IEEE Signal Processing - March 2018 - 166

Life ScienceS
Emad S. Ebbini, Claudio Simon,
and Dalong Liu

real-Time Ultrasound Thermography
and Thermometry

his article describes the basic principles of ultrasound thermography
(UST) and its real-time implementation using graphics processing unit
(GPU)-enabled software architecture.
In medicine, the term thermography
is mostly associated with heat-sensing
infrared cameras for recording surface
temperature changes. In this article,
we use this term to describe the qualitative noninvasive imaging of tissue
temperature change using any imaging
modality. Examples of these modalities
include microwave radiometry, magnetic resonance imaging (MRI), US
imaging, and photoacoustic tomography. Of these imaging methods, US and
MR are the most widely investigated.
Table S1 in "Real-Time Thermography
in Medicine" lists some US- and MRbased methods for tissue thermography
reported in recent literature.
The methods listed in Table S1 have
been validated experimentally in various media, including in vivo [1], [3], [5].
As indicated in the table, all of these
methods exhibit some level of tissue
dependence. To measure quantitative
thermometry data, the changes in the
measured temperature-sensitive parameter need to be adjusted for tissue type
such as muscle, fat, liver, etc. This can
be challenging in some tissues such as
cirrhotic liver or breast.

Digital Object Identifier 10.1109/MSP.2017.2773338
Date of publication: 7 March 2018

166

Image guidance offers the promise of revolutionizing surgery in the
21st century. For example, focused US
(FUS) has been shown to produce localized thermal therapy in deep tissue
targets without the need for resection.
However, while the principle of this surgery was well demonstrated in the early
1950s, it never gained clinical acceptance until after the successful demonstration of MR-guided FUS (MRgFUS)
surgery. In particular, the implementation of MR thermometry [1] was the key
enabling image guidance technology for
MRgFUS to gain clinical approval from
the U.S. Food and Drug Administration
(FDA). Given US's high accessibility,
cost-effectiveness, and clinical utility
in initial diagnosis, the robust implementation of UST on real-time scanners could significantly impact the use
of image-guided FUS worldwide, with
or without MRI.
Noninvasive thermometry remains
a sought-after goal in medical imaging with numerous applications. In the
context of image guidance, quantitative
thermometry will provide the tools for delivering and evaluating the treatment as a
prescription in addition to the monitoring
and guidance provided by real-time UST
(rtUST). More significantly, noninvasive
thermometry will open the door for applications such as imaging inflammation
and metabolic rate. These applications require high sensitivity and specificity due
to the need to detect very small changes
in temperature at the inflammation site,
IEEE SIgnal ProcESSIng MagazInE | March 2018 |

possibly in the presence of large tissue
motion and deformation.
The feasibility of two-dimensional
(2-D) temperature imaging using diagnostic pulse-echo US was demonstrated
almost two decades ago [2]. The approach
that received the most attention is based
on the detection and estimation of minute temperature-induced echo shifts,
which could be processed to generate the
desired 2-D maps of temperature change.
The principle of measurement has been
confirmed by a number of groups in tissuemimicking phantoms, ex vivo models
and, recently, in vivo small animal
models [3]. Furthermore, GPU-enabled
rtUST systems have been developed and
demonstrated in preclinical settings.
The advent of rtUST, together with
validation studies by numerous groups
worldwide, has improved the odds of its
clinical acceptance, but important challenges remain. One challenge is the reliable echo-shift estimation in the presence
of large tissue deformations in organs
such as the liver and the heart. Tissue
deformations result in echo decorrelation, which increases the variance of the
estimation error. This is partially mitigated by the development of rtUST software architectures capable of supporting
hundreds of frames per second (fps)
thus minimizing frame-to-frame echo
decorrelations. Another challenge is the
distortion of the US imaging beams as
they traverse regions undergoing temperature change. This introduces errors
in the echo-shift estimation distal to the

Table of Contents for the Digital Edition of IEEE Signal Processing - March 2018