Acoustic noise reduction in T 1- and proton-density-weighted turbo spin-echo imaging.

OBJECTIVE: To reduce acoustic noise levels in T 1-weighted and proton-density-weighted turbo spin-echo (TSE) sequences, which typically reach acoustic noise levels up to 100 dB(A) in clinical practice. MATERIALS AND METHODS: Five acoustic noise reduction strategies were combined: (1) gradient ramps and shapes were changed from trapezoidal to triangular, (2) variable-encoding-time imaging was implemented to relax the phase-encoding gradient timing, (3) RF pulses were adapted to avoid the need for reversing the polarity of the slice-rewinding gradient, (4) readout bandwidth was increased to provide more time for gradient activity on other axes, (5) the number of slices per TR was reduced to limit the total gradient activity per unit time. We evaluated the influence of each measure on the acoustic noise level, and conducted in vivo measurements on a healthy volunteer. Sound recordings were taken for comparison. RESULTS: An overall acoustic noise reduction of up to 16.8 dB(A) was obtained by the proposed strategies (1-4) and the acquisition of half the number of slices per TR only. Image quality in terms of SNR and CNR was found to be preserved. CONCLUSIONS: The proposed measures in this study allowed a threefold reduction in the acoustic perception of T 1-weighted and proton-density-weighted TSE sequences compared to a standard TSE-acquisition. This could be achieved without visible degradation of image quality, showing the potential to improve patient comfort and scan acceptability.

Acoustic-noise-optimized diffusion-weighted imaging.

OBJECTIVE: This work was aimed at reducing acoustic noise in diffusion-weighted MR imaging (DWI) that might reach acoustic noise levels of over 100 dB(A) in clinical practice. MATERIALS AND METHODS: A diffusion-weighted readout-segmented echo-planar imaging (EPI) sequence was optimized for acoustic noise by utilizing small readout segment widths to obtain low gradient slew rates and amplitudes instead of faster k-space coverage. In addition, all other gradients were optimized for low slew rates. Volunteer and patient imaging experiments were conducted to demonstrate the feasibility of the method. Acoustic noise measurements were performed and analyzed for four different DWI measurement protocols at 1.5T and 3T. RESULTS: An acoustic noise reduction of up to 20 dB(A) was achieved, which corresponds to a fourfold reduction in acoustic perception. The image quality was preserved at the level of a standard single-shot (ss)-EPI sequence, with a 27-54% increase in scan time. CONCLUSIONS: The diffusion-weighted imaging technique proposed in this study allowed a substantial reduction in the level of acoustic noise compared to standard single-shot diffusion-weighted EPI. This is expected to afford considerably more patient comfort, but a larger study would be necessary to fully characterize the subjective changes in patient experience.

Sequence-based acoustic noise reduction of clinical MRI scans.

PURPOSE: Clinical MRI patients typically experience elevated acoustic noise levels of 80-110 dB(A). In this study, standard clinical turbo spin echo (TSE) and gradient echo (GRE) sequences were optimized for reduced acoustic noise at preserved diagnostic image quality. METHODS: The physical sources of acoustic noise generation in an MRI gradient coil were analyzed. A sequence conversion algorithm was derived that optimized the gradient time scheme for an arbitrary MRI sequence, preserving the governing spin physics. The algorithm was applied to generate "quiet" versions of standard clinical TSE and GRE sequences. RESULTS: The first volunteer images indicated that contrast-to-noise ratio and perceived diagnostic image quality remained on the same level for the algorithmic optimization. Additional careful TSE- and GRE-specific protocol adaptions yielded total acoustic noise reductions of up to 14.4 dB(A) for the TSE and up to 16.8 dB(A) for the GRE. CONCLUSION: A physical sound pressure reduction of 81% (TSE) and 86% (GRE) for MRI patients was achieved. The results can be used to render MRI scans more patient-friendly in clinical practice, particularly for patients who are young, scared, or elderly.

Value-based noise reduction for low-dose dual-energy computed tomography.

We introduce a value-based noise reduction method for Dual-Energy CT applications. It is based on joint intensity statistics estimated from high- and low-energy CT scans of the identical anatomy in order to reduce the noise level in both scans. For a given pair of measurement values, a local gradient ascension algorithm in the probability space is used to provide a noise reduced estimate. As a consequence, two noise reduced images are obtained. It was evaluated with synthetic data in terms of quantitative accuracy and contrast to noise ratio (CNR)-gain. The introduced method allows for reducing patient dose by at least 30% while maintaining the original CNR level. Additionally, the dose reduction potential was shown with a radiological evaluation on real patient data. The method can be combined with state-of-the-art filter-based noise reduction techniques, and makes low-dose Dual-Energy CT possible for the full spectrum of quantitative CT applications.

Single and dual energy attenuation correction in PET/CT in the presence of iodine based contrast agents.

In present positron emission tomography (PET)/computed tomography (CT) scanners, PET attenuation correction is performed by relying on the information given by a single CT scan. The scaling of the linear attenuation coefficients from CT x-ray energy to PET 511 keV gamma energy is prone to errors especially in the presence of CT contrast agents. Attenuation correction based upon two CT scans at different energies but performed at the same time and patient position should reduce such errors and therefore improve the accuracy of the reconstructed PET images at the cost of introduced additional noise. Such CT scans could be provided by future PET/CT scanners that have either dual source CT or energy sensitive CT. Three different dual energy scaling methods for attenuation correction are introduced and assessed by measurements with a modified NEMA 1994 phantom with different CT contrast agent concentrations. The scaling is achieved by differentiating between (1) Compton and photoelectric effect, (2) atomic number and density, or (3) water-bone and water-iodine scaling schemes. The scaling method (3) is called hybrid dual energy computed tomography attenuation correction (hybrid DECTAC). All three dual energy scaling methods lead to a reduction of contrast agent artifacts with respect to single energy scaling. The hybrid DECTAC method resulted in PET images with the weakest artifacts. Both, the hybrid DECTAC and Compton/photoelectric effect scaling resulted also in images with the lowest PET background variability. Atomic number/density scaling and Compton/photoelectric effect scaling had problems to correctly scale water, hybrid DECTAC scaling and single energy scaling to correctly scale Teflon. Atomic number/density scaling and hybrid DECTAC could be generalized to reduce these problems.