Mika W. Vogel

B1 Mapping by Bloch-Siegert Shift

A novel method for amplitude of radiofrequency field (B  1+ ) mapping based on the Bloch‐Siegert shift is presented. Unlike conventionally applied double‐angle or other signal magnitude–based methods, it encodes the B1 information into signal phase, resulting in important advantages in terms of acquisition speed, accuracy, and robustness. The Bloch‐Siegert frequency shift is caused by irradiating with an off‐resonance radiofrequency pulse following conventional spin excitation. When applying the off‐resonance radiofrequency in the kilohertz range, spin nutation can be neglected and the primarily observed effect is a spin precession frequency shift. This shift is proportional to the square of the magnitude of B  12 . Adding gradient image encoding following the off‐resonance pulse allows one to acquire spatially resolved B1 maps. The frequency shift from the Bloch‐Siegert effect gives a phase shift in the image that is proportional to B  12 . The phase difference of two acquisitions, with the radiofrequency pulse applied at two frequencies symmetrically around the water resonance, is used to eliminate undesired off‐resonance effects due to amplitude of static field inhomogeneity and chemical shift. In vivo Bloch‐Siegert B1 mapping with 25 sec/slice is demonstrated to be quantitatively comparable to a 21‐min double‐angle map. As such, this method enables robust, high‐resolution B  1+ mapping in a clinically acceptable time frame. Magn Reson Med 63:1315–1322, 2010.

Acoustical properties of selected tissue phantom materials for ultrasound imaging

This note summarizes the characterization of the acoustic properties of four materials intended for the development of tissue, and especially breast tissue, phantoms for the use in photoacoustic and ultrasound imaging. The materials are agar, silicone, polyvinyl alcohol gel (PVA) and polyacrylamide gel (PAA). The acoustical properties, i.e., the speed of sound, impedance and acoustic attenuation, are determined by transmission measurements of sound waves at room temperature under controlled conditions. Although the materials are tested for application such as photoacoustic phantoms, we focus here on the acoustic properties, while the optical properties will be discussed elsewhere. To obtain the acoustic attenuation in a frequency range from 4 MHz to 14 MHz, two ultrasound sources of 5 MHz and 10 MHz core frequencies are used. For preparation, each sample is cast into blocks of three different thicknesses. Agar, PVA and PAA show similar acoustic properties as water. Within silicone polymer, a significantly lower speed of sound and higher acoustical attenuation than in water and human tissue were found. All materials can be cast into arbitrary shapes and are suitable for tissue-mimicking phantoms. Due to its lower speed of sound, silicone is generally less suitable than the other presented materials.

Small-tip-angle spokes pulse design using interleaved greedy and local optimization methods.

Current spokes pulse design methods can be grouped into methods based either on sparse approximation or on iterative local (gradient descent‐based) optimization of the transverse‐plane spatial frequency locations visited by the spokes. These two classes of methods have complementary strengths and weaknesses: sparse approximation‐based methods perform an efficient search over a large swath of candidate spatial frequency locations but most are incompatible with off‐resonance compensation, multifrequency designs, and target phase relaxation, while local methods can accommodate off‐resonance and target phase relaxation but are sensitive to initialization and suboptimal local cost function minima. This article introduces a method that interleaves local iterations, which optimize the radiofrequency pulses, target phase patterns, and spatial frequency locations, with a greedy method to choose new locations. Simulations and experiments at 3 and 7 T show that the method consistently produces single‐ and multifrequency spokes pulses with lower flip angle inhomogeneity compared to current methods. Magn Reson Med, 2012.

Local specific absorption rate in high-pass birdcage and transverse electromagnetic body coils for multiple human body models in clinical landmark positions at 3T

To use electromagnetic (EM) simulations to study the effects of body type, landmark position, and radiofrequency (RF) body coil type on peak local specific absorption rate (SAR) in 3T magnetic resonance imaging (MRI).

Fast radiofrequency flip angle calibration by Bloch–Siegert shift

In a recent work, we presented a novel method for B  1+ field mapping based on the Bloch–Siegert shift. Here, we apply this method to automated fast radiofrequency transmit gain calibration. Two off‐resonance radiofrequency pulses were added to a slice‐selective spin echo sequence. The off‐resonance pulses induce a Bloch–Siegert phase shift in the acquired signal that is proportional to the square of the radiofrequency field magnitude B12. The signal is further spatially localized by a readout gradient, and the signal‐weighted average B1 field is calculated. This calibration from starting system transmit gain to average flip angle is used to calculate the transmit gain setting needed to produce a desired imaging sequence flip angle. A robust implementation is demonstrated with a scan time of 3 s. The Bloch–Siegert‐based calibration was used to predict the transmit gain for a 90° radiofrequency pulse and gave a flip angle of 88.6 ± 3.42° when tested in vivo in 32 volunteers. Magn Reson Med, 2011.

