to as DESPOT2, with full modeling (DESPOT2‐FM), for deriving T₂, as well as off‐resonance frequency, from dual flip angle bSSFP data acquired with two RF phase increments.

Results

The method is demonstrated in vivo through the acquisition of whole‐brain, 1 mm³ isotropic T₂ maps at 3T and shown to provide near artifact‐free maps, even in areas with steep susceptibility‐induced gradients.

Conclusion

DESPOT2‐FM offers an efficient method for acquiring high spatial resolution, whole‐brain T₂ maps at 3T with high precision and free of artifact. J. Magn. Reson. Imaging 2009;30:411–417. © 2009 Wiley‐Liss, Inc.     

Bi-exponential T2* analysis of healthy and diseased Achilles tendons: an in vivo preliminary magnetic resonance study and correlation with clinical score

Objective

To compare mono- and bi-exponential T₂* analysis in healthy and degenerated Achilles tendons using a recently introduced magnetic resonance variable-echo-time sequence (vTE) for T₂* mapping.

Methods

Ten volunteers and ten patients were included in the study. A variable-echo-time sequence was used with 20 echo times. Images were post-processed with both techniques, mono- and bi-exponential [T₂*_m, short T₂* component (T₂*_s) and long T₂* component (T₂*_l)]. The number of mono- and bi-exponentially decaying pixels in each region of interest was expressed as a ratio (B/M). Patients were clinically assessed with the Achilles Tendon Rupture Score (ATRS), and these values were correlated with the T₂* values.

Results

The means for both T₂*_m and T₂*_s were statistically significantly different between patients and volunteers; however, for T₂*_s, the P value was lower. In patients, the Pearson correlation coefficient between ATRS and T₂*s was ?0.816 (P?=?0.007).

Conclusion

The proposed variable-echo-time sequence can be successfully used as an alternative method to UTE sequences with some added benefits, such as a short imaging time along with relatively high resolution and minimised blurring artefacts, and minimised susceptibility artefacts and chemical shift artefacts. Bi-exponential T₂* calculation is superior to mono-exponential in terms of statistical significance for the diagnosis of Achilles tendinopathy.

Key Points

? Magnetic resonance imaging offers new insight into healthy and diseased Achilles tendons ? Bi-exponential T2* calculation in Achilles tendons is more beneficial than mono-exponential ? A short T2* component correlates strongly with clinical score ? Variable echo time sequences successfully used instead of ultrashort echo time sequences