Magnetic resonance: an introduction to ultrashort TE (UTE) imaging

The background underpinning the clinical use of ultrashort echo-time (UTE) pulse sequences for imaging tissues or tissue components with short T2s is reviewed. Tissues properties are discussed, and tissues are divided into those with a majority of short T2 relaxation components and those with a minority. Features of the basic physics relevant to UTE imaging are described including the fact that when the radiofrequency pulse duration is of the order T2, rotation of tissue magnetization into the transverse plane is incomplete. Consequences of the broad line-width of short T2 components are also discussed including their partial saturation by off-resonance fat suppression pulses as well as multislice and multiecho imaging. The need for rapid data acquisition of the order T2 is explained. The basic UTE pulse sequence with its half excitation pulse and radial imaging from the center of k-space is described together with options that suppress fat and/or long T2 components. Image interpretation is discussed. Clinical features of the imaging of cortical bone, tendons, ligaments, menisci, and periosteum as well as brain, liver, and spine are illustrated. Short T2 components in all of these tissues may show high signals. Possible future developments are outlined as are technical limitations.     

Contrast enhancement of short T2 tissues using ultrashort TE (UTE) pulse sequences

AIM: To review the effects of contrast administration on tissues with short T2s using a pulse ultrashort echo time (UTE) sequence. MATERIALS AND METHODS: Pulse sequences were implemented with echo times of 0.08 ms and three later gradient echoes. A fat-suppression option was used and later echo images were subtracted from the first echo image. Contrast enhancement with gadodiamide (0.3 mmol/kg) was used for serial studies in a volunteer. The images of 10 patients were reviewed for evidence of contrast enhancement in short T2 tissues. RESULTS: Contrast enhancement was seen in normal meninges, falx, tendons, ligaments, menisci, periosteum and cortical bone. In addition more extensive enhancement than with conventional pulse sequences was seen in meningeal disease, intervertebral disc disease, periligamentous scar tissue and periosteum after fracture. Subtraction of an image taken with a longer TE from the first image was of value in differentiating enhancement in short T2 tissues from that in long T2 tissues or blood. CONCLUSION: Contrast enhancement can be identified in tissues with short T2s using UTE pulse sequences in health and disease.     

Magnetic resonance imaging of the Achilles tendon using ultrashort TE (UTE) pulse sequences

AIM: To assess the potential value of imaging the Achilles tendon with ultrashort echo time (UTE) pulse sequences. MATERIALS AND METHODS: Four normal controls and four patients with chronic Achilles tendinopathy were examined in the sagittal and transverse planes. Three of the patients were examined before and after intravenous gadodiamide. RESULTS: The fascicular pattern was clearly demonstrated within the tendon and detail of the three distinct fibrocartilaginous components of an "enthesis organ" was well seen. T2* measurements showed two short T2* components. Increase in long T2 components with reduction in short T2 components was seen in tendinopathy. Contrast enhancement was much more extensive than with conventional sequences in two cases of tendinopathy but in a third case, there was a region of reduced enhancement. CONCLUSION: UTE pulse sequences provide anatomical detail not apparent with conventional sequences, demonstrate differences in T2* and show patterns of both increased and decreased enhancement in tendinopathy.     

Magnetic resonance imaging of entheses using ultrashort TE (UTE) pulse sequences

The attachment of tendons, ligaments, and joint capsule to bone (entheses) is reviewed and new options for visualizing key components of entheses provided by ultrashort TE (UTE) pulse sequences are described. Many features of entheses are adapted to the dispersion of stress at the boundary between tendons/ligaments and bone. Of particular interest is fibrocartilage, which has mechanical properties different from those of both "pure" tendon/ligament and bone. Features typical of entheses can also be seen at sites where tendons or ligaments are in contact with (but not attached to) bone, and the concept of a "functional enthesis" has been developed to emphasize the similarities. The enthesis concept has also been broadened to include the idea of an "enthesis organ" in which many tissues play a role in dissipating stress concentration. UTE pulse sequences can specifically identify the calcified and uncalcified fibrocartilage tissue components of entheses and differentiate these from fibrous connective tissue and bone. These tissues cannot be separately visualized at entheses with conventional pulse sequences. Entheses are involved in overuse syndromes and seronegative spondyloarthropathies (SpA) and there are important issues related to tissue repair and healing following surgery.     

