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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_605_Библиотеки_им_академика_М_И_Перельмана.pdf
Diagnostic Procedures
Figure 5.23 (A) Normal blood flow of the kidney; (B) complete interruption of blood
inflow into the kidney due to thrombosis. All: Courtesy: Dr. E. Matevossian, Klinikum
rechts der Isar.
121
In visceral medicine, it is of outstanding importance to assess the
blood flow in the hepatic artery and portal vein (e.g., in case of portal
hypertension or after kidney transplantation).
A continuous wave Doppler (CW Doppler) continuously generates and
receives US waves. A CW Doppler uses two transducers. One transducer
continuously transmits and one transducer continuously receives signals.
In contrast to CW Doppler systems, PW Doppler Systems use a single
transducer for transmission and reception. Pulsed wave (PW) Doppler
systems transmit short pulses of US into the tissue. The pulse travels for a
given time until it is reflected back. It then returns to the transducer over
the same time interval, but at a shifted frequency. A timing mechanism
controls the range gating that samples the returning Doppler shift data
from a given region. Only Doppler shift data from inside that area is displayed. The transducer alternates transmission and reception of US.
Doppler signals can be acquired from a known depth. Today, pulsed
Doppler systems for spectral Doppler measurements are typically used in
combination with US B-mode imaging, which is known as duplex US.
Duplex scanners use arrays of elements to produce both the B-mode
image and the Doppler spectrum. This facilitates accurate anatomical
location of the blood flow under investigation.
An advantage of CW Doppler systems is their capability for measuring
movements with high velocity, which is needed in applications like cardiography, where high blood flow rates occur. A CW Doppler cannot provide range resolution because it is unable to separate Doppler signals that
arise from different points along the transmitted US beam. If two blood
vessels intersect the US beam, it will not be possible to separate velocities
122 Biomedical Engineering in Gastrointestinal Surgery
at the different points along the beam, so it cannot be used to produce
color flow images.
One main advantage of pulsed Doppler is its ability to provide
Doppler shift data selected from a small segment along the US beam,
referred to as the “sample volume.” The location of the sample volume
can be controlled by the operator.
Recent Developments and Current Research
There are recent developments in measuring blood flow via lowpeak frequency-modulated continuous wave (FMCW) and stepped
frequency-modulated continuous waves (s tep-FMCW) US Doppler
systems. Low-peak FMCW us es a new demodulation technique:
the Doppler signals are demodulated with a reference FMCW signal to
adjust delay times so that they are equal to propagation times between
the transmitter and the receiver. Doppler signals can be obtained from
a selected position, as with a sample volume in PW Doppler
systems
[49].
Step-FMCW leads to a high SNR and a high range resolution compared to traditional pulse-echo signals. In step-FMCW ultrasonic ranging,
the phase and magnitude differences at stepped frequencies are used to
sample the frequency domain. Step-FMCW features lower peak power,
wider dynamic range, lower noise figure, and simpler electronics in comparison to pulse-echo systems
[50].
Technologies using the Doppler-effect with pulsed US waves are
widely matured. Recent developments are not based on developments of
the CW Doppler technology but mostly on combinations of Doppler
visualizations with other modifications of US, like elastography.
5.4.5 US Elastography
US elastography is an imaging technique to evaluate the mechanical
properties of soft tissue by applying strain on the organ examined and
detecting differences in its tissue density and stiffness
found useful in showing abnormalities of muscle and breast tissue, and in
the detection of tumors
[53].
The stiffness of the liver correlates, e.g., with its content of fibrous tissue. A fibrotic liver is harder in the case of fibrosis, and even harder in
cirrhosis than a healthy liver. Most malignant tumors are, likewise, harder
than the surrounding tissue, a feature which was always used in medicine
to detect pathological findings (manual palpation).
[51,52]. It has been
123Diagnostic Procedures
The following techniques of US elastography will be considered in
detail below: acoustic radiation force impulse imaging (ARFI), shear
wave elastography (SWE), and shear wave dispersion US vibrometry
(SDUV).
5.4.5.1 Acoustic Radiation Force Impulse Imaging
ARFI imaging is a form of strain elastography which is mainly used for
liver, thyroid, and breast imaging. In this technique, the tissue is excited
internally by a focused US pulse. ARFI is based on the principle that as the
US pulse passes a tissue, the displacement of soft tissues is larger than the
displacement of hard tissues. As a result, this technique offers a qualitative
color-coded or grayscale elastogram representing relative tissue stiffness
[51].
Characteristics of the Modification
ARFI transmits a brief acoustic radiation force (0.0031 ms) to generate a localized displacement in tissue. This technique utilizes a single
transducer for both transmitting the radiation force and tracking the
resulting displacement of tissue.
