Modeling the interference between shear and longitudinal waves under high intensity focused ultrasound propagation in bone.

Magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) is a noninvasive thermal technique that enables rapid heating of a specific area in the human body. Its clinical relevance has been proven for the treatments of soft tissue tumors, like uterine fibroids, and for the treatments of solid tumors in bone. In MR-HIFU treatment, MR-thermometry is used to monitor the temperature evolution in soft tissue. However, this technique is currently unavailable for bone tissue. Computer models can play a key role in the accurate prediction and monitoring of temperature. Here, we present a computer ray tracing model that calculates the heat production density in the focal region. This model accounts for both the propagation of shear waves and the interference between longitudinal and shear waves. The model was first compared with a finite element approach which solves the Helmholtz equation in soft tissue and the frequency-domain wave equation in bone. To obtain the temperature evolution in the focal region, the heat equation was solved using the heat production density generated by the raytracer as a heat source. Then, we investigated the role of the interaction between shear and longitudinal waves in terms of dissipated power and temperature output. The results of our model were in agreement with the results obtained by solving the Helmholtz equation and the frequency-domain wave equation, both in soft tissue and bone. Our results suggest that it is imperative to include both shear waves and their interference with longitudinal waves in the model when simulating high intensity focused ultrasound propagation in solids. In fact, when modeling HIFU treatments, omitting the interference between shear and longitudinal waves leads to an over-estimation of the temperature increase in the tissues.

Modelling the temperature evolution of bone under high intensity focused ultrasound.

Magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) has been clinically shown to be effective for palliative pain management in patients suffering from skeletal metastasis. The underlying mechanism is supposed to be periosteal denervation caused by ablative temperatures reached through ultrasound heating of the cortex. The challenge is exact temperature control during sonication as MR-based thermometry approaches for bone tissue are currently not available. Thus, in contrast to the MR-HIFU ablation of soft tissue, a thermometry feedback to the HIFU is lacking, and the treatment of bone metastasis is entirely based on temperature information acquired in the soft tissue adjacent to the bone surface. However, heating of the adjacent tissue depends on the exact sonication protocol and requires extensive modelling to estimate the actual temperature of the cortex. Here we develop a computational model to calculate the spatial temperature evolution in bone and the adjacent tissue during sonication. First, a ray-tracing technique is used to compute the heat production in each spatial point serving as a source term for the second part, where the actual temperature is calculated as a function of space and time by solving the Pennes bio-heat equation. Importantly, our model includes shear waves that arise at the bone interface as well as all geometrical considerations of transducer and bone geometry. The model was compared with a theoretical approach based on the far field approximation and an MR-HIFU experiment using a bone phantom. Furthermore, we investigated the contribution of shear waves to the heat production and resulting temperatures in bone. The temperature evolution predicted by our model was in accordance with the far field approximation and agreed well with the experimental data obtained in phantoms. Our model allows the simulation of the HIFU treatments of bone metastasis in patients and can be extended to a planning tool prior to MR-HIFU treatments.

[Detection of focal lung lesions with magnetic resonance tomography using T2-weighted ultrashort turbo-spin-echo-sequence in comparison with spiral computerized tomography]. / Detektion fokaler Lungenläsionen mit der Magnetresonanz-Tomographie mittels T2-gewichteter Ultrashort-Turbo-Spin-Echo-Sequenz im Vergleich zur Spiral-Computer-Tomographie.

PURPOSE: To compare spiral CT and MRT for the detection of focal pulmonary lesions. PATIENTS AND METHODS: 50 patients with focal pulmonary lesions confirmed by spiral CT were examined using a T2-weighted UTSE sequence (TE: 90 ms, TR: 1500-3000 ms, echo interval 9 ms, 8 mm slice thickness, diastolic triggering, expiratory breath gating). Image quality was compared using a 4-stage scale. Lesions with a minimum size of 2 mm were counted and measured in the CT image. The results were compared with the MRT images. RESULTS: The image quality in CT examinations with an average value of 1.22 better than that in MRT (1.78). In total 163 pulmonary lesions with a size of 2-115 mm were found by CT. MRT found 151/163 lesions (92.6%). Of the 12 lesions not detected, 9 were smaller than 4 mm, 1 corresponded to a 12 mm large, completely calcified granuloma. In 2 cases there was a 4 or 5 mm large unspecific scar. Thus, 160/163 (98.1%) of all lesions larger than 3 mm were detected. CONCLUSIONS: MRT with use of a suitable UTSE sequence is an alternative to CT for the detection of focal pulmonary lesions with a size larger than 3 mm.

