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.     

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.     

Dental MRI: Imaging of soft and solid components without ionizing radiation

Purpose:

To evaluate the ability of conventional and ultra‐short or zero echo time MRI for imaging of soft and solid dental components in and ex vivo.

Materials and Methods:

Turbo spin echo (TSE), ultra‐short echo time (UTE), and zero echo time (ZTE) MRI were performed on extracted (human and equine) teeth and in vivo using whole‐body and small‐bore MR systems at 3 T, 7T, and 9.4T, respectively.

Results:

At an isotropic resolution of (600 μm)³, strong signal of soft‐tissue, e.g., mucosa and nerves with excellent contrast was achieved using TSE at 3T in vivo. No signal, though, was obtained from solid components, e.g., teeth (due to short T₂). In contrast, dentin, cementum as well as enamel of extracted teeth were readily depicted using UTE and ZTE at a resolution of ≈ (150 μm)³ at 7T and 9.4T. In particular, ZTE provided higher signal in enamel.

Conclusion:

As an alternative to X‐ray based methods like cone‐beam computed tomography (CT) or conventional CT, the presented results demonstrate the potential of ZTE and UTE MRI as a radiation‐free imaging modality, delivering contrast of soft and solid components at the same time. J. Magn. Reson. Imaging 2012;36:841–846. © 2012 Wiley Periodicals, Inc.     

Sensitivity of quantitative UTE MRI to the biomechanical property of the temporomandibular joint disc

Purpose

To quantify MR properties of discs from cadaveric human temporomandibular joints (TMJ) using quantitative conventional and ultrashort time-to-echo magnetic resonance imaging (UTE MRI) techniques and to corroborate regional variation in the MR properties with that of biomechanical indentation stiffness.

Methods

This study was exempt from the institutional review board approval. Cadaveric (four donors, two females, 74?±?10.7 years) TMJs were sliced (n?=?14 slices total) sagittally and imaged using quantitative techniques of conventional spin echo T2 (SE T2), UTE T2*, and UTE T1rho. The discs were then subjected to biomechanical indentation testing, which is performed by compressing the tissue with the blunt end of a small solid cylinder. Regional variations in MR and indentation stiffness were correlated. TMJ of a healthy volunteer was also imaged to show in vivo feasibility.

Results

Using the ME SE T2 and the UTE T1rho techniques, a significant (each p?<?0.0001) inverse relation between MR and indentation stiffness properties was observed for the data in the lower range of stiffness. However, the strength of correlation was significantly higher (p?<?0.05) for UTE T1rho (R²?=?0.42) than SE T2 (R²?=?0.19) or UTE T2* (R²?=?0.02, p?=?0.1) techniques.

Conclusion

The UTE T1rho technique, applicable in vivo, facilitated quantitative evaluation of TMJ discs and showed a high sensitivity to biomechanical softening of the TMJ discs. With additional work, the technique may become a useful surrogate measure for loss of biomechanical integrity of TMJ discs reflecting degeneration.     

Ultrashort echo time imaging of normal middle ear ossicles: a feasibility study

Objective

The aim of this study was to assess the feasibility of ultrashort echo time (UTE) imaging in the visualization of middle ear ossicles in normal subjects.

Methods

12 young adult volunteers (males/females = 6/6, age 25–44 years, mean 30.3 years) with normal hearing levels underwent MRI studies using a 3.0 T clinical unit with an eight-channel SENSE head coil. For each subject, the whole head was imaged using a three-dimensional dual-echo UTE imaging sequence with radial trajectory and the following parameters: field of view, 240 × 240 × 240 mm; matrix, 320 × 320; flip angle, 7°; repetition time/echo time (TE)1/TE2, 8.0 ms/0.14 ms/1.8 ms; acquisition voxel size, 0.75 × 0.75 × 0.75 mm; number of signals averaged, 1; imaging time, 27 min 20 s. Subsequently, subtraction images were obtained by subtracting long TE (1.8 ms) images from short TE (0.14 ms) images. By using these three images, the visibility of the bilateral middle ear ossicles was evaluated. Moreover, as a reference for the UTE findings, CT images of the temporal bone were obtained in one volunteer.

