J Korean Soc Magn Reson Med.  2014 Jun;18(2):87-106. 10.13104/jksmrm.2014.18.2.87.

Towards Routine Clinical Use of Radial Stack-of-Stars 3D Gradient-Echo Sequences for Reducing Motion Sensitivity

• Block KT
• Chandarana H
• Milla S
• Bruno M
• Mulholland T
• Fatterpekar G
• Hagiwara M
• Grimm R
• Geppert C
• Kiefer B
• Sodickson

• KMID: 2099886
• DOI: http://doi.org/10.13104/jksmrm.2014.18.2.87

Abstract

PURPOSE
To describe how a robust implementation of a radial 3D gradient-echo sequence with stack-of-stars sampling can be achieved, to review the imaging properties of radial acquisitions, and to share the experience from more than 5000 clinical patient scans.
MATERIALS AND METHODS
A radial stack-of-stars sequence was implemented and installed on 9 clinical MR systems operating at 1.5 and 3 Tesla. Protocols were designed for various applications in which motion artifacts frequently pose a problem with conventional Cartesian techniques. Radial scans were added to routine examinations without selection of specific patient cohorts.
RESULTS
Radial acquisitions show significantly lower sensitivity to motion and allow examinations during free breathing. Elimination of breath-holding reduces failure rates for non-compliant patients and enables imaging at higher resolution. Residual artifacts appear as streaks, which are easy to identify and rarely obscure diagnostic information. The improved robustness comes at the expense of longer scan durations, the requirement for fat suppression, and the nonexistence of a time-to-center value. Care needs to be taken during the configuration of receive coils.
CONCLUSION
Routine clinical use of radial stack-of-stars sequences is feasible with current MR systems and may serve as substitute for conventional fat-suppressed T1-weighted protocols in applications where motion is likely to degrade the image quality.

Keyword

Radial sampling; Abdominal MRI; Pediatric imaging; Head and neck imaging; Motion robustness, vibe

MeSH Terms

Artifacts
Cohort Studies
Humans
Respiration

Figure

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  Fig. 1 (a) "Stack-of-stars" trajectory, which acquires the kx-ky plane along radial spokes and the kz direction with Cartesian sampling. The angle of the radial spokes can be ordered (b) using an equidistant scheme or (c) using the golden-angle scheme. Numbers indicate the temporal order of the acquisition.

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  Fig. 2 Basic pulse sequence diagram for one TR repetition cycle, starting with a RF excitation pulse (a), which is used either without or with slab-selection gradient (b). Gradient pulse (c) combines slab-selection rewinding and Cartesian phase encoding along the SLICE axis. Gradient pulses (d) act as dephasing, (e) readout, and (g) spoiling gradients in the READ-PHASE plane. Gradients (d, e, g) are played simultaneously in the READ and PHASE direction with amplitudes modulated according to Eq. (1), resulting in radial k-space traversal. The signal is recorded using an ADC event (f) during the flat-top time of the readout gradients (e). Gradient pulse (h) spoils residual magnetization along the SLICE axis. Gray color indicates that gradient amplitudes are changing per repetition.

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  Fig. 3 Free-breathing abdominal exam using (a) conventional Cartesian sampling and (b) radial sampling (with otherwise identical sequence parameters and matching acquisition time). While the Cartesian scan suffers from strong "ghosting" artifacts, the radial acquisition provides diagnostic image quality.

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  Fig. 4 Scan with reduced FOV size using (a) Cartesian sampling and (b) radial sampling. Due to the large object extent, aliasing occurs with Cartesian sampling along the phase-encoding direction. Because readout oversampling can be applied in both directions, aliasing does not appear with radial sampling.

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  Fig. 5 (Left) Radial point-spread-function and (right) exemplary imaging simulation for (top) mild and (bottom) strong undersampling. The PSF has a central peak (c), which reflects the obtained spatial resolution and whose width is independent of the undersampling. It is surrounded by a dense disc with minor circular Gibbs ringing. Beyond the Nyquist radius (a), which depends on the number of acquired spokes, it shows radially propagating streak patterns (b). The high-intensity circle of the simulation object therefore leads to streaks in the reconstructed image at Nyquist distance. A streak-free image is obtained if the largest object diameter is smaller than the Nyquist radius.

