Whole‐body imaging at 7T: Preliminary results

Vaughan, J. Thomas; Snyder, Carl J.; DelaBarre, Lance; Bolan, Patrick J.; Tian, Jinfeng; Bolinger, Lizann; Adriany, Gregor; Andersen, Peter M.; Strupp, John; Uğurbil, Kâmil

doi:10.1002/mrm.21751

Cited by 254 publications

(271 citation statements)

References 16 publications

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“…This has both advantages and disadvantages. Recent studies have shown that homogeneous transmit fields can be produced using transmit arrays and sophisticated B 1 shimming routines (29). Although this represents the optimum strategy, it does require hardware that is not yet standard on most commercial MRI scanners, and it currently necessitates the development of sophisticated hardware and software interfaces.…”

Section: Discussionmentioning

confidence: 99%

Initial results on in vivo human coronary MR angiography at 7 T

Elderen

Versluis

Webb

et al. 2009

Magnetic Resonance in Med

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Seven tesla (T) MR imaging is potentially promising for the morphologic evaluation of coronary arteries because of the increased signal-to-noise ratio compared to lower field strengths, in turn allowing improved spatial resolution, improved temporal resolution, or reduced scanning times. However, there are a large number of technical challenges, including the commercial 7 T systems not being equipped with homogeneous body radiofrequency coils, conservative specific absorption rate constraints, and magnified sample-induced amplitude of radiofrequency field inhomogeneity. In the present study, an initial attempt was made to address these challenges and to implement coronary MR angiography at 7 T. A singleelement radiofrequency transmit and receive coil was designed and a 7 T specific imaging protocol was implemented, including significant changes in scout scanning, contrast generation, and navigator geometry compared to current protocols at 3 T. With this methodology, the first human coronary MR images were successfully obtained at 7 T, with both qualitative and quantitative findings being presented. Coronary magnetic resonance angiography (MRA) has been shown to be a promising tool for the noninvasive identification of significant proximal coronary artery disease (1,2). The most commonly used magnetic field strength for coronary MRA is 1.5 T, involving many years of sequence, parameter, and radiofrequency (RF) coil optimization. In common with many MRI applications, the use of higher magnetic field strengths for coronary MRA is attractive, with advantages arising from increases in signal-to-noise ratio (SNR), smaller voxel sizes, a higher temporal resolution, and/or shortened scanning times. Individually or in combination, these improvements are likely to result in improved image quality and ultimately better access to small diameter and branching vessels. As a first step in this direction, implementation of coronary MRA at 3 T has been found to result in increased SNR, increased contrast-to-noise ratio, and increased measured coronary vessel lengths compared to 1.5 T (3-5). Voxel sizes as low as 0.35 ϫ 0.35 ϫ 1.5 mm 3 have been obtained at 3 T (6) in selected cases. Although initial studies comparing 1.5 T coronary MRA with 3 T coronary MRA and the gold standard x-ray coronary angiography showed little advantage of 3 T for the identification of significant luminal coronary artery disease, a more recent report (4) appears much more promising, and other studies that take advantage of new high-field specific improvements are ongoing. Notably, the increase in magnetic field strength between 1.5 and 3 T produces a series of challenges, including adequate electrocardiograph (ECG) triggering (7), sophisticated higherorder shimming (8) to account for increased magnetic field susceptibility, and spatially homogeneous magnetization preparation for contrast generation (9).Given the promising indicators at 3 T, the recent availability of commercial human whole-body high-field 7 T MR systems might offer great potential for c...

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Section: Discussionmentioning

confidence: 99%

Initial results on in vivo human coronary MR angiography at 7 T

Elderen

Versluis

Webb

et al. 2009

Magnetic Resonance in Med

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“…For measurements inside the MR imager, the checkerboard phantom was placed centrally in a custom-built 8-channel head coil (13), so that the checkerboard pattern was visible in the axial slice. To provoke strong B 1 inhomogeneities, the checkerboard phantom was inserted into a water-filled canister of 15 Â 15 Â 20 cm 3 . For comparison with a nearly homogeneous transmit field, the experiment was repeated without the water-filled canister.…”

Section: Contrast Measurementsmentioning

confidence: 99%

“…At higher field strengths, an increased signal-to-noise ratio and new contrasts can be obtained; however, high static field strengths of 7 T and above lead to severe radiofrequency (RF) homogeneity problems (1,2), because the operational frequency is proportional to the static field strength, and hence, the wavelength is shortened. Especially, whole-body imaging at 7 T and higher is therefore challenging (3). Multichannel transmit approaches to tackle these problems have been proposed in the literature.…”

