Figure 4

Typical examples of MRT artifacts. The arrows point out MRT artifacts in magnitude (M) images, temperature (T) images, and phase (P) images. The red dashed triangles indicate the expected HIFU cone. (a) A respiratory time-varying field inhomogeneity MRT artifact (treatment 5, sagittal slice, supine position, osteolytic lesion in the pubic bone). The artifact causes periodical “blinking” of the temperature map. The arrow points out an additional non-respiratory time-varying field inhomogeneity MRT artifact. (b) A non-respiratory time-varying field inhomogeneity MRT artifact (treatment 9, sagittal slice, supine position, mixed lesion in the sacrum). The artifact is caused by an air cavity, which can also be seen in the magnitude image. The local changes in the phase image around the location of the air cavity verify that the air cavity is the source. (c) An arterial ghosting MRT artifact (treatment 5, sagittal slice, supine position, osteolytic lesion in the pubic bone) caused by the femoral artery, which can also be seen in the magnitude image. (d) Muscle contraction MRT artifacts (treatment 5, transverse slice, prone position, mixed lesion in the pubic bone). The artifacts occur not only at the location of the contracting muscles (two arrows at the most right) but also around the rectum. The three arrowheads point out additional arterial ghosting MRT artifacts. (e) A gross body movement MRT artifact (treatment 6, transverse slice, supine position, mixed lesion in the sacrum, affects the whole temperature image drastically. In the dashed ellipses in (c) and (d), heating due to the HIFU treatment can be observed. The image shown in (b) was acquired before HIFU sonication started; thus, no HIFU heating was expected to be observed. In (a) and (e), the visualization of potential HIFU heating was hampered due to the presence of the artifact.