Michael Carl

Qualitative and quantitative ultrashort echo time (UTE) imaging of cortical bone.

We describe the use of two-dimensional ultrashort echo time (2D UTE) sequences with minimum TEs of 8 μs to image and quantify cortical bone on a clinical 3T scanner. An adiabatic inversion pulse was used for long T(2) water and fat signal suppression. Adiabatic inversion prepared UTE acquisitions with varying TEs were used for T(2) measurement. Saturation recovery UTE acquisitions were used for T(1) measurement. Bone water concentration was measured with the aid of an external reference phantom. UTE techniques were evaluated on cadaveric specimens and healthy volunteers. A signal-to-noise ratio of around 30, contrast-to-noise ratio of around 27/20 between bone and muscle/fat were achieved in tibia in vivo with a nominal voxel size of 0.23 × 0.23 × 6.0 mm(3) in a scan time of 5 min. A mean T(1) of 223 ± 11 ms and mean T(2) of 390 ± 19 μs were found. Mean bone water concentrations of 23.3 ± 1.6% with UTE and 21.7 ± 1.3% with adiabatic inversion prepared UTE sequences were found in tibia in five normal volunteers. The results show that in vivo qualitative and quantitative evaluation of cortical bone is feasible with 2D UTE sequences.

Short T2 contrast with three-dimensional ultrashort echo time imaging.

There is increasing interest in imaging short T2 species which show little or no signal with conventional magnetic resonance (MR) pulse sequences. In this paper, we describe the use of three-dimensional ultrashort echo time (3D UTE) sequences with TEs down to 8 μs for imaging of these species. Image contrast was generated with acquisitions using dual echo 3D UTE with echo subtraction, dual echo 3D UTE with rescaled subtraction, long T2 saturation 3D UTE, long T2 saturation dual echo 3D UTE with echo subtraction, single adiabatic inversion recovery 3D UTE, single adiabatic inversion recovery dual echo 3D UTE with echo subtraction and dual adiabatic inversion recovery 3D UTE. The feasibility of using these approaches was demonstrated in in vitro and in vivo imaging of calcified cartilage, aponeuroses, menisci, tendons, ligaments and cortical bone with a 3-T clinical MR scanner. Signal-to-noise ratios and contrast-to-noise ratios were used to compare the techniques.

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 T2 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 T2* and fractions of the short and long T2 components. The tissues studies showed two distinct components with short T2*s ranging from 0.3 ms for bovine cortical bone to 2.1 ms for menisci, and long T2*s ranging from 2.9 ms for bovine cortical bone to 35.0 ms for articular cartilage. The short T2* 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 T2 water components in vitro in musculoskeletal tissues. Magn Reson Med, 2012.

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.

Anatomic Evaluation of 3-Dimensional Ultrashort-Echo-Time Bone Maps for PET/MR Attenuation Correction

Ultrashort-echo-time (UTE) sequences have been proposed in the past for MR-based attenuation correction of PET data, because of their ability to image cortical bone. In the present work we assessed the limitations of dual-echo UTE imaging for bone segmentation in head and neck imaging. Sequentially acquired MR and PET/CT clinical data were used for this purpose. Methods: Twenty patients referred for a clinical oncology examination were scanned using a trimodality setup. Among the MR sequences, a dual-echo UTE acquisition of the head was acquired and used to create tissue R2 maps. The different undesired structures present in these maps were identified by an experienced radiologist. Global and local measurements of the overlap between R2-based and CT-based bone masks were computed. Results: UTE R2 maps displayed a nonfunctional relation with CT data. The obtained bone masks showed acceptable overlap with the corresponding CT data, in the case of the skull itself (e.g., 47% mismatch for the parietal region), with decreased performance in the base of the skull and in the neck (e.g., 78% for the maxillary region). Unwanted structures were detected, both anatomic (e.g., sternocleidomastoid, temporal, and masseter muscles) and artifactual (e.g., dental implants and air–tissue interfaces). Conclusion: It is indeed possible to estimate the anatomic location of bone tissue using UTE sequences. However, using pure parametric maps for attenuation correction may lead to bias close to certain anatomic structures and areas of high magnetic field inhomogeneity. More sophisticated approaches are necessary to compensate for these effects.

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.

Optimization of RF excitation to maximize signal and T2 contrast of tissues with rapid transverse relaxation.