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  Fig. 6 (a) Without correction of gradient delays, radial scans suffer from intensity modulations and streaks around the object. (b) Corrected radial scan after applying the described adaptive calibration approach.

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  Fig. 7 (Top) Off-resonant fat signal leads to the well-known chemical-shift artifact along the readout direction for Cartesian trajectories. (Middle) Due to the varying readout orientation, the chemical-shift effect translates into signal blurring for radial sampling. (Bottom) The effect can be avoided by using radial sampling with fat-suppression.

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  Fig. 8 (a) Severe streak artifacts appear in the thorax (arrows), which are caused by an improperly selected coil element with main sensitivity localization in the pelvic area (circle). (b) After disabling the coil, the streak artifacts disappear with negligible loss of the overall signal intensity in the region-of-interest.

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  Fig. 9 (a, c) Cartesian 3D GRE exam of the liver in patients unable to suspend respiration, compared to (b, d) examination during free breathing using the radial sequence. A lesion in the spleen (arrow in b) is only visible on the radial exam. An enlarged lymph node (arrow in d) is clearly identified on the radial exam but difficult to appreciate on the Cartesian exam.

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  Fig. 10 (a, c) The achievable resolution with Cartesian scans is limited by the typical breath-hold capacity of less than 20 sec. (b, d) Radial acquisition during free shallow breathing enables to obtain significantly higher spatial resolution. In the example of late-phase exam after injection of EOVIST, it provides much sharper depiction of the biliary duct (arrows).

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  Fig. 11 Pediatric abdominopelvic exams are degraded by respiration artifacts when performed with conventional sequences during sedation (a, c), which limit the effective resolution. Significantly improved image quality is achieved with the radial sequence (b, d). In the case of a patient with Tuberous Sclerosis (top), the radial sequence reveals small cysts in the kidney (arrows).

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  Fig. 12 Brain exam of a 4-week-old active patient using (a) 2D TSE, (b) MP-RAGE, (c) 2D FLASH, and (d) the radial sequence. While all Cartesian sequences were affected by a certain level of motion artifacts, the radial sequence provided acceptable diagnostic image quality.

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  Fig. 13 Imaging of the neck (a, b) and upper chest (c, d) using a conventional 2D TSE sequence (a, c) and the radial sequence (b, d). Due to strong motion and flow artifacts, a mass in the thyroid gland is not visible on the 2D TSE exam but appreciated on the radial scan (arrow in b). Evaluation of the sternum in a patient complaining of pain was not possible on the 2D TSE scan (c), but unproblematic with the radial sequence (d).

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  Fig. 14 Multi-planar reconstructions of a radial neck scan with 1.0 mm isotropic resolution, which are unaffected from swallowing or respiration artifacts and which can be used as replacement for separate 2D TSE acquisitions in these orientations. Of note is a failure of spectral fat saturation in the back of the neck, which is a common observation in the proximity of skin folds. Furthermore, the outer slices show slight aliasing along kz, caused by imperfections of the slab-selective excitation pulse. Both effects are not related to the radial scheme.

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  Fig. 15 (Top) Imaging of the orbits using (a) 2D TSE, (b) MP-RAGE, and (c) the radial sequence. Both Cartesian scans are affected by motion artifacts that limit the clarity of the optic nerves. The radial sequence provides sharp visualization of the optic nerves and surrounding vessels (arrow). (Bottom) Imaging of the full brain using (d) MR-RAGE and (e) the radial sequence, which shows significantly lower motion artifacts and higher image clarity.

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  Fig. 16 Spine imaging in patients with metastasis using (a, c) conventional 2D TSE sequences without fat suppression and (b, d) the radial sequence. The radial sequence shows clear contrast enhancement of the metastasis (arrows), while the enhancement is more difficult to see on the TSE scans because of low contrast difference to normal bone marrow and because of ghosting artifacts arising from blood flow and organ movement.

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