Magnetic resonance imaging with ultrashort TE (UTE) PULSE sequences: Technical considerations

Tyler, Damian J.; Robson, Matthew D.; Henkelman, R. Mark; Young, I. R.; Bydder, Graeme M.

doi:10.1002/jmri.20851

Cited by 202 publications

(157 citation statements)

References 43 publications

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“…Although COMPOSER outperformed the other reference‐free reconstruction approaches significantly, slight further improvement (to parity with Roemer) could be achieved using selective excitation or increasing the imaging slab in the head‐foot direction. Other short TE sequences could also be deployed, such as UTE 54, PETRA 55, or simply a GE scan with the shortest possible TE. Our preparatory experiments suggest that a conventional low‐resolution GE is adequate, although some reduction in phase contrast would be expected if the echo times of the SER and the target scan would be similar (eg, in imaging very short T2* species).…”

Section: Discussionmentioning

confidence: 99%

Combining phase images from array coils using a short echo time reference scan (COMPOSER)

Robinson

Dymerska

Bogner

et al. 2015

Magnetic Resonance in Med

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PurposeTo develop a simple method for combining phase images from multichannel coils that does not require a reference coil and does not entail phase unwrapping, fitting or iterative procedures.Theory and MethodsAt very short echo time, the phase measured with each coil of an array approximates to the phase offset to which the image from that coil is subject. Subtracting this information from the phase of the scan of interest matches the phases from the coils, allowing them to be combined. The effectiveness of this approach is quantified in the brain, calf and breast with coils of diverse designs.ResultsThe quality of phase matching between coil elements was close to 100% with all coils assessed even in regions of low signal. This method of phase combination was similar in effectiveness to the Roemer method (which needs a reference coil) and was superior to the rival reference‐coil‐free approaches tested.ConclusionThe proposed approach—COMbining Phase data using a Short Echo‐time Reference scan (COMPOSER)—is a simple and effective approach to reconstructing phase images from multichannel coils. It requires little additional scan time, is compatible with parallel imaging and is applicable to all coils, independent of configuration. Magn Reson Med 77:318–327, 2017. © 2015 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine

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Section: Discussionmentioning

confidence: 99%

Combining phase images from array coils using a short echo time reference scan (COMPOSER)

Robinson

Dymerska

Bogner

et al. 2015

Magnetic Resonance in Med

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“…A UTE sequence provides the possibility to obtain MR images at echo time below 1 ms, 12 but this sequence has some intrinsic problems, like errors from eddy-currents generated by the gradient ramps, which need to be corrected by k-space trajectory measurements; additionally, the ramp reduces the signal to noise ratio. 21 Therefore, the use of ZTE imaging, which samples signal without an echo time delay, was explored to visualize bone and CPC at optimal signal strength.…”

Section: Discussionmentioning

confidence: 99%

Zero Echo Time Magnetic Resonance Imaging of Contrast-Agent-Enhanced Calcium Phosphate Bone Defect Fillers

Sun

Ventura²,

Oosterwijk³

et al. 2013

Tissue Engineering Part C: Methods

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Calcium phosphate cements (CPCs) are widely used bone substitutes. However, CPCs have similar radiopacity as natural bone, rendering them difficult to be differentiated in classical X-ray and computed tomography imaging. As conventional magnetic resonance imaging (MRI) of bone is cumbersome, due to low water content and very short T 2 relaxation time, ultra-short echo time (UTE) and zero echo time (ZTE) MRI have been explored for bone visualization. This study examined the possibility to differentiate bone and CPC by MRI. T 1 and T 2 * values determined with UTE MRI showed little difference between bone and CPC; hence, these materials were difficult to separate based on T 1 or T 2 alone. Incorporation of ultra-small particles of iron oxide and gadopentetatedimeglumine (Gd-DTPA; 1 weight percentage [wt%] and 5 wt% respectively) into CPC resulted in visualization of CPC with decreased intensity on ZTE images in in vitro and ex vivo experiments. However, these additions had unfavorable effects on the solidification time and/or mechanical properties of the CPC, with the exception of 1% Gd-DTPA alone. Therefore, we tested this material in an in vivo experiment. The contrast of CPC was enhanced at an early stage postimplantation, and was significantly reduced in the 8 weeks thereafter. This indicates that ZTE imaging with Gd-DTPA as a contrast agent could be a valid radiation-free method to visualize CPC degradation and bone regeneration in preclinical experiments.