Comparison of inversion-recovery gradient- and spin-echo and fast spin-echo techniques in the detection and characterization of liver lesions.

PURPOSE: To compare respiratory-triggered inversion-recovery (IR) gradient- and spin-echo (GRASE) magnetic resonance (MR) imaging with respiratory-triggered T2-weighted fast spin-echo (SE) imaging in the diagnosis of liver metastases. MATERIALS AND METHODS: In this prospective study, two radiologists independently identified focal hepatic lesions on respiratory-triggered IR GRASE and respiratory-triggered fast SE MR images in 28 consecutive patients with 186 (135 malignant and 51 benign) proved lesions. A combination of findings at surgery, intraoperative ultrasonography (US), and histologic examination served as the standard of reference. Contrast-to-noise ratios (CNRs) were obtained from 86 lesions larger than 10 mm. RESULTS: The sensitivity in the detection of liver metastases was, independent of lesion size and observer, higher for IR GRASE imaging (55%) than for fast SE imaging (44%-50%) (observer 1, P = .014; observer 2, P = .21). Confidence levels with IR GRASE imaging were higher, but not significantly so, than those with fast SE imaging (P < .098). Both observers characterized liver lesions better with IR GRASE than with fast SE imaging (observer 1, P = .04; observer 2, P = .48). The metastasis-liver CNR was significantly higher (P = .012) with IR GRASE imaging. CONCLUSION: The respiratory-triggered IR GRASE sequence is a fast alternative to the respiratory-triggered fast SE sequence in the evaluation of suspected liver metastases.

Interventional breast MR imaging: clinical use of a stereotactic localization and biopsy device.

PURPOSE: To evaluate the usefulness of preoperative magnetic resonance (MR) imaging-guided stereotactic localization and core biopsy of suspicious breast lesions that are visible at breast MR imaging alone (ie, that are clinically, mammographically, and ultrasonographically occult), with the goal of integrating this technique into the diagnostic and therapeutic work-up of MR-suspicious lesions in a clinical setting. MATERIALS AND METHODS: A stereotactic breast biopsy device was used for needle placement in and guide wire localization of 97 lesions in 66 patients or core biopsy of five lesions in five patients; all lesions were visible at MR imaging. Interventions were performed with MR guidance on a 0.5- or 1.5-T system. RESULTS: Lesion localization and resection were successful in 95 of the 97 lesions; two of the lesions were not resected in spite of correct guide wire localization. In this series, 53 (55%) of 97 lesions proved malignant (11 [21%] in situ; 42 [79%] invasive). Lesions were 4-19 mm (mean, 8.7 mm); all invasive cancers corresponded to a pT1 tumor stage. Location of the lesion in the parenchyma (retroareolar or prepectoral) did not interfere with accessibility. CONCLUSION: MR imaging-guided stereotactic hook-wire placement and excisional biopsy are accurate and effective in managing lesions identified at only breast MR imaging. MR imaging-guided core biopsy holds promise for allowing a definite work-up of these lesions.

Breast neoplasms: T2* susceptibility-contrast, first-pass perfusion MR imaging.

PURPOSE: To evaluate the differentiation of benign from malignant breast tumors with T2*-weighted perfusion magnetic resonance (MR) imaging (blood volume imaging) versus that with dynamic T1-weighted contrast agent-enhanced MR imaging. MATERIALS AND METHODS: Ten healthy adult volunteers and 18 adult patients with benign or malignant lesions underwent both conventional T1-weighted dynamic contrast-enhanced breast MR imaging and repetitive first-pass, single-section, dynamic T2*-weighted perfusion MR imaging. Images were obtained before, during, and after injection of 20 mL of gadopentetate dimeglumine; peak gadopentetate dimeglumine concentrations were calculated from the maximal signal intensity loss on T2*-weighted images. RESULTS: No perfusion effect was detectable in healthy breast parenchyma. A strong susceptibility-mediated signal intensity loss occurred in malignant breast tumors. No or only minor perfusion effects were seen in fibroadenomas, in spite of their rapid enhancement at T1-weighted dynamic imaging. Perfusion imaging was possible after conventional dynamic contrast-enhanced breast MR imaging. CONCLUSION: T2*-weighted perfusion imaging exploits the susceptibility-mediated signal intensity loss of a first-pass bolus of gadopentetate dimeglumine within the capillary bed. First-pass perfusion imaging of breast lesions is feasible. It is promising in the differentiation of benign from malignant, rapidly enhancing lesions.

