Three‐dimensional radial ultrashort echo‐time imaging with <i>T</i><sub>2</sub> adapted sampling

Rahmer, Jürgen; Börnert, Peter; Groen, J.P.; Bos, Clemens

doi:10.1002/mrm.20868

Cited by 255 publications

(325 citation statements)

References 26 publications

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“…For a trapezoidal gradient with shortest possible ramp time (highest possible slew rate), our results do not deviate from the results Rahmer et al (33) obtained analytically neglecting ramp sampling. They calculated the total PSF by convolving the image space blurring function with the idealized PSF, which is the 3D Fourier transform of the sampling volume.…”

Section: Discussioncontrasting

confidence: 50%

“…For imaging of short T 2 specimens that are not SNR limited to such an extent (e.g., musculoskeletal imaging), one would use the optimal readout length as presented in Fig. 6b and stated by Rahmer et al (33).…”

Section: Discussionmentioning

confidence: 99%

“…To quantify the resulting blurring and loss of signal, the point-spread functions P tot (x) (PSF) were simulated for different ratios of T RO /T * 2 . The blurring was quantified by the full width at half maximum (FWHM) of the PSFs and the SNR by ͱT RO P tot (33). Note that these SNR calculations only hold for the case of small (point source) objects.…”

Section: Analysis Of the Point-spread Functionmentioning

confidence: 99%

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Sodium MRI using a density‐adapted 3D radial acquisition technique

Nagel

Laun

Weber

et al. 2009

Magnetic Resonance in Med

244

256

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A density-adapted three-dimensional radial projection reconstruction pulse sequence is presented which provides a more efficient k-space sampling than conventional three-dimensional projection reconstruction sequences. The gradients of the density-adapted three-dimensional radial projection reconstruction pulse sequence are designed such that the averaged sampling density in each spherical shell of k-space is constant. Due to hardware restrictions, an inner sphere of k-space is sampled without density adaption. This approach benefits from both the straightforward handling of conventional three-dimensional projection reconstruction sequence trajectories and an enhanced signal-to-noise ratio (SNR) efficiency akin to the commonly used three-dimensional twisted projection imaging trajectories. Benefits for low SNR applications, when compared to conventional three-dimensional projection reconstruction sequences, are demonstrated with the example of sodium imaging. In simulations of the point-spread function, the SNR of small objects is increased by a factor 1.66 for the densityadapted three-dimensional radial projection reconstruction pulse sequence sequence. Using analytical and experimental phantoms, it is shown that the density-adapted three-dimensional radial projection reconstruction pulse sequence allows higher resolutions and is more robust in the presence of field inhomogeneities. High-quality in vivo images of the healthy human leg muscle and the healthy human brain are acquired. For equivalent scan times, the SNR is up to a factor of 1.8 higher and anatomic details are better resolved using density-adapted three-dimensional radial projection reconstruction pulse sequence. Key words: sodium magnetic resonance imaging; densityadapted sampling; radial imaging; projection reconstruction; sampling density; field inhomogeneities Sodium ( 23 Na) ions play an important role in cellular homeostasis and cell viability. In healthy tissue, the extracellular sodium concentration ([Na ϩ ] ex ϭ 145 mM) is about 10 times higher than the intracellular concentration ([Na ϩ ] in ϭ 10-15 mM) (1). Using sodium MRI, volume-and relaxation-weighted signal of these compartments can be measured. Thus, sodium MRI is a promising diagnostic tool since pathologic processes can alter this ion gradient.Many studies investigating the usefulness of sodium MRI in human pathologies have been performed recently. Brain neoplasia and sustained cell depolarization, a precursor of cell division, lead to an increase of the intracellular sodium concentration and to a rise in the average tissue sodium concentration (2). Furthermore, the application of sodium MRI has been shown to be valuable for muscular channelopathies (3,4), brain tumors (5), the human kidney (6), myocardial infarction (7), and cerebral ischemia (8,9) diagnostics.However, sodium MRI remains a challenging technique for several reasons. The sodium nucleus exhibits a fast biexponential transversal relaxation in the extreme narrowing limit, i.e., if the correlation time is much shorter...

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Section: Discussionmentioning

confidence: 99%

Section: Analysis Of the Point-spread Functionmentioning

confidence: 99%

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Sodium MRI using a density‐adapted 3D radial acquisition technique

Nagel

Laun

Weber

et al. 2009

Magnetic Resonance in Med

244

256

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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.…”

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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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“…Partial volume effect may significantly degrade its depiction of lesions, which may be better depicted using three-dimensional UTE acquisition [42]. Another disadvantage of this technique is that the 2D acquisition is sensitive to eddy currents, field homogeneity and gradient non-linearity.…”

Section: Discussionmentioning

confidence: 99%

Ultrashort TE (UTE) Imaging of the Knee Cartilage at 3T

Gao¹

2013

OMICS J Radiol

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Three‐dimensional radial ultrashort echo‐time imaging with T₂ adapted sampling

Cited by 255 publications

References 26 publications

Sodium MRI using a density‐adapted 3D radial acquisition technique

Sodium MRI using a density‐adapted 3D radial acquisition technique

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

Ultrashort TE (UTE) Imaging of the Knee Cartilage at 3T

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Three‐dimensional radial ultrashort echo‐time imaging with T2 adapted sampling

Cited by 255 publications

References 26 publications

Sodium MRI using a density‐adapted 3D radial acquisition technique

Sodium MRI using a density‐adapted 3D radial acquisition technique

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

Ultrashort TE (UTE) Imaging of the Knee Cartilage at 3T

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Three‐dimensional radial ultrashort echo‐time imaging with T₂ adapted sampling

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