Comparison of optimized soft-tissue suppression schemes for ultrashort echo time MRI

Ultrashort echo time (UTE) imaging with soft-tissue suppression reveals short-T(2) components (typically hundreds of microseconds to milliseconds) ordinarily not captured or obscured by long-T(2) tissue signals on the order of tens of milliseconds or longer. Therefore, the technique enables visualization and quantification of short-T(2) proton signals such as those in highly collagenated connective tissues. This work compares the performance of the three most commonly used long-T(2) suppression UTE sequences, i.e., echo subtraction (dual-echo UTE), saturation via dual-band saturation pulses (dual-band UTE), and inversion by adiabatic inversion pulses (IR-UTE) at 3 T, via Bloch simulations and experimentally in vivo in the lower extremities of test subjects. For unbiased performance comparison, the acquisition parameters are optimized individually for each sequence to maximize short-T(2) signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) between short- and long-T(2) components. Results show excellent short-T(2) contrast which is achieved with these optimized sequences. A combination of dual-band UTE with dual-echo UTE provides good short-T(2) SNR and CNR with less sensitivity to B(1) homogeneity. IR-UTE has the lowest short-T(2) SNR efficiency but provides highly uniform short-T(2) contrast and is well suited for imaging short-T(2) species with relatively short T(1) such as bone water.     

Magnetic resonance imaging of short T2 components in tissue

The most widely used clinical magnetic resonance imaging techniques for the diagnosis of parenchymal disease employ heavily T(2)-weighted sequences to detect an increase or decrease in the signal from long T(2) components in tissue. Tissues also contain short T(2) components that are not detected or only poorly detected with conventional sequences. These components are the majority species in tendons, ligaments, menisci, periosteum, cortical bone and other related tissues, and the minority in many other tissues that have predominantly long T(2) components.The development and clinical application of techniques to detect short T(2) components are just beginning. Such techniques include magic angle imaging, as well as short echo time (TE), and ultrashort TE (Ute) pulse sequences. Magic angle imaging increases the T(2) of highly ordered, collagen-rich tissues such as tendons and ligaments so signal can be detected from them with conventional pulse sequences. Ute sequences detect short T(2) components before they have decayed, both in tissues with a majority of short T(2) components and those with a minority. In the latter case steps usually need to be taken to suppress the signal from the majority of long T(2) components. Fat suppression of different types may also be helpful. Once signal from short T(2) components has been detected, different pulse sequences can be used to determine increases or decreases in T(1) and T(2) and study contrast enhancement.Using these approaches, signals have been detected from normal tissues with a majority of short T(2) components such as tendons, ligaments, menisci, periosteum, cortical bone, dentine and enamel (the latter four tissues for the first time) as well as from the other tissues in which short T(2) components are a minority. Some diseases such as chronic fibrosis, gliosis, haemorrhage and calcification may increase the signal from short T(2) components while others such as loss of tissue, loss of order in tissue and an increase in water content may decrease them. Changes of these types have been demonstrated in tendonopathy, intervertebral disc disease, ligament injury, haemachromatosis, pituitary perivascular fibrosis, gliomas, multiple sclerosis and angiomas.Use of these techniques has reduced the limit of clinical detectability of short T(2) components by about two orders of magnitude from about 10 ms to about 100 micros. As a consequence it is now possible to study tissues that have a majority of short T(2) components with both "bright" and "dark" approaches, with the bright (high signal) approach offering options for developing tissue contrast of different types, as well as the potential for tissue characterization. In addition, tissues with a minority of short T(2) components may demonstrate changes in disease that are not apparent with conventional heavily T(2)-weighted sequences.     

Magnetic resonance imaging of the liver with ultrashort TE (UTE) pulse sequences

PURPOSE: To assess the feasibility of imaging the liver in volunteers and patients with ultrashort echo time (UTE) pulse sequences. MATERIALS AND METHODS: Seven normal controls as well as 12 patients with biopsy-proven generalized liver disease and three patients with focal disease were examined using pulse sequences with initial TEs of 0.08 msec followed by three later echoes, with or without frequency-based fat suppression. T(2)* values were calculated from regions of interest in the liver. RESULTS: Good image quality was obtained in each subject. There was a highly significant difference in the mean T(2)* values between the normal controls and patients with generalized liver disease (P = 0.001). T(2)* was significantly decreased in hemochromatosis (P = 0.002) and increased in cirrhosis (P = 0.04), compared with controls. T(2)* also correlated with functional status assessed by Child's grade (P = 0.001). A hepatocellular carcinoma showed reduced short T(2) components in the region of thermal ablation and evidence of a subcapsular hematoma which were not apparent with conventional imaging. CONCLUSIONS: Imaging of the liver with UTE sequences showed good image quality and tolerance of abdominal motion. T(2)* was specifically correlated with the presence of hemochromatosis, cirrhosis, and functional grade. Imaging of short T(2) relaxation components may provide useful information in disease.     

