PURPOSEAcoustic noise during magnetic resonance imaging (MRI) is the main source for patient discomfort and leads to verbal communication problems, difficulties in sedation, and hearing impairment. Silent Scan technology uses less changes in gradient excitation levels, which is directly related to noise levels. Here, we report our preliminary experience with this technique in neuroimaging with regard to subjective and objective noise levels and image quality. MATERIALS AND METHODS Ten patients underwent routine brain MRI with 3 TeslaMR750w system and 12-channel head coil. T1-weighted gradient echo (BRAVO) and Silenz pulse sequence (TE=0, 3D radial center-out k-space filling and data sampling with relatively small gradient steps) were performed. Patients rated subjective sound impression for both sequences on a 6-point scale. Objective sound level measurements were performed with a dedicated device in gantry at different operation modes. Image quality was subjectively assessed in consensus by two radiologists on a 3-point scale. RESULTSReaders rated image quality as fully diagnostic in all patients. Measured mean noise was reduced significantly with Silenz sequence (68.8 dB vs. 104.65 dB with BRAVO, P = 0.024) corresponding to 34.3% reduction in sound intensity and 99.97% reduction in sound pressure. No significant difference was observed between Silenz sound levels and ambient sounds (i.e., background noise in the scanner room, 68.8 dB vs. 68.73 dB, P = 0.5). The patients' subjective sound level score was lower for Silenz compared with conventional sequence (1.1 vs. 2.3, P = 0.003). CONCLUSION T1-weighted Silent Scan is a promising technique for acoustic noise reduction and improved patient comfort.
Purpose: To assess the performance of a three-dimensional (3D) non-contrast respiratory-triggered steady state free precession (SSFP) pulse sequence for detection of renal artery stenosis. Materials and Methods:A total of 64 patients who had non-contrast MR angiography (NC MRA) and 3D contrastenhanced MRA (CE MRA) performed during the same exam and three patients who had NC MRA followed by conventional catheter angiography within one month of the MRI exam were included in this retrospective study. Two blinded readers evaluated NC MRA images for the presence of significant renal artery stenosis and also rated their diagnostic confidence and evaluated the images for artifact. A similar analysis was performed for CE MRA images by two additional blinded readers, and discrepancies were resolved by consensus reading. Results:The 67 patients had 168 main and accessory renal arteries, with significant (>50%) stenosis in 34 arteries on CE MRA or conventional angiography. The two NC MRA readers had sensitivity and specificity for detection of significant stenosis of 94%/82% and 82%/87% respectively on a per renal artery basis. Conclusion:There was good agreement between CE MRA and NC MRA for detection of significant renal artery stenosis. This technique should prove useful in evaluating patients with suspected renovascular hypertension who are unable to undergo CE MRA. THREE-DIMENSIONAL gadolinium contrast-enhanced MR angiography (CE MRA) is a sensitive and accurate technique for detection of renal artery stenosis, and is useful as a primary or confirmatory non-invasive test in patients with suspected renal artery stenosis (1-5). Recent reports linking the use of gadolinium-based MR contrast agents with nephrogenic systemic fibrosis (NSF) (6-9), however, have led to a dramatic reduction in the number of renal MRA exams ordered and performed, since many patients with refractory hypertension and suspected renal artery stenosis also have renal insufficiency.While duplex sonography is effective in many patients, complete visualization of the renal arteries can be limited. Computed tomography (CT) angiography involves the use of iodinated contrast, also problematic in patients with renal insufficiency, and requires ionizing radiation. This state of affairs has stimulated many attempts to reinvestigate non-contrast MRA (NC MRA) methods. While phase contrast and time-of-flight MRA pulse sequences have proven useful in assessment of the renal arteries in previous investigations, they have limitations as a primary technique for non-contrast renal MRA (10-13); more recent investigations have focused predominantly on modified steady state free precession (SSFP) pulse sequences. SSFP sequences are attractive for their high vascular signal to noise ratio, fairly rapid acquisition times, and inherent flow compensation (14-26). Modifications of the basic 3D SSFP pulse sequence for improved visualization of the renal arteries have included arterial spin labeling (14,15), navigator or respiratory gating (14,15,17,18,(20)(21)(22)(23)(24), ...
The capability of magnetic resonance imaging (MRI) to produce spatially resolved estimation of tissue electrical properties (EPs) in vivo has been a subject of much recent interest. In this work we introduce a method to map tissue EPs from low-flip-angle, zero-echo-time (ZTE) imaging. It is based on a new theoretical formalism that allows calculation of EPs from the product of transmit and receive radio-frequency (RF) field maps. Compared to conventional methods requiring separation of the transmit RF field (B1+) from acquired MR images, the proposed method has such advantages as: (i) reduced theoretical error, (ii) higher acquisition speed, and (iii) flexibility in choice of different transmit and receive RF coils. The method is demonstrated in electrical conductivity and relative permittivity mapping in a salt water phantom, as well as in-vivo measurement of brain conductivity in healthy volunteers. The phantom results show the validity and scan-time efficiency of the proposed method applied to a piece-wise homogeneous object. Quality of in-vivo EP results was limited by reconstruction errors near tissue boundaries, which highlights need for image segmentation in EP mapping in a heterogeneous medium. Our results show the feasibility of rapid EP mapping from MRI without B1+ mapping.
Large and spatially-linear phase errors along the frequencyencode direction may be induced by several common and hardto-avoid system imperfections such as eddy currents. For data acquired in dual-echo Dixon techniques, the linear phase error can be more aggravated when compared to that acquired in a single echo and can pose challenges to a phase-correction algorithm necessary for successful Dixon processing. In this work, we propose a two-step process that first corrects the linear component of the phase errors with a modified Ahn-Cho algorithm (Ahn CB and Cho ZH, IEEE Trans. Med. Successful phase-error correction is essential for Dixon water/fat separation (1-3) and other phase-sensitive MRI techniques (4,5). In general, the spatial dependency of the phase errors in the images acquired in these techniques is quite complex. However, spatially-linear phase errors along the frequency encode direction are common and may be induced by many technical factors (such as uncompensated linear eddy currents with time constants of less than a millisecond or substantially shorter than the readout window, gradient RF subsystem delay, or incorrect setting of the data acquisition window or the currents used for magnetic field shimming). For dual-echo Dixon techniques (6,7), in which two echoes with water and fat in-phase and out-of-phase are consecutively acquired with two readout gradients of alternating polarity, a large gradient ramp is needed for the gradient polarity switch immediately before the second echo readout. As a result, it is difficult to completely eliminate the eddy current-induced linear phase errors in the image corresponding to the second echo. The problem is typically exacerbated with increased readout gradient amplitudes (which are needed for imaging at higher spatial resolution) or at higher receiver bandwidth (RBW) (which is often required at higher field strength because of the shorter in-phase and out-of-phase time separation). Irrespective of the underlying causes, unusually large linear phase errors in the presence of other phase errors and noise fluctuations may present challenges to phase correction because they introduce a systematic bias to the overall phase error distribution and because most phase-correction algorithms are based on an implicit assumption that phase-error variations are spatially slow and smooth (6,8 -11).Recently, Yeo et al. (12) proposed a calibration approach in which the linear phase errors in the dual-echo images are determined and corrected by comparing phase distribution of the phantom images from a dual-echo acquisition with that from single-echo acquisitions with the same echo times. In their study, the linear phase difference is assumed to arise only from the gradient polarity switch and to be constant during all subsequent patient scanning. The method is reportedly successful when applied over a period of 2 years on a single scanner and has the advantage of not using an elaborate phase-correction algorithm. However, it is conceivable that changes in the scanner settin...
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