[MR mammography at 0.5 tesla. II. The capacity to differentiate malignant and benign lesions in MR mammography at 0.5 and 1.5 T]. / MR-Mammographie bei 0.5 Tesla. Teil II: Differenzierbarkeit maligner und benigner Läsionen in der MR-Mammographie bei 0.5 und 1.5 T.

PURPOSE: To determine whether medium field strength (0.5 T) MR mammography is able to differentiate between benign and malignant lesions in the same way as a 1.5 T standard technique. MATERIAL: In 40 consecutive female patients with nodular lesions, examinations were carried out at 1.5 T (2D-FFE, TR/TE/FA 200/3.9/80) and 0.5 T (3D-FFE, 24/3.4/40). RESULTS: There was wide spread of the speed of enhancement of malignant tumours for both techniques (44%-145% with signal increase in the first minute of 42%-189%). In 15 out of 17 carcinomas, the rapidity of uptake and final degree of enhancement after contrast was higher during 0.5 T/3D measurements than it was for 1.5 T/2D images. Fibroadenomas and mastopathies showed similar enhancement characteristics for both techniques. Sharply defined "wash-out" was seen only in malignant lesions; it appeared ten times more frequently in the 0.5 T/3D examinations than at 1.5 T/2D. CONCLUSIONS: The distinction between benign and malignant lesions can be made with more certainty using medium field strength and 3D-FFE sequence than using the 2D high-field standard technique.

[MR mammography at 0.5 tesla. I. Comparison of image quality and sensitivity of MR mammography at 0.5 and 1.5 T]. / MR-Mammographie bei 0.5 Tesla. Teil I: Vergleich von Bildqualität und Sensitivität der MR-Mammographie bei 0.5 und 1.5 T.

PURPOSE: To determine whether dynamic MR mammography is possible on midfield systems without loss of diagnostic sensitivity when compared to the standard highfield technique. MATERIALS AND METHODS: 42 consecutive patients were examined twice: Once using the standard dynamic 2D gradient echo technique at 1.5 T; a second examination was performed on a 0.5 T system. For the midfield examinations a 3D sequence with optimized T1 contrast was used to compensate for the shorter T1 relaxation times at 0.5 T. Subtraction images were calculated to improve detectability of enhancing lesions. RESULTS: Image quality was comparable on both systems. Mean enhancement of lesions was higher at 0.5 T/3D as compared to 1.5 T/2D (161% versus 112%). In malignant lesions, enhancement at 0.5 T/3D surpassed that at 1.5 T/2D in 88% of cases; average maximum signal intensity increase of cancers was significantly higher at 0.5 T/3D as compared to 1.5 T/2D (183% versus 108% relative to baseline). One satellite lesion of a recurrent carcinoma was detected on the 0.5 T/3D images only. CONCLUSION: A 3D gradient echo pulse sequence can be used to compensate for the T1 shortening effect of the lower field strength. With a 3D sequence, sensitivity of MRM at 0.5 T is even superior to that of the standard 2D highfield technique.

Calculation of electron beam dose distributions for arbitrarily shaped fields.

A method for the calculation of absorbed dose distributions of arbitrarily shaped electron beams is presented. Isodose distributions and output factors of treatment fields can be predicted with good accuracy, without the need for any dose measurement in the actual field. A Gaussian pencil beam model is employed with two different pencil beams for each electron beam energy. The values of the parameters of the pencil beam dose distributions are determined from a set of measurements of broad beam distributions; in this way the influence of electrons scattered by the applicator walls is taken into account. The dose distribution of electrons scattered from high atomic number metal frames, which define the treatment field contour at the skin, is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for beams with 6, 10, 14 and 20 MeV electron energy. The distance between calculated and measured isodose lines with values between 10 and 90% is under 0.3 cm. The difference between calculated and measured output factors does not exceed 2%.

Dose calculations for arbitrarily shaped electron beams.

A method for the calculation of absorbed dose distributions of arbitrarily shaped electron beams is described. Isodose distributions and the output factor of a newly designed treatment field can be predicted with good accuracy, without the need for any dose measurement in the actual field. Two different Gaussian pencil beams are used as building elements for the treatment beams of each electron energy. The dose distributions of the pencil beams are derived from measurements of broad beam dose distributions; in this way the influence of electrons scattered by the applicator walls is taken into account. The contribution to the dose by electrons scattered from a high Z metal frame which defines the treatment field contour is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for electron beams with 6, 10, 14 and 20 MeV nominal energy. The distance between calculated and measured isodose lines with values between 90 and 10 per cent of the maximum dose did not exceed a limit of 0.3 cm. The difference between calculated and measured output factors remained within 2 per cent.