The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
John M Pauly - One of the best experts on this subject based on the ideXlab platform.
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maximum linear phase spectral spatial radioFrequency pulses for fat suppressed proton Resonance Frequency shift mr thermometry
Magnetic Resonance in Medicine, 2009Co-Authors: William A Grissom, John M Pauly, Adam B Kerr, Andrew B Holbrook, Kim ButtspaulyAbstract:Conventional spectral-spatial pulses used for water-selective excitation in proton Resonance Frequency–shift MR thermometry require increased sequence length compared to shorter wideband pulses. This is because spectral-spatial pulses are longer than wideband pulses, and the echo time period starts midway through them. Therefore, for a fixed echo time, one must increase sequence length to accommodate conventional spectral-spatial pulses in proton Resonance Frequency–shift thermometry. We introduce improved water-selective spectral-spatial pulses for which the echo time period starts near the beginning of excitation. Instead of requiring increased sequence length, these pulses extend into the long echo time periods common to PRF sequences. The new pulses therefore alleviate the traditional tradeoff between sequence length and fat suppression. We experimentally demonstrate an 11% improvement in frame rate in a proton Resonance Frequency imaging sequence compared to conventional spectral-spatial excitation. We also introduce a novel spectral-spatial pulse design technique that is a hybrid of previous model- and filter-based techniques and that inherits advantages from both. We experimentally validate the pulses' performance in suppressing lipid signal and in reducing sequence length compared to conventional spectral-spatial pulses. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc.
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improved shim method based on the minimization of the maximum off Resonance Frequency for balanced steady state free precession bssfp
Magnetic Resonance in Medicine, 2009Co-Authors: Michael Lustig, John M PaulyAbstract:In this article, a shim method that minimizes the maximum off-Resonance Frequency (min-max shim) in balanced steady-state free precession (bSSFP) is tested for brain imaging at 3 T with contrast and linear shim terms. The method demonstrates improvement of spatial coverage and banding artifact reduction over standard least-squares shimming. In addition, a new method (modified min-max shim) is introduced. This method reduces boundary band regions where the artifact is inevitable due to the excessive off-Resonance Frequency distribution. In comparison to standard least-squares shimming, the min-max based shim method either eliminate or reduce the size of banding artifacts. The method can be used to increase the SNR in bSSFP imaging or to increase the functional contrast in bSSFP fMRI by allowing a longer usable repetition time (TR).
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triggered navigated multi baseline method for proton Resonance Frequency temperature mapping with respiratory motion
Magnetic Resonance in Medicine, 2003Co-Authors: Karl K Vigen, Bruce L Daniel, John M Pauly, Kim ButtsAbstract:A technique is presented for the acquisition of temperature maps in the presence of variable respiratory motion using the proton Resonance Frequency (PRF) shift. The technique uses respiratory triggering, diaphragm position determination with a navigator echo, and the collection of multiple baseline images to generate temperature maps. Laser ablations were performed in an ex vivo liver phantom undergoing variable simulated respiratory motion and in vivo in four porcine livers, demonstrating a reduction of artifacts in the computed temperature maps compared with conventional single baseline techniques, both uncorrected and corrected for motion.
Masaki Fukunaga - One of the best experts on this subject based on the ideXlab platform.
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improving contrast to noise ratio of Resonance Frequency contrast images phase images using balanced steady state free precession
NeuroImage, 2011Co-Authors: Masaki Fukunaga, Jeff H DuynAbstract:Abstract Recent MRI studies have exploited subtle magnetic susceptibility differences between brain tissues to improve anatomical contrast and resolution. These susceptibility differences lead to Resonance Frequency shifts which can be visualized by reconstructing the signal phase in conventional gradient echo (GRE) acquisition techniques. In this work, a method is proposed to improve the contrast to noise ratio per unit time (CNR efficiency) of anatomical MRI based on Resonance Frequency contrast. The method, based on the balanced steady-state free precession (bSSFP) MRI acquisition technique, was evaluated in its ability to generate contrast between gray and white matter in human brain at 3 T and 7 T. The results show substantially improved CNR efficiency of bSSFP phase images (2.85 ± 0.21 times at 3 T and 1.71 ± 0.11 times at 7 T) compared to the GRE data in a limited spatial area. This limited spatial coverage is attributed to the sensitivity of bSSFP to macroscopic B0 inhomogeneities. With this CNR improvement, high resolution bSSFP phase images (resolution = 0.3 × 0.3 × 2 mm3, acquisition time = 10 min) acquired at 3 T had sufficient CNR to allow the visualization of cortical laminar structures in in vivo human primary visual cortex. Practical application of the proposed method may require improvement of B0 homogeneity and stability by additional preparatory scans and/or compensation schemes such as respiration and drift compensation. Without these additions, the CNR benefits of the method may be limited to studies at low field or limited regions of interest.
