Fast and quiet MRI using a swept radiofrequency

doi:10.1016/j.jmr.2006.05.014

Journal of Magnetic Resonance

Volume 181, Issue 2, August 2006, Pages 342-349

https://doi.org/10.1016/j.jmr.2006.05.014 Get rights and content

Abstract

A novel fast and quiet method of magnetic resonance imaging (MRI) is introduced which creates new opportunities for imaging in medicine and materials science. The method is called SWIFT, sweep imaging with Fourier transformation. In SWIFT, time-domain signals are acquired in a time-shared manner during a swept radiofrequency excitation of the nuclear spins. With negligible time between excitation and signal acquisition, new possibilities exist for imaging objects consisting of spins with extremely fast transverse relaxation rates, such as macromolecules, semi-solids, and quadrupolar nuclei. The field gradient used for spatial-encoding is not pulsed on and off, but rather is stepped in orientation in an incremental manner, which results in low acoustic noise. This unique acquisition method is expected to be relatively insensitive to sample motion, which is important for imaging live objects. Additionally, the frequency-swept excitation distributes the signal energy in time and thus dynamic range requirements for proper signal digitization are reduced compared with conventional MRI. For demonstration, images of a plastic object and cortical bone are shown.

Introduction

NMR signal is observed following one of three basic types of radiofrequency (RF) excitation: sequential, simultaneous, and random. Accordingly, three different types of NMR techniques have been developed: continuous wave (CW) [1], [2], pulsed [3], [4], [5], and stochastic [6], [7]. Historically, the first high resolution NMR spectra of liquid samples were acquired using the CW technique. NMR spectroscopy using the CW technique became an important tool in the field of chemistry until the early 1970’s. Then, shortly after realizing the efficiency of pulsed Fourier transform (FT) spectroscopy [8], pulsed FT supplanted CW as the main spectroscopic technique. Nowadays, pulsed FT spectroscopy continues to dominate the field, while the CW and stochastic NMR techniques are mainly used only for niche applications.

Magnetic resonance imaging (MRI) has additional technical requirements over high-resolution NMR spectroscopy of liquids. Because the object to be imaged is usually much larger than a sample tube, inevitably the static and RF fields used in MRI are more inhomogeneous than those used in high resolution NMR. Furthermore, MRI requires high power RF amplifiers to attain a sufficiently intense RF field to excite all spins simultaneously. In a search for ways to circumvent such difficulties, some researchers have reconsidered old, almost forgotten NMR techniques. For this reason, the “sleeping beauty” [9], stochastic NMR, was reawakened [10] and found to have remarkable features; most notably, the peak RF power required for spin excitation is at least two orders of magnitude lower than that required by conventional pulsed schemes. Very recently, a CW MRI system using low peak RF power has also been developed and applied to study a variety of heterogeneous materials exhibiting extremely fast spin–spin relaxation [11]. Another example is the extensive application in modern MRI of adiabatic sweep pulses, which evolved from the traditional CW technique. An advantage of the adiabatic sweep pulse is its ability to accomplish uniform rotation over a broad band of resonance frequencies [12].

This article presents a new MRI method that can be considered as a combination of all three basic NMR techniques. As in CW NMR, this new method uses swept RF excitation, but the sweep rate far exceeds the CW sweep rate even during a rapid-scan [13], [14]. Unlike the CW method in which the signal is acquired in the frequency domain, here the signal is treated as a function of time, as in the pulsed FT method. Finally, the method uses correlation, identical to that used in stochastic NMR, to extract the signal arising from the spin system. This novel method is dubbed SWIFT, for sweep imaging with Fourier transformation. The concept of using swept RF excitation instead of a monochromatic RF pulse or stochastic excitation and then reconstructing the NMR spectrum using the correlation method, was mentioned more than three decades ago [13], but was never put into practice.