Photoacoustic Image Reconstruction - A Quantitative Analysis

Photoacoustic imaging is a promising new way to generate unprecedented contrast in ultrasound diagnostic imaging. It differs from other medical imaging approaches, in that it provides spatially resolved information about optical absorption of targeted tissue structures. Because the data acquisition process deviates from standard clinical ultrasound, choice of the proper image reconstruction method is crucial for successful application of the technique. In the literature, multiple approaches have been advocated, and the purpose of this paper is to compare four reconstruction techniques. Thereby, we focused on resolution limits, stability, reconstruction speed, and SNR. We generated experimental and simulated data and reconstructed images of the pressure distribution using four different methods: delay-and-sum (DnS), circular backprojection (CBP), generalized 2D Hough transform (HTA), and Fourier transform (FTA). All methods were able to depict the point sources properly. DnS and CBP produce blurred images containing typical superposition artifacts. The HTA provides excellent SNR and allows a good point source separation. The FTA is the fastest and shows the best FWHM. In our study, we found the FTA to show the best overall performance. It allows a very fast and theoretically exact reconstruction. Only a hardware-implemented DnS might be faster and enable real-time imaging. A commercial system may also perform several methods to fully utilize the new contrast mechanism and guarantee optimal resolution and fidelity.

Nonuniform and multidimensional Shinnar‐Le Roux RF pulse design method

The Shinnar‐Le Roux (SLR) radiofrequency (RF) pulse design algorithm is widely used for designing slice‐selective RF pulses due to its intuitiveness, optimality, and speed. SLR is limited, however, in that it is only capable of designing one‐dimensional pulses played along constant gradients. We present a nonuniform SLR RF pulse design framework that extends most of the capabilities of classical SLR to nonuniform gradient trajectories and multiple dimensions. Specifically, like classical SLR, the new method is a hard pulse approximation‐based technique that uses filter design relationships to produce the lowest power RF pulse that satisfies target magnetization ripple levels. The new method is validated and compared with methods conventionally used for nonuniform and multidimensional large‐tip‐angle RF pulse design. Magn Reson Med, 2012.

Spiral imaging artifact reduction: a comparison of two k-trajectory measurement methods.

To compare an external sensor‐based k‐space calibration technique with a routine precalibration method for quantification of method accuracy and reduction of spiral imaging artifacts to obtain improved image quality.

OPUS : Optoacoustic imaging combined with conventional ultrasound for breast cancer detection

Besides x-ray imaging, sonography is the most common method for breast cancer screening. The intention of our work is to develop optoacoustical imaging as an add-on to a conventional system. While ultrasound imaging reveals acoustical properties of tissue, optoacoustics generates an image of the distribution of optical absorption. Hence, it can be a valuable addition to sonography, because acoustical properties of different tissues show only a slight variation whereas the optical properties may differ strongly. Additionally, optoacoustics gives access to physiological parameters, like oxygen saturation of blood. For the presented work, we combine a conventional ultrasound system to a 100 Hz laser. The laser system consists of a Nd:YAG-laser at a wavelength of 532 nm with 7 ns pulse duration, coupled to a tunable Optical Parametric Oscillator (OPO) with a tuning rage from 680 nm to 2500 nm. The tunable laser source allows the selection of wavelengths which compromising high spectral information content with high skin transmission. The laser pulse is delivered fiber-optically to the ultrasound transducer and coupled into the acoustical field of view. Homogeneous illumination is crucial in order to achieve unblurred images. Furthermore the maximum allowed pulse intensities in accordance with standards for medical equipment have to be met to achieve a high signal to noise ration. The ultrasound instrument generates the trigger signal which controls the laser pulsing in order to apply ultrasound instruments imaging procedures without major modifications to generate an optoacoustic image. Detection of the optoacoustic signal as well as of the classical ultrasound signal is carried out by the standard medical ultrasound transducer. The characterization of the system, including quantitative measurements, performed on tissue phantoms, is presented. These phantoms have been specially designed regarding their acoustical as well as their optical properties.

First Practical Experiences with the Optoacoustic/Ultrasound System OPUS

The OPUS (OPtoacoustic UltraSound) system combines a conventional ultrasound (US) system with a specially designed OPO (Optical Parametrical Oscillator) laser system to generate and detect optoacoustical (OA) signals at multiple wavelengths. The intention of this combination was to demonstrate that a conventional ultrasound system can be transformed into an optoacoustic module without major modifications. To offer operational ease of use similar to those of the conventional US instrumentation, i.e. slow moving of the US transducer over the examined tissue area, a high repetition rate of the laser is required. A repetition rate of 100 Hz of the laser system enables a fast image frame rate. Different approaches for the presentation of the two types of images to the operator are compared. For an optimum applicability of the system we found it essential to provide both, the well-known US image and the OA image of the same tissue section to the user. The operator has now the possibility to overlay both images on one screen and thus to extract the desired information from each imaging mode.