To obtain displacement information for a spatial location through ARFI
imaging, a reference line is first acquired by a conventional US pulse.
Afterward, a radiation force impulse is induced in the same location to create a slight displacement. Thereafter, with the help of conventional US, a
series of tracking lines are acquired for monitoring displacement and recovery of the tissue. To observe the recovery of tissue as it returns to its original configuration, approximately 46 ms of tracking are required. The
repetition of this reference push-track procedure over other spatial locations
allows the creation of a 2D image by aligning the displacements of each
location in time relative to its pushing pulse
[54].
Strengths and Weaknesses
A general strength of ARFI is its ability to image deeper tissue, which
is not reachable by external compression
[51]. Moreover, ARFI requires
merely slight hardware modifications to add an alternative imaging mode
similar to Doppler or M-mode
[54] (Table 5.3).
Table 5.3 Key facts on ARFI
Typical
applications
Oncology
Endocrinology
Strengths and
weaknesses
High depth
penetration
No spatial
resolution
Recent
developments
n/a Real-time
Research potential
and future trends
scanning
Heat management
124 Biomedical Engineering in Gastrointestinal Surgery
A weakness of ARFI imaging is its inability to identify the tissue’s
composition. Nowadays, it is only possible to detect differences in the
tissue’s softness or stiffness in comparison with adjacent tissue. A further limitation is often encountered as a result of the obscur ity of the
acoustic force, as its influencing factor s—the acoustic intensity and
the degree of attenuation to the acoustic pulse—are normally
unknown.
Recent Developments and Current Research
Technical challenges fo r this imaging technique involve the
implementation of real-time scanning into a clinical US unit and its
heat management. The former may easily be achieved because the
imaging sequence of ARFI fits with the software and hardware performances of c linical scanners. The latter, which is mainly influenced by ARFI pushes, may be addressed by using parallel beamforming.
5.4.5.2 Shear Wave Elastography
SWE is a type of US elastography that uses shear waves to assess tissue
elasticity and display it in a quantitative manner. Unlike acoustic compressive waves, which spread in the same direction as the particle compression, the propagation of shear waves proceeds orthogonal to the
stimulated displacement. Shear waves can result from an acoustic radiation
force, mechanical punch, or external sources
imaging
[56].
[55]. SWE is used for breast
Characteristics of the Modification
The attenuation of shear waves is approximately 10,000 times more
rapid than conventional US
[51]. SWE provides a quantitative real-time
elastogram, whereby elasticity can be depicted as a superimposition of a
color-coded image measured in kPa over a B-mode image. In the images,
stiffer tissues appear in red while softer tissues are coded in blue. The
image resolution remains around 1 mm
[56].
Strengths and Weaknesses
SWE is currently the only approach with the ability to provide quan-
titative and local elastic information in real time.
A limitation of this technique is the weakness of the generated shear
waves due to dissipation, which occurs after spreading a few millimeters.
Stronger shear waves need increased US power, which causes overheating
in the hardware and concerns over acoustic power
[56] (Table 5.4).
Table 5.4 Key facts on SWE
Typical
applications
Strengths and
weaknesses
Diagnostic Procedures
Recent
developments
125
Research potential
and future trends
Oncology, e.g.,
liver tumors
Detection of
liver cirrhosis
Figure 5.24 SWE: Focal nodular hyperplasia of the liver (A) as compared to normal
tissue (B). Courtesy: Prof. K. Stock, Klinikum rechts der Isar.
Quantitative and
local elasticity
information
Low power of
shear waves
n/a Increasing
acoustic power
Currently, the detection of liver diseases is the main clinical applica-
tion (
Fig. 5.24).
Recent Developments and Current Research
Future challenges must deal with creating an upswing in the shear
waves’ amplitude in order to increase their ability to travel through tissue
while still limiting the acoustic power to safe levels.
5.4.5.3 Shear Wave Dispersion Ultrasound Vibrometry
In contrast to other imaging techniques mentioned earlier, shear wave
dispersion ultrasound vibrometry (SDUV) quantifies not only the elasticity of tissue, but also its viscosity. This technique utilizes shear wave propagation speed, which is measured in tissue at multiple frequencies within
the range of hundreds of Hertz. One sample application of this technique
is liver fibrosis staging.
126 Biomedical Engineering in Gastrointestinal Surgery
In general, SDUV is a fast imaging technique, especially when
repeated pulses are used. Accordingly, only 50200 ms are needed to
achieve measurements
[57].
Unlike ARFI, which is based on transient shear waves, SDUV uses
periodic shear waves and the dispersion of their velocity to qualify viscosity. The estimation of shear wave speed is based on the phase differences
and multiple cycles of shear wave vibration.
