M.Sc Thesis

M.Sc StudentHugeri Malka
SubjectFeasibility Study of MRI Assessment of Tissue Mechanical
Properties Using HIFU Induced Acoustic Radiation
DepartmentDepartment of Biomedical Engineering
Supervisor PROF. Haim Azhari
Full Thesis textFull thesis text - English Version


Palpation has significant importance in pathological diagnostic since the bio-mechanical properties, especially stiffness, increase and are highly correlated with the physiological and pathological state of the tissue. However, the limitations of the subjective manual palpation call for remote palpation techniques development enabling quantitative and non-invasive tissue stiffness assessment.

Acoustic radiation force imaging (ARFI) has been suggested as a tool for remote palpation.  In this study two MR-ARFI sequences are introduced for assessment of local tissue mechanical properties.

The first method, named as MR-FEMT, provides fast relative stiffness evaluation by measuring the integrative displacement resulting from a single HIFU burst. The MR-ARFI sequence, synchronized to the HIFU activation, was designed to include the transient and relaxation phases of the displacement. Combining this measurement with the Kelvin-Voight viscoelastic tissue model, the local stiffness index was evaluated. The method was implemented on a gel phantom, ex-vivo bovine brain and chicken liver specimens. The results have demonstrated the ability to evaluate the relative local stiffness in 600ms and to distinguish between different tissues on the basis of their stiffness levels.

The second method included development of MR-ARFI sequence which enables displacement measurement with high temporal resolution (1ms). The temporal displacement of a gel phantom and an ex-vivo bovine brain induced by HIFU pulse transmission was measured. The results have demonstrated the ability to monitor fast repeatable tissue motion over time.

In addition, the second method was used to derive the elastic properties indirectly. Comparing the stiffness index, E*, values, the average discrepancy of the MR-FEMT was 5.6% in the phantom and only 2% in the "aged" bovine brain, validating the results obtained by the fast method.

The two different approaches presented in this study create two complimentary methods and demonstrate the feasibility of using MRI combined with HIFU for non-invasive assessment of tissue mechanical properties. The MR-FEMT technique enables fast relative stiffness evaluation with a high spatial resolution and therefore can be used to diagnose areas of suspected pathological tissues and also can be used for real-time monitoring of tissue stiffness to evaluate time based stiffness changes. This protocol can potentially serve as a mechanical feedback option during minimally invasive treatments. The second MR-ARFI technique provides high temporal resolution displacement measurements and therefore can be used to monitor fast repeatable motion and may be utilized as research tool to match tissue models for viscosity estimation.