Multilevel wavelet-transform encoding in MRI

Compressed-sensing MRI with random encoding

Compressed sensing (CS) has the potential to reduce magnetic resonance (MR) data acquisition time. In order for CS-based imaging schemes to be effective, the signal of interest should be sparse or compressible in a known representation, and the measurement scheme should have good mathematical properties with respect to this representation. While MR images are often compressible, the second requirement is often only weakly satisfied with respect to commonly used Fourier encoding schemes. This paper investigates the use of random encoding for CS-MRI, in an effort to emulate the "universal" encoding schemes suggested by the theoretical CS literature. This random encoding is achieved experimentally with tailored spatially-selective radio-frequency (RF) pulses. Both simulation and experimental studies were conducted to investigate the imaging properties of this new scheme with respect to Fourier schemes. Results indicate that random encoding has the potential to outperform conventional encoding in certain scenarios. However, our study also indicates that random encoding fails to satisfy theoretical sufficient conditions for stable and accurate CS reconstruction in many scenarios of interest. Therefore, there is still no general theoretical performance guarantee for CS-MRI, with or without random encoding, and CS-based methods should be developed and validated carefully in the context of specific applications.

PSF-choice: a novel MRI method for shaping point-spread functions in phase-encoding dimensions

An imaging method to obtain arbitrary point-spread functions (PSFs) in phase-encoding dimensions is described. This method, called PSF-Choice, is particularly relevant for applications, such as spectroscopic imaging, in which only a very few phase encodes are acquired and ringing artifact can be a serious problem. PSF-Choice uses partial 2D RF excitations to produce aliased excitations that are encoded using standard phase-encoding gradients. Theoretically, the PSF of the reconstructed result depends only on the RF excitation profile. Simulations demonstrate that a Gaussian-like PSF can be achieved, eliminating the side lobes that are associated with ringing artifact. It is further shown that neither the spatial resolution (as represented by the width of the PSF) nor the signal-to-noise ratio (SNR) of the method is adversely affected when compared to standard phase encoding. In the sense that the same number of encodes are required as with standard phase encoding, temporal resolution is also maintained. Phantom experiments demonstrate the initial feasibility of the method to eliminate ringing artifact.

MR-guided core biopsies using a closed 1.0 T imager. First clinical results

OBJECTIVE

High soft-tissue contrast and multiplanar imaging capabilities of MRI may be advantageous in biopsy guidance compared to CT. We report our first results with MR-guided core biopsies using a closed 1.0 T MR imager.

METHODS AND PATIENTS

In ten patients, seven liver lesions and one lesion each in the muscle of the back, the gluteal muscle and in the breast were biopsied under MR guidance using MR-compatible needles (Tru-Cut type, 18G and 14G). For control scans T1-weighted turbo-spin-echo (TSE), gradient-echo and T2-weighted TSE sequences were used.

RESULTS

In all patients, the suspicious lesions and the biopsy needle were exactly delineated in MR control scans. In nine out of ten patients, the suspicious lesion was clarified histologically. Controls of needle position in a second plane were performed twice. Pushing the inner stylet alone resulted in a distortion of the needle in several cases in its flat area. The small diameter of the MR gantry was inconvenient for a few patients. One complication (intrahepatic bleeding) was observed, which healed up without consequences.

CONCLUSION

Using a closed 1.0 T MR imager MR-guided core biopsies can be conducted efficiently. Core biopsies should be taken by pulling and pushing the outer cannula. Advantageous compared to CT are the multiplanar imaging capabilities, while the smaller gantry is disadvantageous.

Dynamic imaging with multiple resolutions along phase-encode and slice-select dimensions

An implementation is reported of an imaging method to obtain MUltiple Resolutions along Phase-encode and Slice-select dimensions (MURPS), which enables dynamic imaging of focal changes using a graded, multiresolution approach. MURPS allows one to trade spatial resolution in part of the volume for improved temporal resolution in dynamic imaging applications. A unique method of Hadamard slice encoding is used, enabling the varying of the phase encode and slice resolution while maintaining a constant effective TR throughout the entire 3-D volume. MURPS was implemented using a gradient-recalled echo sequence, and its utility was demonstrated for MR temperature monitoring. In this preliminary work, it has been shown that changes throughout a large volume can be effectively monitored in times that would normally only permit dynamic imaging in one or a very few slices.

Magnetic resonance image-guided biopsy and aspiration

Recent advances in magnet design and magnetic resonance (MR) system technology coupled with the development of fast gradient-echo pulse sequences have contributed to the increasing interest in interventional magnetic resonance imaging (MRI). Minimally invasive diagnostic and therapeutic image-based intervention can now be performed under near real-time MR guidance, taking advantage of the high tissue contrast, spatial resolution, vascular conspicuity and multiplanar capabilities of MRI to achieve safe and precise needle placement. This is particularly advantageous for needle navigation in regions of complex anatomy, such as the suprahyoid neck. This article discusses the theoretical concepts and clinical applications of MR for guidance for biopsy and aspiration, and highlights the technical developments that provide the foundation for interventional MRI.

