Combining rheology and MRI: Imaging healthy and tumorous brains based on mechanical properties.

PURPOSE: It is well known that pathological changes in tissue alter its mechanical properties. This holds also true for brain tissue. In case of the brain, however, obtaining information about these properties is hard due to the surrounding cranial bone. In this paper a novel technique to create an imaging contrast based on the aforementioned properties is presented. METHODS: The method is based on an excitation of the brain induced by a short fall. The response of the brain tissue is measured using a motion sensitive MRI sequence. RESULTS: The new method is tested by measurements on phantom material as well as on healthy volunteers. In a proof of principle experiment the capability of the approach to identify local alterations in the mechanical properties is shown by means of measurements on meningioma patients. CONCLUSION: The presented results show the feasibility of the novel method. Even in this early state of the proposed method, comparisons of measurements on meningioma patients with intraoperative palpation suggest that meningioma tissue responds differently to the excitation depending on their mechanical properties. Magn Reson Med 78:930-940, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Acoustic radiation force contrast in MRI: detection of calcifications in tissue-mimicking phantoms.

PURPOSE: Mammography is a widely used tool for the screening of breast cancer, and calcifications are a common finding in most mammograms. The location, size, number, morphology, and distribution of calcifications are an important information to differentiate a benign lesion from probably malignant pathologies. Calcifications are not detectable with a standard dynamic contrast enhanced breast MRI. The authors present a novel method for the detection and imaging of calcifications in breast tissue without ionizing radiation or contrast agents. METHODS: Measurements of localized tissue displacement in phantoms due to applied acoustic radiation force were performed. This displacement was imaged with a displacement sensitive spin-echo MRI sequence. Pieces of eggshell that represent calcifications were embedded in tissue-mimicking agarose phantoms. The sizes of the calcifications were 0.8 x 0.8 x 0.4, 1.5 x 1.5 x 0.4, and 2 x 3 x 0.4 mm3. The calcifications were scanned with ultrasound (U.S.) at 2.5 MHz and intensities up to I(spta) =7.18 W/cm2. The U.S. beam was moved inside the phantom by a computer-controlled three-dimensional hydraulic positioning system. The U.S. beam was scanned over the two smaller calcifications with the displacement sensitivity of the MRI sequence parallel to the U.S. beam path. Grayscale coded maps of the displacement scans are presented. For the 0.8 x 0.8 x 0.4 mm3 calcification, the U.S. intensities were varied. Finite element simulations were performed to verify if the experiments complied with theory. RESULTS: The authors found that the displacement caused by the U.S. is increased at the position of the calcification. The area of increased displacement is at least twice as large as the calcification itself. The simulations show this increase in displacement and area at the position of the calcification. When changing the displacement sensitivity direction to perpendicular to the U.S. beam, a crossed black and white four-leaf clover is visible at the position of the calcification. CONCLUSIONS: The U.S. is scattered and reflected by the calcifications. This leads to the increased displacement which is transmitted to the surrounding material because of the elastic coupling between the calcification and the agarose material. Due to the high differences in acoustic impedance and elastic properties between the surrounding tissue and the calcification, even the detection of pieces smaller than the resolution of the MRI scanner is possible. The acoustic radiation force contrast in MR phase-difference images offers a positive signal for calcifications from a smooth background in phantoms. This method offers a possibility of differentiating qualitatively and quantitatively hard calcifications from stiffer inclusions such as tumors.