Jeff H Duyn - One of the best experts on this subject based on the ideXlab platform.
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improving contrast to noise ratio of Resonance Frequency contrast images phase images using balanced steady state free precession
NeuroImage, 2011Co-Authors: Masaki Fukunaga, Jeff H DuynAbstract:Abstract Recent MRI studies have exploited subtle magnetic susceptibility differences between brain tissues to improve anatomical contrast and resolution. These susceptibility differences lead to Resonance Frequency shifts which can be visualized by reconstructing the signal phase in conventional gradient echo (GRE) acquisition techniques. In this work, a method is proposed to improve the contrast to noise ratio per unit time (CNR efficiency) of anatomical MRI based on Resonance Frequency contrast. The method, based on the balanced steady-state free precession (bSSFP) MRI acquisition technique, was evaluated in its ability to generate contrast between gray and white matter in human brain at 3 T and 7 T. The results show substantially improved CNR efficiency of bSSFP phase images (2.85 ± 0.21 times at 3 T and 1.71 ± 0.11 times at 7 T) compared to the GRE data in a limited spatial area. This limited spatial coverage is attributed to the sensitivity of bSSFP to macroscopic B0 inhomogeneities. With this CNR improvement, high resolution bSSFP phase images (resolution = 0.3 × 0.3 × 2 mm3, acquisition time = 10 min) acquired at 3 T had sufficient CNR to allow the visualization of cortical laminar structures in in vivo human primary visual cortex. Practical application of the proposed method may require improvement of B0 homogeneity and stability by additional preparatory scans and/or compensation schemes such as respiration and drift compensation. Without these additions, the CNR benefits of the method may be limited to studies at low field or limited regions of interest.
N Meredith - One of the best experts on this subject based on the ideXlab platform.
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a comparison between cutting torque and Resonance Frequency measurements of maxillary implants a 20 month clinical study
International Journal of Oral and Maxillofacial Surgery, 1999Co-Authors: Bertil Friberg, Lars Sennerby, N Meredith, Ulf LekholmAbstract:Oral implant treatment ad modum Branemark was undertaken in nine patients with edentulous maxillae. Cutting torque measurements and Resonance Frequency analyses (RFA) were conducted at implant placement and the corresponding values were subjected to correlation analyses. The implants were also evaluated with RFA at abutment connection and at one-year follow-up in order to identify possible changes in implant stability. A total of 61 implants were inserted, of which 49 were of the Mk II self-tapping type. Two implants were lost during the study period. The cumulative torque was presented as a mean value for the upper/crestal, the middle and the lower/apical third of the implant site respectively, as well as an overall value for the whole site. The highest correlation (r = 0.84, P<0.05) was found when comparing the mean torque values of the upper/crestal portion with the Resonance Frequency values at implant placement. The Mk II implant sites were divided into three groups based on the values of the cutting torque, i.e. soft (group 1), medium (group 2) and dense bone (group 3). The mean value of each group was plotted against the corresponding mean value of Resonance Frequency measured at implant insertion. Statistical analysis showed significant differences in Resonance Frequency at implant insertion between groups 1 and 2 (P = 0.047) and between groups 1 and 3 (P = 0.002). When repeating the Resonance Frequency analyses at second stage surgery and at one-year follow-up, no significant differences were detected between any of the groups. It was shown that the stability of implants placed in softer bone seemed to "catch up" over time with more dense bone sites.
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the application of Resonance Frequency measurements to study the stability of titanium implants during healing in the rabbit tibia
Clinical Oral Implants Research, 1997Co-Authors: N Meredith, Lars Sennerby, F Shagaldi, D N Alleyne, P CawleyAbstract:The aim of this investigation was to measure the Resonance Frequency of a number of implants placed in the rabbit tibia at insertion and at predetermined periods thereafter and to correlate the results with histomorphometric measurements made when the animals were sacrificed. Ten mature New Zealand White rabbits were used in the study. Two c.p. threaded titanium implants were placed in the right tibia of each animal. Resonance Frequency measurements were made by screwing a small transducer onto a standard abutment mounted on each fixture. Measurements were repeated with the transducer oriented perpendicular and parallel to the long axis of the tibia for all proximal implants 14 and 28 days after placement and in 6 implants additionally at 42, 56, 93, 122 and 168 days after which all animals were sacrificed. Histomorphometric analysis comprised 2 parts; measurement of bone-implant contact area and height. A significant increase in Resonance Frequency was observed after 14 (405 Hz, +/- 234 Hz) and 28 (658 Hz, +/- 332 Hz) days. The increase in Resonance Frequency levelled after approximately 40 days and little further change was observed. The variation in bone-implant contact area was relatively small (1.8-4.9 mm2) and the range of bone-implant contact heights was also narrow (-1.5 (-)+ 1.5 mm). Values for Resonance frequencies plotted against contact area and height were grouped around 10 kHz. In conclusion, it was shown that Resonance Frequency measurements can be made at placement and during healing in vivo and changes may be related to the increase in stiffness of an implant in the surrounding tissues.