Unaliasing by fourier-encoding the overlaps using the temporal dimension (UNFOLD), applied to cardiac imaging and fMRI

In several applications, MRI is used to monitor the time behavior of the signal in an organ of interest; e.g., signal evolution because of physiological motion, activation, or contrast-agent accumulation. Dynamic applications involve acquiring data in a k-t space, which contains both temporal and spatial information. It is shown here that in some dynamic applications, the t axis of k-t space is not densely filled with information. A method is introduced that can transfer information from the k axes to the t axis, allowing a denser, smaller k-t space to be acquired, and leading to significant reductions in the acquisition time of the temporal frames. Results are presented for cardiac-triggered imaging and functional MRI (fMRI), and are compared with data obtained in a conventional way. The temporal resolution was increased by nearly a factor of two in the cardiac-triggered study, and by as much as a factor of eight in the fMRI study. This increase allowed the acquisition of fMRI activation maps, even when the acquisition time for a single full time frame was actually longer than the paradigm cycle period itself. The new method can be used to significantly reduce the acquisition time of the individual temporal frames in certain dynamic studies. This can be used, for example, to increase the temporal or spatial resolution, increase the spatial coverage, decrease the total imaging time, or alter sequence parameters e.g., repetition time (TR) and echo time (TE) and thereby alter contrast. Magn Reson Med 42:813-828, 1999.

Partial wavelet encoding: a new approach for accelerating temporal resolution in contrast-enhanced MR imaging

We propose a new approach using wavelet encoding to improve temporal resolution in contrast-enhanced magnetic resonance (MR) imaging. Exploiting the unique property of wavelets localized in space and frequency, we construct an efficient encoding scheme to capture signal changes due to contrast agent uptake, which in general is spatially localized with low- and mid-range frequency components. On the basis of space-frequency analysis, we describe mathematical formulations of our method and discuss its theoretical advantages over Fourier-based phase-encoding methods (the keyhole and reduced-encoding imaging by generalized-series reconstruction [RIGR] techniques). The results obtained in computer simulations and a phantom study demonstrate the feasibility and practical advantages of our approach.

Dynamic tracking in interventional MRI using wavelet-encoded gradient-echo sequences

This work describes a newly developed magnetic resonance imaging (MRI) data-acquisition strategy which replaces the standard Fourier phase-encoding with the spatially localized coefficients of wavelet-encoding and offers a new technique for image guidance when combined with a dynamic tracking algorithm. By using this new technique, only a specific fraction of the entire raw data set needs to be updated and reconstructed to visualize the movement of an interventional device during an MR guided procedure. The combination of wavelet-encoding and a dynamic tracking algorithm was implemented in two-dimensional and three-dimensional gradient-echo sequences on a 0.2-T open C-arm-shaped MR system (Siemens, Erlangen Germany) and tested in phantom and in vitro experiments. When applying the wavelet-encoding direction parallel to the movement of a straight interventional device, only those spatially localized wavelet-coefficients mainly affected by the interventional device are updated. This led to potential increases of the image frame rate by a factor of up to seven.

Digital wavelet-encoded MRI: a new wavelet-encoding methodology

A new digital wavelet-encoding method for MRI is described. The method differs from previously described wavelet-encoding approaches, because the point-spread function is made independent of the wavelet basis used. This has a significant practical advantage, because wavelet bases can now be considered that would otherwise be excluded due to the difficulty of precisely exciting wavelet-shaped RF profiles. The method has been implemented on a clinical MRI system, and human images are presented.

Wavelet encoding for improved SNR and retrospective slice thickness adjustment

A method to encode the slice-select direction in magnetic resonance imaging through the use of a discrete wavelet transform is presented. The paper reports the first implementation of wavelet encoding using the Haar basis. Theoretically, the signal-to-noise ratio (SNR) for two levels of wavelet encoding is expected to be approximately 1.6 times higher than for conventional multislice imaging in which each slice is excited separately. A slightly lower SNR is encountered experimentally because of slice interference. The design of radiofrequency (RF) pulses to excite slices approximating Haar wavelet basis functions is discussed. Wavelet encoding also provides an efficient method for balancing slice thickness and SNR after an image has been acquired. The experiments have been conducted at 1.5 Tesla. Wavelet encoding of slices should be especially suitable for low-field MRI, in which the SNR is lower and the characteristically lower T1 values favor the use of shorter TR, which sometimes forces the slices to be acquired in multiple passes.

Non-Fourier encoding with multiple spin echoes

The advantages and limitations of multiple spin-echo sequences for non-Fourier encoding are investigated. Complications caused by improper encoding of alternate magnetization pathways due to imperfect refocusing pulses are analyzed. It is shown that mirror image ghosts result if the encoding RF pulse matrix is real-valued. These ghosts can be avoided as long as the rows of the RF pulse matrix are conjugate symmetric, which implies that spatial profiles are real valued. Non-Fourier encoding using bases derived from wavelet, Hadamard, and other real-valued orthogonal functions does not result in a mirror ghost artifact. A RARE sequence for non-Fourier encoding has been implemented on a clinical imaging system and successfully applied for brain imaging.

Wavelet encoding for 3D gradient-echo MR imaging

An implementation of wavelet encoding to resolve one spatial direction of a three-dimensional gradient-echo magnetic resonance image is described. This is the first report of the use of wavelet encoding with a relatively short repetition time for rapid image acquisition. An important feature of this implementation is that it allows wavelet-encoded images to be reconstructed without phase correction. The images show no discernible degradation compared with images acquired with phase encoding or scaling-function encoding instead of wavelet encoding. In addition to describing the implementation, typical sources of phase errors are investigated both theoretically and experimentally.