Results

In all subjects, the middle ear ossicles were clearly visualized as a high signal intensity spot surrounded by a signal void of air on short TE images bilaterally, while they were not visible in long TE images in any of the subjects. The subtraction images provided better contrast of the ossicles.

Conclusion

We demonstrated the feasibility of UTE imaging of the middle ear ossicle in normal subjects.     

Ultra‐short echo time (UTE) MR imaging of the lung: Comparison between normal and emphysematous lungs in mutant mice

Purpose:

To investigate the utility of ultra‐short echo time (UTE) sequence as pulmonary MRI to detect non‐uniform disruption of lung architecture that is typical of emphysema.

Materials and Methods:

MRI of the lungs was conducted with a three‐dimensional UTE sequence in transgenic mice with severe emphysema and their wild‐type littermates in a 3 Tesla clinical MR system. Measurements of the signal intensity (SI) and transverse relaxation time (T2*) of the lung parenchyma were performed with various echo times (TEs) ranging from 100 μs to 2 ms.

Results:

Much higher SI of the lung parenchyma was observed at an UTE of 100 μs compared with longer TEs. The emphysematous lungs had reduced SIs and T2* than the controls, in particular at end‐expiratory phase. The results suggested that both SI and T2* in lung parenchyma measured with the method represent fractional volume of lung tissue.

Conclusion:

The UTE imaging provided MR signal from the lung parenchyma. Moreover, the UTE sequence was sensitive to emphysematous changes and may provide a direct assessment of lung parenchyma. UTE imaging has the potential to assist detection of localized pathological destruction of lung tissue architecture in emphysema. J. Magn. Reson. Imaging 2010;32:326–333. © 2010 Wiley‐Liss, Inc.     

Longitudinal and multi-echo transverse relaxation times of normal breast tissue at 3 Tesla

Purpose

To measure longitudinal (T₁) and multi‐echo transverse (T₂) relaxation times of healthy breast tissue at 3 Tesla (T).

Materials and Methods

High‐resolution relaxation time measurements were made in six healthy female subjects. Inversion recovery images were acquired at 10 inversion times between 100 ms and 4000 ms, and multiple spin echo images were acquired at 16 echo times between 10 ms and 160 ms.

Results

Longitudinal relaxation times T₁ were measured as 423 ± 12 ms for adipose tissue and 1680 ± 180 ms for fibroglandular tissue. Multi‐echo transverse relaxation times T₂ were measured as 154 ± 9 ms for adipose tissue and 71 ± 6 ms for fibroglandular tissue. Histograms of the voxel‐wise relaxation times and quantitative relaxation time maps are also presented.

Conclusion

T₁ and multi‐echo T₂ relaxation times in normal human breast tissue are reported. These values are useful for pulse sequence design and optimization for 3T breast MRI. Compared with the literature, T₁ values are significantly longer at 3T, suggesting that longer repetition time and inversion time values should be used for similar image contrast. J. Magn. Reson. Imaging 2010;32:982–987. © 2010 Wiley‐Liss, Inc.     

Simulation study of susceptibility gradients leading to focal myocardial signal loss

Purpose

To assess the cause of a “bite”‐shaped signal void artifact often seen in 1.5 Tesla (T) and 3T gradient echo MR images in myocardium along the infero‐apical border of the heart, MRI simulation was used to conduct experiments impossible in reality. Two previous studies attempting to explain the origin of this artifact came to different conclusions. One suggested deoxygenated blood in the posterior vein of the left ventricle (PVLV) leads to a susceptibility gradient that causes the artifact. The other suggested the difference in susceptibility between lung tissue and myocardium was responsible. This study assessed the relative effect of each possible cause.

Materials and Methods

Anthropometric phantoms were developed for use with a previously reported MRI simulator. The images were simulated at 3T with gradient echo scans using TE = 4 ms, TR = 25 ms, and θ = 25°.

Results

The simulations indicate that both susceptibility differences can lead to signal losses in the area of the artifact with contributions from the PVLV being more localized while lung tissue effects are stronger but more spatially distributed.