The main advantage of SWIFT originates in its nearly simultaneous excitation and acquisition scheme. In conventional MRI, excitation and acquisition events are separated by the length of time known as the echo time (T_E), which is typically >1 ms. This length of time is too long to allow detection of slowly tumbling nuclei with short spin–spin relaxation time (T₂). By comparison, SWIFT allows T_E ≈ 0, because signal acquisition can begin within a few microseconds after excitation. Thus, SWIFT is a powerful tool for imaging objects having a broad distribution of relaxation times, including very short T₂ values. In particular, the method is expected to find extensive application for imaging and spectroscopy of semi-solid objects such as bone, macromolecules, and quadrupolar nuclei such as sodium-23 [15], potassium-39 [16], and boron-11 [17]. Here, a description of the new technique is provided, and proof-of-principle is shown by simulated and experimental data.

Section snippets

Sweeping technique

The simplest realization of the SWIFT method is presented in Fig. 1a. The scheme employs a sequence of RF pulses, each having a duration T_p typically in the millisecond range. In the present implementation, the RF pulse is one of the family of adiabatic hyperbolic secant (HSn) pulses [12], [18], [19] which utilizes both amplitude and frequency modulation. The amplitude of the pulse is denoted by the variable ω₁ (t) (= γB₁ (t), where γ is the gyromagnetic ratio and B₁ (t) is the time-dependent

Pulsed FT NMR

In the pulsed FT technique, the excitation pulse is generally assumed to be short (a delta function), so that the FT of this input function can be considered approximately equal to unity, and thus, the system spectrum (H (ω)) can be obtained directly via FT of the impulse response (h (t)). In MRI, the RF pulse can approach a delta function only by restricting the flip angle to ≪90°. As the pulse length increases, the pulsed FT spectrum suffers from increasing phase and baseline distortions.

Image contrast

In the implementation of SWIFT, the magnetic field variation created by the applied gradient G can readily exceed other potential contributions (e.g., magnetic susceptibility differences and an inhomogeneous B₀), so their effects are not perceivable in images. SWIFT images are also essentially unaffected by transverse relaxation, provided T₂ > 10dw (see Fig. 4). Under these conditions, the signal intensity depends only on T₁ and spin density (M₀) as [8], [28] $S = M_{0} \frac{1 - E_{1}^{SWIFT}}{1 - E_{1}^{SWIFT} \cos (θ)} \sin (θ) .$ For

Experiments

Although many new applications are anticipated for this novel MRI technique, here the utility of SWIFT is demonstrated for imaging spins with extremely short T₂ values (Fig. 5, Fig. 6). The first test object is thermoplastic, which is invisible by conventional MRI because the T₂ value is extremely short (∼0.3 ms). As can be seen from Fig. 5, SWIFT produced a highly resolved 3D image of the object. It is worth noting that these images were obtained using a standard MRI scanner (4.7 T) which is not

Conclusions

In conclusion, the SWIFT technique has many novel and beneficial properties for MRI:

(a) Fast. The method avoids not only delays associated with refocusing pulses or gradient inversion, but also time for an excitation pulse, which is combined with the acquisition period. Of course, like all fast imaging sequences, SWIFT is limited by existing imaging system hardware and chosen compromise between acquisition speed, spatial resolution and S/N.

(b) Sensitive to short T₂. The method is sensitive to

Acknowledgments

This research was supported by NIH Grants RR008079 and CA92004. The authors thank Dr. Ivan Tkac for helping with reconstruction software implementation and Dr. Jutta Ellermann for assistance in conducting the experiments.

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CommunicationFast and quiet MRI using a swept radiofrequency

Abstract

Introduction

Section snippets

Sweeping technique

Pulsed FT NMR

Image contrast

Experiments

Conclusions

Acknowledgments

J. Magn. Reson.

J. Magn. Reson.

Magn. Reson. Imaging

J. Magn. Reson.

J. Magn. Reson.

J. Magn. Reson.

J. Magn. Reson.

J. Magn. Reson. A

Adv. Magn. Reson.

Magn. Reson. Imaging

Resonance absorption by nuclear magnetic moments in a solid

Phys. Rev.

Nuclear induction

Phys. Rev.

The nuclear induction experiment

Phys. Rev.

Transient nutations in nuclear magnetic resonance

Phys. Rev.

Spin echoes

Phys. Rev.

Application of Fourier transform spectroscopy to magnetic resonance

Rev. Sci. Instrum.

Communication
Fast and quiet MRI using a swept radiofrequency