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confidence: 99%

Time‐interleaved acquisition of modes: An analysis of SAR and image contrast implications

Orzada

Maderwald

Poser

et al. 2011

Magnetic Resonance in Med

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As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole-body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called timeinterleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B þ 1 modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time-interleaved acquisition of mode on image contrast as well as on time-averaged specific absorption rate is addressed in detail. Time-interleaved acquisition of mode is superior in B þ 1 homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time-interleaved acquisition of modes can enable almost homogeneous high-field imaging throughout the entire field of view in PD, T 2 , and T 2 *-weighted imaging and, if a specified homogeneity criterion is met, in Key words: 7 Tesla; ultra-high field; body imaging; parallel transmission; RF shimmingSince the early days of magnetic resonance imaging, a steady drive to higher field strengths has been apparent. At higher field strengths, an increased signal-to-noise ratio and new contrasts can be obtained; however, high static field strengths of 7 T and above lead to severe radiofrequency (RF) homogeneity problems (1,2), because the operational frequency is proportional to the static field strength, and hence, the wavelength is shortened. Especially, whole-body imaging at 7 T and higher is therefore challenging (3). Multichannel transmit approaches to tackle these problems have been proposed in the literature. The most notable are static RF shimming (4,5) and Transmit SENSE (6).In RF shimming, each element in an array is driven with its own constant amplitude and phase, while the pulse profile is identical for all elements. By choosing a suitable set of amplitudes and phases, the resulting B þ 1 can be shaped within certain limitations to achieve a more homogeneous field excitation in an extended area or a localized field of view (5,7).Transmit SENSE and equivalent approaches (8) use spatially tailored RF pulses. The amplitude and phase of each element in a transmit array are varied during transmission, leading to element-dependent pulse profiles. This approach yields excellent results but presupposes precise knowledge of element transmission profiles as well as expensive and complicated hardware.Recently, an acquisition scheme called time-interleaved acquisition of modes (TIAMO) (9) has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B þ 1 modes using static RF shimming in an interleaved acquisition, whe...

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“…MRI accidents are listed as number 9 of the top 10 risks in modern medicine [1][2]. With the advent of (ultra)high field MR systems [3][4][5][6][7][8][9][10][11][12][13][14] this risk, which is commonly known as the missile or projectile effect is even more pronounced. These projectiles usually consist of common office and hospital items that contain a fair amount of ferromagnetic metal including for example hospital beds, intra venous poles, oxygen tanks, and conventional ECG devices used for patient monitoring, computer displays, ventilator etc.…”

Section: Introductionmentioning

confidence: 99%

“…Recent designs have typically been laid out as strip line elements on rigid or semiflexible frames [7]. A strategy employing small magnetic field alert sensors which can be attached to ferromagnetic objects that are commonly used in a clinical environment is conceptually appealing for the pursuit of reducing the risk of ferromagnetic projectile accidents.…”

Section: Introductionmentioning

confidence: 99%

Development and evaluation of a small and mobile Magneto Alert Sensor (MALSE) to support safety requirements for magnetic resonance imaging

et al. 2011

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Objective: The purpose of this study is to (i) design a small and mobile Magnetic field ALert SEnsor (MALSE), (ii) to carefully evaluate its sensors to their consistency of activation/deactivation and sensitivity to magnetic fields, and (iii) to demonstrate the applicability of MALSE in 1.5 T, 3.0 T and 7.0 T MR fringe field environments.Methods: MALSE comprises a set of reed sensors, which activate in response to their exposure to a magnetic field. The activation/deactivation of reed sensors was examined by moving them in/out of the fringe field generated by 7TMR. Results:The consistency with which individual reed sensors would activate at the same field strength was found to be 100% for the setup used. All of the reed switches investigated required a substantial drop in ambient magnetic field strength before they deactivated.Conclusions: MALSE is a simple concept for alerting MRI staff to a ferromagnetic object being brought into fringe magnetic fields which exceeds MALSEs activation magnetic field. MALSE can easily be attached to ferromagnetic objects within the vicinity of a scanner, thus creating a barrier for hazardous situations induced by ferromagnetic parts which should not enter the vicinity of a MR-system to occur.

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Whole‐body imaging at 7T: Preliminary results

Cited by 254 publications

References 16 publications

Initial results on in vivo human coronary MR angiography at 7 T

Initial results on in vivo human coronary MR angiography at 7 T

Time‐interleaved acquisition of modes: An analysis of SAR and image contrast implications

Development and evaluation of a small and mobile Magneto Alert Sensor (MALSE) to support safety requirements for magnetic resonance imaging

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