MR thermometry in the human prostate gland at 3.0T for transurethral ultrasound therapy

Abstract

Purpose: To investigate the spatial, temporal, and temperature resolution of a segmented gradient echo echo-planar imaging (EPI) technique as applied to proton resonance frequency (PRF) shift thermometry at 3 T in the human prostate gland, and to determine appropriate sequence parameters for magnetic resonance imaging (MRI)-controlled transurethral ultrasound thermal therapy.

Materials and methods: Eleven healthy volunteers (age range 23-58) were scanned at 3 T with a 16-channel torso coil to study the behavior of a gradient echo EPI thermometry sequence. The temperature stability and geometric distortion were assessed for 11 different parameter sets. In a further five volunteers, the prostate T2* was measured.

Results: For all scan parameters investigated, the temperature standard deviation within the prostate was less than 1°C, while the distortion was less than 1 mm. Temperature stability was best with higher TE values (up to 25 msec), larger voxel sizes and lower EPI factors, but this had to be balanced against requirements for good spatial and temporal resolution. Prostate T2* values ranged from 30-50 msec.

Conclusion: A good balance between temperature stability and temporal/spatial resolution is obtained with TE = 15 msec, voxel size = 1.14 mm, and EPI factor = 9, resulting in a dynamic scan time of 7.2 seconds for the nine slices.

Keywords: MR thermometry; interventional MRI; proton resonance frequency shift; segmented EPI; thermal therapy.

Copyright © 2013 Wiley Periodicals, Inc.

Figures

Figure 1

a) Sagittal image of the pelvic region, showing typical location and orientation of imaging slices. b) Magnitude and c) phase thermometry images cropped to show region around prostate.

Figure 2

Typical temperature standard deviation maps for a series of prostate slices (top row) and equivalent T2w anatomical images (bottom row). The white arrows indicate locations in bladder and rectum where the temperature standard deviation is high due to physiologic motion or susceptibility from air.

Figure 3

Average temperature standard deviation as a function of (a) TE and (b) EPI factor. Distortion as a function of EPI factor is shown in (c). In these plots, each symbol color represents results for a different volunteer, averaged in each case over a prostate volume ROI. Values for constant parameters are indicated on each plot.

Figure 4

Temperature standard deviation maps with in-plane spatial resolutions of (a) 2.00 mm, (b) 1.45 mm and (c) 1.14 mm. A plot of temperature standard deviation as a function of voxel dimension is shown in (d). The different colours indicate data from separate volunteers. In all cases, TE = 15 ms and the EPI factor is 9.

Figure 5

Average prostate T2* for five different volunteers (left). Typical T2* map (right).