Conclusion

The data support the conclusion that both differences together, rather than one or the other, are responsible for the artifact. J. Magn. Reson. Imaging 2008;28:1402–1408. © 2008 Wiley‐Liss, Inc.     

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.     

Hyaline articular cartilage: relaxation times,pulse-sequence parameters and MR appearance at 1.5 T

In order to optimize the parameters for the best visualization of the internal architecture of the hyaline articular cartilage a study both ex vivo and in vivo was performed. Accurate T1 and T2 relaxation times of articular cartilage were obtained with a particular mixed sequence and then used for the creation of isocontrast intensity graphs. These graphs subsequently allowed in all pulse sequences (spin echo, SE and gradient time (TR), echo time (TE) and flip angle (FA) for optimization of signal differences between MR cartilage zones. For SE sequences maximum contrast between cartilage zones can be obtained by using a long TR (> 1,500 ms) with a short TE (< 30 ms), whereas for GRE sequences maximum contrast is obtained with th shortest TE (< 15 ms) combined with a relatively long TR (> 400 ms) and an FA greater than 40°. A trilaminar appearance was demonstrated with a superficial and deep hypointense ozne in all sequences and an intermediate zone that was moderately hyperintense on SET1-weighted images, slightly more hyperintense on proton density Rho and SE T2-weighted images and even more hyperintense on GRE images.     

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.     

Adrenal magnetic resonance imaging in addison’s disease

Magnetic resonance imaging of the adrenal glands was performed in 9 patients with Addison’s disease to evaluate the role of magnetic resonance (MR) in this entity. All patients had bilateral adrenal masses demonstrated by computed tomography (CT); etiologies included adrenal hemorrhage (2 patients), granulomatous disease (1 patient), adrenal lymphoma (3 patients), and adrenal metastases (3 patients). Spin-echo axial images were obtained at repetition times (TR) 0.5, 2.0 s and TE 28, 56 ms, using a Diasonics superconducting magnet operating at 0.35 T. In the patients with lymphoma, metastases, and granulomatous disease, the adrenal masses appeared hypointense or isointense with liver on the T₁-weighted images (TR 0.5 s, TE 28 ms). In cases of adrenal hemorrhage, areas of hyperintensity were seen on TR 0.5, TE 56 ms sequences, due to shortening of T₁ values. In both groups of patients the masses were hyperintense on T₂ weighted sequences. Mean calculated T₁ of the hemorrhagic glands was 449 ms, compared with a mean of 782 ms for mestastases and lymphoma. While MR is not capable of distinguishing between acute inflammatory and metastatic disases of the adrenal glands, it may be equally efficacious as CT in suggesting the diagnosis of adrenal hemorrhage in patients with Addison’s disease.     

<Superscript>1</Superscript>H MR chemical shift imaging detection of phenylalanine in patients suffering from phenylketonuria (PKU)

Short echo time single voxel methods were used in previous MR spectroscopy studies of phenylalanine (Phe) levels in phenylketonuria (PKU) patients. In this study, apparent T ₂ relaxation time of the 7.3-ppm Phe multiplet signal in the brain of PKU patients was assessed in order to establish which echo time would be optimal. ¹H chemical shift imaging (CSI) examinations of a transverse plain above the ventricles of the brain were performed in 10 PKU patients and 11 persons not suffering from PKU at 1.5 T, using four echo times (TE 20, 40, 135 and 270 ms). Phe was detectable only when the signals from all CSI voxels were summarized. In patients suffering from PKU the T ₂ relaxation times of choline, creatine and N-acetyl aspartate (NAA) were similar to those previously reported for healthy volunteers (between 200 and 325 ms). The T ₂ of Phe in brain tissue was 215±120 ms (standard deviation). In the PKU patients the brain tissue Phe concentrations were 141±69 M as opposed to 58±23 M in the persons not suffering from PKU. In the detection of Phe, MR spectroscopy performed at TE 135 or 270 ms is not inferior to that performed at TE 20 or 40 ms (all previous studies). Best results were obtained at TE=135 ms, relating to the fact that at that particular TE, the visibility of a compound with a T ₂ of 215 ms still is good, while interfering signals from short-TE compounds are negligible.     

