Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
E1 E2E3 E4
10 mm
10 mm
10 mm
FIGURE 11.4
Transurethral ultrasound applicator with rotating multi-element planar transducer for MR guided prostate ablation: (a) distal
end of applicator with transducer array; (b) sagittal image and alignment of MRI slices for monitoring temperature delivery and providing closed
loop treatment control during rotation to conform to (c) preset outer boundary; (d) cumulative temperature and thermal dose map correlating to
completed coagulation within target zone obtained in clinical pilot study. (Reprinted with permission from Chopra R. et al.
Int J Hyperthermia,
26, 8, 2010.)
with the applicator and within an ancillary endorectal cooling
device to thermally protect the rectum and urethral mucosa.
The planar applicator without rotation can generate thermal
lesions ~3 mm wide and extending 5-20 mm deep from the ure-
thra. During treatment rotation, speed and applied power are
both automatically controlled with an adaptive feedback control
loop based upon the MR thermometry and target boundary.
Once positioned within the prostate, the transducer assembly
can be rotated (on average 15°/min) to sweep the heating zone
(54 -55°C outer contour line) and conform the coagulation to
a larger target volume, emanating from the urethra outward
toward the capsule boundary. The rotation rate is driven by an
MRI compatible piezoceramic motor under computer control.
Treatment modeling and canine experiments indicate that mon-
itoring temperature with multiple slices through the target and
automatic feedback control can maintain better than 1-3 mm
accuracy in ablation zones [58,59]. A clinical feasibility study
has recently been undertaken using this device, limited to two
transducer elements active and one 10 mm MR monitoring slice,
to target 30% of the prostate gland prior to prostatectomy. The
results have indicated that transurethral ultrasound under MR
treatment control is feasible and has potential to be a precise
method to ablate prostate tissue [56,57].
11.4 Interstitial and Intraluminal
Devices
11.4.1 Interstitial with tubular Sources
Interstitial ultrasound applicators using tubular transducer arrays
have been developed for applying hyperthermia in conjunction
with brachytherapy and investigated for high-temperature ther-
mal therapy alone [60-65]. As shown in Figure 11.5a, devices con-
sist of a linear array of small tubular transducers, each wired for
independent power control, which are inserted within a plastic
implant catheter. These closed-ended plastic catheters are inserted
directly into the target region during a surgical procedure, and
can be either single or multiple catheter implants depending
upon the size of the target region. Water flow within the catheter
couples the ultrasound energy and extends penetration of the
maximum temperature away from the catheter surface. The ultra-
sound energy emanating from each transducer section, typically
6.5-9 MHz, is collimated within the borders of each segment so
that the axial length of the therapeutic temperature zone remains
well defined by the number of active elements over a large range
of treatment duration and applied power levels [63, 66]. The rela-
tive power levels to each transducer section can be adjusted dur-
ing the treatment to tailor the heating zone along the applicator