Ultrashort echo time MRI requires specialized pulse sequences to overcome the short T2 of the MR signal encountered in tissues such as ligaments, tendon, or cortical bone. Theoretical work is presented, supported by simulations and experimental data on optimizing the radiofrequency excitation to maximize signal‐to‐noise ratio and contrast‐to‐noise ratio. The theoretical calculations and simulations are based on the classic Bloch equations and lead to a closed form expression for the optimal radiofrequency pulse parameters to maximize the MR signal in the presence of rapid T2 decay. In the steady state, the spoiled gradient recalled echo signal amplitude in response to the radiofrequency excitation pulses is not maximized by the classic Ernst angle but by a more general criterion we call “generalized Ernst angle.” Finally, it is shown that T2 contrast is maximized by flipping the magnetization at the Ernst angle with a radiofrequency pulse duration proportional to the targeted T2. Experimental studies on short T2 phantoms confirm these optimization criteria for both signal‐to‐noise ratio and contrast‐to‐noise ratio. Magn Reson Med, 2010.

Ultrashort echo time (UTE) magnetic resonance imaging of the short T2 components in white matter of the brain using a clinical 3T scanner.

White matter of the brain contains a majority of long T2 components as well as a minority of short T2 components. These are not detectable using clinical magnetic resonance imaging (MRI) sequences with conventional echo times (TEs). In this study we used ultrashort echo time (UTE) sequences to investigate the ultrashort T2 components in white matter of the brain and quantify their T2*s and relative proton densities (RPDs) (relative to water with a proton density of 100%) using a clinical whole body 3T scanner. An adiabatic inversion recovery prepared dual echo UTE (IR-dUTE) sequence was used for morphological imaging of the ultrashort T2 components in white matter. IR-dUTE acquisitions at a constant TR of 1000 ms and a series of TIs were performed to determine the optimal TI which corresponded to the minimum signal to noise ratio (SNR) in white matter of the brain on the second echo image. T2*s of the ultrashort T2 components were quantified using mono-exponential decay fitting of the IR-dUTE signal at a series of TEs. RPD was quantified by comparing IR-dUTE signal of the ultrashort T2 components with that of a rubber phantom. Nine healthy volunteers were studied. The IR-dUTE sequence provided excellent image contrast for the ultrashort T2 components in white matter of the brain with a mean signal to noise ratio of 18.7 ± 3.7 and a contrast to noise ratio of 14.6 ± 2.4 between the ultrashort T2 white matter and gray matter in a 4.4 min scan time with a nominal voxel size of 1.25 × 1.25 × 5.0mm(3). On average a T2* value of 0.42 ± 0.08 ms and a RPD of 4.05 ± 0.88% were demonstrated for the ultrashort T2 components in white matter of the brain of healthy volunteers at 3T.

UTE imaging with simultaneous water and fat signal suppression using a time-efficient multispoke inversion recovery pulse sequence.

The long repetition time and inversion time with inversion recovery preparation ultrashort echo time (UTE) often causes prohibitively long scan times. We present an optimized method for long T2 signal suppression in which several k‐space spokes are acquired after each inversion preparation.

Dual inversion recovery ultrashort echo time (DIR-UTE) imaging and quantification of the zone of calcified cartilage (ZCC)

OBJECTIVE To develop ultrashort echo time (UTE) magnetic resonance imaging (MRI) techniques to image the zone of calcified cartilage (ZCC), and quantify its T2*, T1 and T1ρ. DESIGN In this feasibility study a dual inversion recovery UTE (DIR-UTE) sequence was developed for high contrast imaging of the ZCC. T2* of the ZCC was measured with DIR-UTE acquisitions at progressively increasing TEs. T1 of the ZCC was measured with saturation recovery UTE acquisitions at progressively increasing saturation recovery times. T1ρ of the ZCC was measured with spin-locking prepared DIR-UTE acquisitions at progressively increasing spin-locking times. RESULTS The feasibility of the qualitative and quantitative DIR-UTE techniques was demonstrated on phantoms and in six cadaveric patellae using a clinical 3 T scanner. On average the ZCC has a short T2* ranging from 1.0 to 3.3 ms (mean ± standard deviation = 2.0 ± 1.2 ms), a short T1 ranging from 256 to 389 ms (mean ± standard deviation = 305 ± 45 ms), and a short T1ρ ranging from 2.2 to 4.6 ms (mean ± standard deviation = 3.6 ± 1.2 ms). CONCLUSION UTE MR based techniques have been developed for high resolution imaging of the ZCC and quantitative evaluation of its T2*, T1 and T1ρ relaxation times, providing non-invasive assessment of collagen orientation and proteoglycan content at the ZCC and the bone cartilage interface. These measurements may be useful for non-invasive assessment of the ZCC, including understanding the involvement of this tissue component in osteoarthritis.