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.     

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 3D ultrashort time-to-echo (UTE) MRI and micro-CT (μCT) evaluation of the temporomandibular joint (TMJ) condylar morphology

Objective

Temporomandibular dysfunction involves osteoarthritis of the TMJ, including degeneration and morphologic changes of the mandibular condyle. The purpose of this study was to determine the accuracy of novel 3D-UTE MRI versus micro-CT (μCT) for quantitative evaluation of mandibular condyle morphology.

Materials and methods

Nine TMJ condyle specimens were harvested from cadavers (2 M, 3 F; age 85?±?10 years, mean?±?SD). 3D-UTE MRI (TR?=?50 ms, TE?=?0.05 ms, 104-μm isotropic-voxel) was performed using a 3-T MR scanner and μCT (18-μm isotropic-voxel) was also performed. MR datasets were spatially registered with a μCT dataset. Two observers segmented bony contours of the condyles. Fibrocartilage was segmented on the MR dataset. Using a custom program, bone and fibrocartilage surface coordinates, Gaussian curvature, volume of segmented regions, and fibrocartilage thickness were determined for quantitative evaluation of joint morphology. Agreement between techniques (MRI vs. μCT) and observers (MRI vs. MRI) for Gaussian curvature, mean curvature, and segmented volume of the bone were determined using intraclass correlation coefficient (ICC) analysis.

Results

Between MRI and μCT, the average deviation of surface coordinates was 0.19?±?0.15 mm, slightly higher than the spatial resolution of MRI. Average deviation of the Gaussian curvature and volume of segmented regions, from MRI to μCT, was 5.7?±?6.5 % and 6.6?±?6.2 %, respectively. ICC coefficients (MRI vs. μCT) for Gaussian curvature, mean curvature, and segmented volumes were 0.892, 0.893, and 0.972, respectively. Between observers (MRI vs. MRI), the ICC coefficients were 0.998, 0.999, and 0.997, respectively. Fibrocartilage thickness was 0.55?±?0.11 mm, as previously described in the literature for grossly normal TMJ samples.

Conclusions

3D-UTE MR quantitative evaluation of TMJ condyle morphology ex-vivo, including surface, curvature, and segmented volume, shows high correlation against μCT and between observers. In addition, UTE MRI allows quantitative evaluation of the fibrocartilaginous condylar component.     

On the measurement of absolute cerebral blood volume (CBV) using vascular‐space‐occupancy (VASO) MRI

Recently, a vascular‐space‐occupancy (VASO) MRI technique was developed for quantitative assessment of cerebral blood volume (CBV). This method uses the T₁‐shortening effect of gadolinium diethylenetriamine pentaacetic acid (Gd‐DTPA) with imaging parameters chosen that null the precontrast blood magnetization but allow the postcontrast blood magnetization to recover to equilibrium. A key advantage of VASO CBV estimation is that it provides a straightforward procedure for converting MR signals to absolute physiologic values. However, as with other T₁‐based steady‐state approaches, several important factors need to be considered that influence the accuracy of CBV values obtained with VASO MRI. Here, the transverse relaxation (T₂/T) effect in VASO MRI was investigated using multiecho spin‐echo and gradient‐echo experiments, resulting in underestimation of CBV by 14.9% ± 1.1% and 16.0% ± 2.5% for spin echo (TE = 10 ms) and gradient echo (TE = 6 ms), respectively. In addition, the influence of contrast agent clearance was studied by acquiring multiple postcontrast VASO images at 2.2‐min intervals, which showed that the concentration of Gd‐DTPA in the first 14 min (single dose) was sufficient for the blood magnetization to fully recover to equilibrium. Finally, the effect of vascular Gd‐DTPA leakage was assessed for scalp tissue, and signal extrapolation as a function of postinjection time was demonstrated to be useful in minimizing the associated errors. Specific recommendations for VASO MRI acquisition and processing strategies are provided. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Measuring T2 in vivo with J-difference editing: application to GABA at 3 Tesla

Purpose:

To develop an experimental approach for determining in vivo transverse relaxation rates (T₂) of metabolites that are detected by spectral editing without using simulations, and to demonstrate this approach to measure the T₂ of γ‐aminobutyric acid (GABA).

