Biomedical Engineering Reference
In-Depth Information
Coronal
Axial
Sagittal
FIGURE 10.3
An example of the treatment planning screen. (Courtesy of InsighTec, Inc.)
10.2.1.1.5 Safety Devices
Safety devices include a sonication lamp, which is installed in
a prominent position on the MRI scanner and lights up during
treatment sonications, and three stop-sonication switches, which
instantaneously stop the delivery of ultrasound energy to the
patient. One is held by the patient who is instructed to squeeze it
in case of sudden discomfort or emergency; another is mounted
on the scanner for use by staff in the treatment room; and the
third switch is located in the control room, next to the operator.
treatment planning system allows the physician and physicist to
define the target, plan energy parameters for each sonication, ver-
ify safe beam path, and edit location and type of individual treat-
ment spots. The physician delineates the tumor and defines safe
treatment pass-zones that will avoid energy passage through sen-
sitive tissue (e.g., scar tissue) or critical structures as mentioned
before. The physician selects an application-specific treatment
protocol that determines the main attributes of the planned treat-
ment. The system then computes a treatment plan, composed of
20-100 sonication (focus position) points that cover the specified
target. Figure 10.3 shows an example of the treatment planning
for a patient with a uterine fibroid.
10.2.1.2 Mr Imaging Guidance
Magnetic resonance imaging (MRI) is used for guidance before,
during, and posttreatment with either a 1.5 T or 3.0 T MR scan-
ner (GE Medical Systems, Milwaukee, Wisconsin). In FUS treat-
ment, MRI provides a tool for precise visualization of the tumor
(or treatment target), other organs, critical structures, and the
ultrasound beam path. MR images enable the physician and
physicist to contour the treatment target and plan the treatment
in three dimensions. The beam path for each focus position and
sonication can be visualized slice by slice to verify that it is not
passing through any undesirable region such as nerve bundles,
major blood vessels, bone, air/gas pockets, and others.
Successful thermal ablation of each focal point is the end point
of a FUS treatment. FDA/CE approved MR thermometry using
a proton resonant frequency shift method (Peters et al. 1998,
Graham et al. 1999) provides the ability to get immediate feed-
back. Real time (with approximately a 3-second delay) thermal
maps generated by the ExAblate and calculated thermal doses
(Sapareto and Dewey 1984) superimposed on the MR anatomical
images enable the treatment team to monitor heating of the target
spot and determine immediately if the sonication was effective. It
also allows the operator to adjust the FUS treatment parameters
for each treatment spot to reach the designated temperature. Thus
MR provides the physician with a wealth of anatomical, geometri-
cal, and thermal information to ensure safety and efficacy.
10.2.1.4 treatment Delivery
Prior to focused ultrasound therapy, the effective focal spot
should be verified with lower energy (sublethal or sub-ablative
sonication) and adjusted using MR thermometry. For each focal
spot, the physician is able to select treatment parameters includ-
ing the acoustic power and delivery duration. The spots are
treated in sequence automatically determined by the ExAblate
system for total treatment time and safety optimization. In
addition, any spot can be picked up and treated manually by
the physician. During a treatment sequence, the robotic system
positions the transducer below the target point and delivers the
planned energy. The ExAblate directs the MR to continuously
acquire thermal images that include the point being treated and
the surrounding anatomy. The system provides real-time moni-
toring of energy deposition and the essential feedback of where
the energy was delivered and the temperature reached. This
quantitative feedback allows the operator to control and adjust
the treatment parameters in real time to ensure that the targeted
tumor is fully treated and the surrounding tissue is spared. The
workstation displays thermal images and computes and overlays
thermal doses on the treated regions.
10.2.1.3 treatment planning
To identify the treatment volume, the ExAblate uses conventional
diagnostic MR images taken at the beginning of treatment. The
10.2.1.5 Quality assurance
Quality Assurance (QA) of the various treatment systems
(hardware, software, and patient safety features) must be done
 
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