MRI超短回波时间序列评价关节软骨的研究进展

关节软骨损伤、变性是骨关节炎(OA)的首要因素,OA早期软骨生化成分最先发生改变,软骨结构逐渐破坏。传统MRI序列的回波时间(TE)较长,采集不到短T_2/T_2~*软骨组织的信号,而超短回波时间(UTE)序列采用超短TE可实现对短T_2/T_2~*的深层软骨和软骨钙化层的直接观察及对其生化组成的定量研究,在探讨OA发生机制及早期诊断中有较好的应用前景。对UTE序列在关节软骨变性早期的定量研究进行综述。     

Ultrashort echo time (UTE) MRI of the spine in thalassaemia

Back pain is common in adult patients with homozygous thalassaemia, and degenerative disc disease is increasingly recognised as a cause. Ultrashort echo time (UTE) pulse sequences, which are sensitive to the presence of short T(2) relaxation components in tissue produced by iron deposition and other processes, were used to examine the lower thoracic and lumbar spine in symptomatic patients with beta-thalassaemia major or intermedia. Three patients were studied with fat suppressed as well as both fat suppressed and long T(2) suppressed UTE (TE=0.08 ms) pulse sequences. Conventional 2D Fourier transformation T(1) and T(2) weighted scans were also performed for comparison. Normal controls showed narrow high signal areas in the region of the end-plate and annulus fibrosus. Patients showed hyperintense bands adjacent to the vertebral end plate in lower thoracic and lumbar spine discs using a UTE sequence with both long T(2) component and fat suppression. The extent of the changes was most marked in the patient with the most severe degenerative change. In the patient with minimal disease, findings of this type were present in discs which did not show evidence of degeneration with conventional MR imaging. High signal changes of a type previously not described were observed in each patient. The effect may be due to organic iron entering the disc and decreasing its T(1) and T(2), but susceptibility effects from iron in the vertebral bodies, fibrosis and other causes also need to be considered.     

MRI超短回波时间序列评价膝关节半月板的研究进展

半月板是膝关节的重要结构,MRI超短回波时间（UTE）序列相比传统MRI序列能够直接采集短T₂/T₂*信号,不仅能够定量测量半月板,还能够显示半月板胶原纤维及钙化成分,区分半月板红白区,以及评估半月板pH值等。简要介绍UTE序列,并就其在半月板中的研究进展予以综述。     

影响短T2成分MR三维超短回波时间双回波脉冲序列成像质量因素的探讨

目的 探讨3D超短回波时间(UTE)舣回波脉冲序列成像的相关成像参数及后处理技术对图像质量的影响.方法 对主要含短T2成分的人于燥股骨标本及一组健康志愿者的胫骨、膝关节、踝部肌腱行MR 3D UTE舣回波脉冲序列成像.通过计算、比较图像的信噪比(SNR)或对比噪声比(CNR)及对图像伪影的分析,探讨系统内部不同轨道延迟时间(-6、-3、-2、-1、0、1、2、3 s)、不同反转角(4°、8°、12°、16°、20°、24°)、不同TE1(0.08、0.16、0.24、0.35 ms)及不同后处理技术(超短回波减影差异图、容积超短回波减影差异图)对图像质量的影响.结果 骨皮质、骨膜、半月板、肌腱、韧带等在UTE图像上表现为高信号.所设的不同轨道延迟时间中,获得最佳SNR的轨道延迟时阳间为2 s.活体人UTE成像的最佳反转角为8°～12°.不同TE1时间的图像质量不同,TE1为0.08 ms时,图像的CNR最佳.随TE1时阳延长,图像伪影逐渐增多.将原始双回波图经多平面重组后再相减(容积超短回波减影差异图),图像SNR明显增加.结论 短T2成分在3D UTE双回波脉冲序列成像上表现为高信号.通过改变反转角和将2次回波图像经MPR后再相减可增加图像SNR.缩短TE1时间可增加图像质量.