Materials and Methods:

The proposed method first determines the TE‐dependence of the edited signals using measurements in a pure phantom solution (10 mM γ‐aminobutyric acid; GABA); the phantom T₂ is also determined. Once the editing echo time (TE) ‐modulation pattern is known, it can then be used to determine T₂ in vivo. The method was applied to measure GABA T₂ in the occipital lobe of five healthy adult subjects at 3T, using a J‐difference editing method. Unwanted macromolecular contributions to the GABA signal were also measured.

Results:

The in vivo T₂ of edited GABA signal was 88 ± 12 ms; this preliminary result is somewhat shorter than other metabolite T₂ values in the literature at this field strength.

Conclusion:

Spectral editing methods are now widely used to detect low concentration metabolites, such as GABA, but to date no edited acquisition methods have been proposed for the measurement of transverse relaxation times (T₂). The method described has been successfully applied to measuring the T₂ of GABA. J. Magn. Reson. Imaging 2012;35:229‐234. © 2011 Wiley Periodicals, Inc.     

Optimal Pulse Sequence for Ferumoxides-Enhanced MR Imaging Used in the Detection of Hepatocellular Carcinoma: A Comparative Study Using Seven Pulse Sequences

Objective

To identify the optimal pulse sequence for ferumoxides-enhanced magnetic resonance (MR) imaging in the detection of hepatocelluar carcinomas (HCCs).

Materials and Methods

Sixteen patients with 25 HCCs underwent MR imaging following intravenous infusion of ferumoxides. All MR studies were performed on a 1.5-T MR system, using a phased-array coil. Ferumoxides (Feridex IV) at a dose of 15 µmol/Kg was slowly infused intravenously, and axial images of seven sequences were obtained 30 minutes after the end of infusion. The MR protocol included fast spin-echo (FSE) with two echo times (TR3333 8571/TE18 and 90-117), singleshot FSE (SSFSE) with two echo times (TR∞/TE39 and 98), T2*-weighted gradient-recalled acquisition in the steady state (GRASS) (TR216/TE20), T2*-weighted fast multiplanar GRASS (FMPGR) (TR130/TE8.4-9.5), and T2*-weighted fast multiplanar spoiled GRASS (FMPSPGR) (TR130/TE8.4-9.5). Contrast-to-noise ratios (CNRs) of HCCs determined during the imaging sequences formed the basis of quantitative analysis, and images were qualitatively assessed in terms of lesion conspicuity and image artifacts. The diagnostic accuracy of all sequences was assessed using receiver operating characteristic (ROC) analysis.

Results

Quantitative analysis revealed that the CNRs of T2*-weighted FMPGR and T2*-weighted FMPSPGR were significantly higher than those of the other sequences, while qualitative analysis showed that image artifacts were prominent at T2*-weighted GRASS imaging. Lesion conspicuity was statistically significantly less clear at SSFSE imaging. In term of lesion detection, T2*-weighted FMPGR, T2*-weighted FMPSPGR, and proton density FSE imaging were statistically superior to the others.

Conclusion

T2*-weighted FMPGR, T2*-weighted FMPSPGR, and proton density FSE appear to be the optimal pulse sequences for ferumoxides-enhanced MR imaging in the detection of HCCs.     

Fast recovery 3D fast spin-echo MR imaging of the inner ear at 3 T

High-resolution MR imaging of the inner ear with a heavily T2-weighted 3D fast spin-echo sequence has been performed successfully at 1.5 T. However, at 3 T, the longer T1 time of CSF necessitates a longer TR, resulting in significantly prolonged imaging times. In this study, the fast recovery 3D fast spin-echo sequence, which permits the TR to be reduced while maintaining T2 contrast, was optimized at 3 T for imaging of the inner ear. The optimized sequence parameters are as follows: 1500/294 (TR/TE); echo spacing, 18.1 ms; bandwidth, 38 kHz at 512 readout; and imaging time, 13 minutes.