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“…Typical minimum TE values for clinical scanners, which depend on the RF transmit-receive switching and settling times, are 40-200 ms (12), while the shortest TE reported is 8 ms (13). In order to achieve a better delineation of the morphology within short T 2 tissues, several long T 2 suppression techniques have been developed, including dual echo subtraction techniques (1,5,14), and long T 2 preparation clusters using either long-duration hard pulses (15-17) or adiabatic pulses (12,18) to saturate or invert long T 2 tissues.When imaging short T 2 tissues that can be on the order of the RF pulse duration, signal decay during excitation cannot be neglected, requiring specialized sequences such as UTE (19,20). The two parameters controlled by the pulse programmer determining the flip angle of a hard RF pulse (as utilized in threedimensional UTE) are the magnetic RF field strength (amplitude of RF field, B 1 ) and the pulse duration t, resulting in a nominal flip angle y ¼ gB 1 t. The reason for the ''nominal'' caveat is that the attained flip angle is generally lower than the nominal flip angle due to T 2 decay during the RF pulse.…”

mentioning

confidence: 99%

“…When imaging short T 2 tissues that can be on the order of the RF pulse duration, signal decay during excitation cannot be neglected, requiring specialized sequences such as UTE (19,20). The two parameters controlled by the pulse programmer determining the flip angle of a hard RF pulse (as utilized in threedimensional UTE) are the magnetic RF field strength (amplitude of RF field, B 1 ) and the pulse duration t, resulting in a nominal flip angle y ¼ gB 1 t. The reason for the ''nominal'' caveat is that the attained flip angle is generally lower than the nominal flip angle due to T 2 decay during the RF pulse.…”

mentioning

confidence: 99%

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

Carl¹,

Bydder

et al. 2010

Magnetic Resonance in Med

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Ultrashort echo time MRI requires specialized pulse sequences to overcome the short T 2 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 T 2 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 T 2 contrast is maximized by flipping the magnetization at the Ernst angle with a radiofrequency pulse duration proportional to the targeted T 2 . Experimental studies on short T 2 phantoms confirm these optimization criteria for both signal-to-noise ratio and contrast-to-noise ratio. Clinical MRI is predominantly geared toward long T 2 species (greater than a few milliseconds). However, tissues containing highly ordered structures such as ligaments (T 2 % 4-10 ms), tendons (T 2 % 2 ms), or cortical bone (T 2 % 0.5 ms) appear dark in images acquired using clinical MR sequences (1,2). This is because the minimum echo times (TEs) for spin echo and gradient echo sequences are approximately 8-10 ms and 1-2 ms, respectively (2). Anomalies in such tissues typically only become apparent when pathology leads to increased signal (e.g., edema) set against the dark signal from short T 2 tissues.With the development of ultrashort TE (UTE) sequences, one is able to directly visualize tissues containing highly ordered structures (1-7). Other potential applications of UTE are imaging of the lung parenchyma, brain, or liver (8-11). Imaging short T 2 tissues is achieved in UTE by acquiring the free induction decay of the MR signal as soon after the end of the radiofrequency (RF) pulse as possible. This is typically accomplished by using a radial center-out k-space trajectory and data sampling of only a few hundred microseconds. Magnitude images are then reconstructed from the (regridded) k-space data. For the special case of UTE imaging, the time between the end of the RF pulse and the beginning of the read gradient (k ¼ 0) is typically defined in the literature as the TE (see for example Rahmer et al. (5)). Typical minimum TE values for clinical scanners, which depend on the RF transmit-receive switching and settling times, are 40-200 ms (12), while the shortest TE reported is 8 ms (13). In order to achieve a better delineation of the morphology within short T 2 tissues, several long T 2 suppression techniques have been developed, including dual echo subtraction techniques (1,5,14), and long T 2 preparation clusters using either long-duration hard pulses (15-17) or adiabatic pulses (12,18)...

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Magnetic resonance imaging with ultrashort TE (UTE) PULSE sequences: Technical considerations

Cited by 202 publications

References 43 publications

Combining phase images from array coils using a short echo time reference scan (COMPOSER)

Combining phase images from array coils using a short echo time reference scan (COMPOSER)

Zero Echo Time Magnetic Resonance Imaging of Contrast-Agent-Enhanced Calcium Phosphate Bone Defect Fillers

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

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Magnetic resonance imaging with ultrashort TE (UTE) PULSE sequences: Technical considerations

Cited by 202 publications

References 43 publications

Combining phase images from array coils using a short echo time reference scan (COMPOSER)

Combining phase images from array coils using a short echo time reference scan (COMPOSER)

Zero Echo Time Magnetic Resonance Imaging of Contrast-Agent-Enhanced Calcium Phosphate Bone Defect Fillers

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

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