Abstract：

Objective To investigate the effect of imaging parameters and postprocessing methods on the quality of MR imaging of short T2 components with 3D ultrashort TE (UTE) double echo pulse sequence. Methods 3D UTE double echo pulse sequence was performed on dry human femoral specimen and the tibial diaphyses, knee joints, and tendons of ankles of a group of healthy volunteers. To investigate the effect of different trajectory delays of the imaging system(-6, -3, -2, - 1,0, 1,2, 3 s), different flip angles(4°, 8°, 12°, 16°, 20°, 24°), different TEs (0. 08, 0. 16, 0. 24, 0. 35 ms)and different postprocessing methods(difference imaging of subtracted volume and non-volume UTE)on the 3D UTE MR imaging quality, the SNR and CNR were calculated and compared, and the artifacts of the images were analysed. Results The cortical bone, periosteum, tendon and meniscus showed high signal intensity on the images of UTE pulse sequence. The best SNR was acquired with 2 s trajectory delay. The best flip angle was 8° to 12° for the human UTE imaging in vivo. The highest CNR was obtained from the TE of 0. 08 ms. The longer the TE was, the more artifacts appeared. The SNR of difference imagewas improved when image subtraction was performed afer multiplanar reconstruction (MPR) of the primary double echo images.Conclusions The short T2 components show high signal intensity on the MRI of 3D UTE double echo pulse sequence. The imaging quality can be improved by shortening TE, using appropriate flip angle and performing subtraction for difference image after MPR of the primary double echo images.     

Validity and reliability of measurements of aponeurosis dimensions from magnetic resonance images

Muscle performance is closely related to the structure and function of tendons and aponeuroses, the sheet‐like, intramuscular parts of tendons. The architecture of aponeuroses has been difficult to study with magnetic resonance imaging (MRI) because these thin, collagen‐rich connective tissues have very short transverse relaxation (T2) times and therefore provide a weak signal with conventional MRI sequences. Here, we validated measurements of aponeurosis dimensions from two MRI sequences commonly used in muscle‐tendon research (mDixon and T1‐weighted images), and an ultrashort echo time (UTE) sequence designed for imaging tissues with short T2 times. MRI‐based measurements of aponeurosis width, length, and area of 20 sheep leg muscles were compared to direct measurements made with three‐dimensional (3D) quantitative microdissection. The errors in measurement of aponeurosis width relative to the mean width were 1.8% for UTE, 3.7% for T1, and 18.8% for mDixon. For aponeurosis length, the errors were 7.6% for UTE, 1.9% for T1, and 21.0% for mDixon. Measurements from T1 and UTE scans were unbiased, but mDixon scans systematically underestimated widths, lengths, and areas of the aponeuroses. Using the same methods, we then found high inter‐rater reliability (intraclass correlation coefficients >0.92 for all measures) of measurements of the dimensions of the central aponeurosis of the human tibialis anterior muscle from T1‐weighted scans. We conclude that valid and reliable measurements of aponeurosis dimensions can be obtained from UTE and from T1‐weighted scans. When the goal is to study the macroscopic architecture of aponeuroses, UTE does not hold an advantage over T1‐weighted imaging.     

MRI of the brain with ultra-short echo-time pulse sequences

As well as the long-T2 relaxation components normally detected with conventional imaging techniques, the brain has short-T2 components. We wished to use ultra-short (0.08 ms) echo time (UTE) pulse sequences to assess the feasibility of imaging these in normal subjects and patients. UTE sequences were employed, with or without fat suppression, 90 degree long-T2 suppression pulses, and selective nulling of long-T2 components using an inversion pulse. Subtraction of later echoes from the first was also used to reduce the signal from long-T2 components. We studied dive normal subjects and 15 patients with various diseases. Short-T2 components were demonstrated in grey and white matter. Increased signal from these components was seen in meningeal disease, probable calcification, presumed cavernomas, melanoma metastases and probable gliosis. Reduced signal was seen in some tumours, infarcts, mild multifocal vascular disease and vasogenic oedema. Further development and evaluation of these pulse sequences is warranted.     

Using adiabatic inversion pulses for long-T2 suppression in ultrashort echo time (UTE) imaging.