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Resonance Frequency measurements of implant stability in vivo a cross sectional and longitudinal study of Resonance Frequency measurements on implants in the edentulous and partially dentate maxilla
Clinical Oral Implants Research, 1997Co-Authors: N Meredith, K Book, Bertil Friberg, Torsten Jemt, Lars SennerbyAbstract:: The aim of this investigation was to evaluate the use of Resonance Frequency measurements in the clinical measurement of implant stability. Resonance Frequency measurements are undertaken by measuring the response of a small transducer attached to an implant fixture or abutment. Two groups of patients were selected for study. Group A comprised 9 patients who had a total of 56 implants placed. Resonance Frequency measurements were made at fixture installation and repeated 8 months later at abutment connection. The Resonance Frequency of the implant/transducer system increased for 50 out of the 56 implants from a mean value of 7473 Hz +/- 127 Hz (P < 0.05) to a mean of 7915 Hz +/- 112 Hz (P < 0.05). Two implants had failed to integrate and the Resonance Frequency of these had fallen. Group B comprised 9 patients who had been provided with fixed prostheses and had a total of 52 implants placed. They were examined 5 years after fixture placement and the prostheses removed. All implants were judged clinically to be osseointegrated. The level of the marginal bone around each implant was calculated by measuring the number of exposed threads on intraoral periapical radiographs and added to the length of each abutment to give a value termed the effective implant length (EIL). Measurements indicated a correlation (R = -0.78, P < 0.01) between EIL and Resonance Frequency. The results support the hypothesis that the Resonance Frequency of an implant/transducer system is related to the height of the implant not surrounded by bone and the stability of the implant/tissue interface as determined by the absence of clinical mobility.
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quantitative determination of the stability of the implant tissue interface using Resonance Frequency analysis
Clinical Oral Implants Research, 1996Co-Authors: N Meredith, D N Alleyne, P CawleyAbstract:Bone anchored implants are now being used in dentistry for supporting intraoral and craniofacial prostheses. Although high success rates have been reported, a small number of implants may fail during the early healing phase or lateral in function. Currently available clinical methods to determine implant stability and osseointegration are relatively crude and may entail percussing a fixture with a blunt instrument. Radiographs are of value, but a standardised technique is necessary to ensure repeatability. This investigation was designed to study the application of a non-invasive test method using Resonance Frequency analysis to make quantitative measurements of the stability of the implant tissue interface in-vitro and in-vivo. The Resonance Frequency of a small transducer was measured when attached to implants embedded at different heights in an aluminum block. A strong correlation (r = 0.94, p < 0.01) was observed between the observed Frequency and the height of implantation fixture exposed. The change in stiffness observed in the bone surrounding an implant during healing was modelled by embedding implants in self-curing polymethylmethacrylate and measuring the Resonance Frequency at periods during polymerisation. A significant increase in Resonance Frequency was observed related to the increase in stiffness. Resonance Frequency measurements were also made on implants in-vivo and the results correlated well with the in-vitro findings.
Michael Lustig - One of the best experts on this subject based on the ideXlab platform.
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improved shim method based on the minimization of the maximum off Resonance Frequency for balanced steady state free precession bssfp
Magnetic Resonance in Medicine, 2009Co-Authors: Michael Lustig, John M PaulyAbstract:In this article, a shim method that minimizes the maximum off-Resonance Frequency (min-max shim) in balanced steady-state free precession (bSSFP) is tested for brain imaging at 3 T with contrast and linear shim terms. The method demonstrates improvement of spatial coverage and banding artifact reduction over standard least-squares shimming. In addition, a new method (modified min-max shim) is introduced. This method reduces boundary band regions where the artifact is inevitable due to the excessive off-Resonance Frequency distribution. In comparison to standard least-squares shimming, the min-max based shim method either eliminate or reduce the size of banding artifacts. The method can be used to increase the SNR in bSSFP imaging or to increase the functional contrast in bSSFP fMRI by allowing a longer usable repetition time (TR).