Ultrashort echo time (UTE) imaging is a technique that can visualize tissues with sub-millisecond T(2) values that have little or no signal in conventional MRI techniques. The short-T(2) tissues, which include tendons, menisci, calcifications, and cortical bone, are often obscured by long-T(2) tissues. This paper introduces a new method of long-T(2) component suppression based on adiabatic inversion pulses that significantly improves the contrast of short-T(2) tissues. Narrow bandwidth inversion pulses are used to selectively invert only long-T(2) components. These components are then suppressed by combining images prepared with and without inversion pulses. Fat suppression can be incorporated by combining images with the pulses applied on the fat and water resonances. Scaling factors must be used in the combination to compensate for relaxation during the preparation pulses. The suppression is insensitive to RF inhomogeneities because it uses adiabatic inversion pulses. Simulations and phantom experiments demonstrate the adiabatic pulse contrast and how the scaling factors are chosen. In vivo 2D UTE images in the ankle and lower leg show excellent, robust long-T(2) suppression for visualization of cortical bone and tendons.     

Ultrashort echo time spectroscopic imaging (UTESI) of cortical bone.

Cortical bone in the mature skeleton has a short T(2)* and produces no detectable signal with conventional MR sequences. A two-dimensional ultrashort echo time (UTE) sequence employing half radio frequency (RF) pulse excitations and radial ramp sampling reduces the effective TE to 8 micros and is capable of detecting signals from cortical bone. We propose a time-efficient UTE spectroscopic imaging (UTESI) technique based on an interleaved variable TE acquisition, preceded by long T(2)* signal suppression using either a 90 degrees pulse and gradient dephasing or an inversion pulse and nulling. The projections were divided into multiple groups with the data for each group being collected with progressively increasing TE and interleaved projection angles. The undersampled projections within each group sparsely covered k-space. A view sharing and sliding window reconstruction algorithm was implemented to reconstruct images at each TE, followed by Fourier transformation in the time domain to generate spectroscopic images. T(2)* was quantified through either exponential fitting of the time domain images or line fitting of the magnitude spectrum. Relative water content and the resonance frequency shift due to bulk susceptibility were also evaluated. The feasibility of this technique was demonstrated with phantom and volunteer studies on a clinical 3T scanner.     

UTE的基本原理及在骨关节系统中的应用进展

骨关节系统主要由短T_2组织构成,在常规MRI检查中常表现为低信号或无信号。超短回波时间(UTE)序列是研究短T_2组织最常用的成像技术,短T_2组织在UTE影像上表现为高信号。对UTE成像技术的基本原理进行介绍,并综述其在骨皮质、骨膜、肌腱和韧带、关节软骨和半月板中的具体应用。     

Ultrashort echo time imaging with bicomponent analysis

Biological tissues frequently contain different water compartments, and these often have distinct transverse relaxation times. Quantification of these may be problematic on clinical scanners because spin echo sequences usually have initial echo times that are too long to accurately quantify shorter relaxation time components. In this study, an ultrashort echo time pulse sequence was used together with bicomponent analysis to quantify both the short and long T(2) components in tissues of the musculoskeletal system. Feasibility studies were performed using numerical simulation, and on phantoms and in vitro tissues including bovine cortical bone, ligaments, menisci, tendons, and articular cartilage. The simulation and phantom studies demonstrated that this technique can quantify T(2) * and fractions of the short and long T(2) components. The tissues studies showed two distinct components with short T(2) *s ranging from 0.3 ms for bovine cortical bone to 2.1 ms for menisci, and long T(2) *s ranging from 2.9 ms for bovine cortical bone to 35.0 ms for articular cartilage. The short T(2) * fraction ranged from 18.5% for patella cartilage to 80.9% for ligaments. The results show that ultrashort echo time imaging with bicomponent analysis can quantify the short and long T(2) water components in vitro in musculoskeletal tissues.     

UTE-MRI常用技术在软骨定量研究中的应用进展

骨关节炎（OA）最初的病理生理变化主要发生在以短T₂成分为主的关节软骨,其信号衰减较快,因此常规MRI序列对早期OA的研究价值有限。MR超短回波时间（UTE）序列可以对软骨进行全面的定量评估。就UTE-T₂* mapping、UTE Adibatic-T_1ρ、UTE-磁化传递（MT）以及酸性化学交换饱和转移（acidoCEST）-UTE等软骨成分定量研究技术的基本原理、优缺点及其应用进展进行综述。     

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.     

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 .