Strengths and Weaknesses
The main benefit of SDUV is the quantification of elasticity with
simultaneous consideration of viscosity. SDUV seems to be more beneficial than ARFI when tissue displacement or SNR is low. The shear wave
propagation depends only on the material properties and not on US
intensity and beam shape. Therefore, measurements are deviceindependent. In addition, the risk of interference due to shear wave echo
is reduced. Finally, short acquisition times of SDUV (about 0.1 s per
acquisition) “also allows fast acquisition of multiple measurements at different locations within the organ of interest to get a comprehensive assessment of tissue state”
[57].
The limitation of SDUV rests in its ability to provide a single-point
measurement. 2D imaging may be theoretically possible, but is timeconsuming under the current SDUV technique (
Table 5.5).
Recent Developments and Current Research
One notable future development for SDUV is an extension in the
field of applications. Another trend is an optimization in radiation force
delivery to create shear waves that produce better information. Beyond
this, there is a need to develop better methods for detection of shear
waves and solving for the viscoelastic material properties of the tissue will
further enhance the performance of SDUV.
Table 5.5 Key facts on SDUV
Typical
applications
Examination
of liver
fibrosis
Strengths and
weaknesses
Quantification of
elasticity and viscosity
of tissue
Long examination of
2D imaging
Recent
developments
n/a Optimizing
Research
potential and
future trends
radiation
force delivery
Extending field
of
applications
127Diagnostic Procedures
Elastography in general is expected to remain in broad use in medical
diagnostics. It is even predicted to gain further relevance in the future. It
combines techniques which have high potential to detect cancerous diseases. Therefore, it is necessary to improve modifications like SWE,
which is currently limited in its performance.
5.4.6 3D/4D Ultrasound
3D US is a volumetric imaging technology that provides a 3D view of
internal str uctures. Dynamic volumetric imag ing, also known as “4D
US” or “real-time 3D US,” extends the visualization with a time
frame so that it is able to display motion instead of a static 3D data
set.
3D data are usually acquired as a large number of consecutive
tomographic images through movement of an US transducer array.
Each tomographic image has to be gathered along wit h its positional
information to constr uct a 3D data set. Accurate positional information is obtained through an electromagnetic position sensor, an electric
gyro attached to t he probe, or by defining previous movement
(
Tab le 5.6 ).
Static 3D images can be acquired manually by moving a 2D transducer across a ROI, or automatically through the use of a 3D transducer
that sweeps a 2D array of beams across the ROI. 3D/4D US requires
rapid automatic sweeps of multiple adjacent 2D cross-sections.
Software
The software is the core of volumetric imaging technologies, especially 4D visualizations, which need highly optimized algorithms. For
applications like scanning a heart, a gated technique is applied to avoid
distortion of a 3D data set due to movement. Tomographic images are
rearranged according to the phase of the cardiac cycle and a 3D data set is
constructed with only tomographic images at the same phase of the
Table 5.6 Key facts on 3D/4D US
Typical
applications
Obstetrics,
cardiovascular
medicine
Visceral medicine
Strengths and
weaknesses
Vague depiction
of internal
structures
Recent
developments
USCT/Warm
bath US
Research potential
and future trends
Optimizing 3D US,
Increasing image
quality
128 Biomedical Engineering in Gastrointestinal Surgery
cardiac cycle. The heart can be seen beating three-dimensionally by
reconstructing many 3D data sets into a single cardiac cycle.
Strengths and Weaknesses
3D images provide examiners with an abundance of information,
reducing the amount of interpretation needed and limiting the probability
of misdiagnoses.
Compared to common US modifications, the amount of data involved
is much higher. The depiction of one ROI demands up to 20 GB of data
storage. Despite the increasing capability of computers, the processing
time of data sets is still a limiting factor.
Recent Developments and Current Research
US travels through soft tissue at an average speed of 1540 m/s, which
limits 3D scanning speed. The parallel receiving technique uses one broad
US beam that is transmitted; its echoes are received as plural ultrasonic
beams. In a 2D array probe, a high degree of parallel receiving is used
and high-speed 3D scanning is possible. As a result, the profound
advancements in 3D/4D imaging are mainly due to a general evolution
of electronics and transducer arrays from linear systems to 1.25D, 1.5D,
1.75D, and 2D arrays and the latest matrix phase transducers, which are a
current field of research
[58].
Currently, there are plans to make 4D US available through handheld
devices, which has already been achieved in high-end devices
[59].
Today, the relevance of 3D/4D US systems is still low due to high
purchasing costs and technological performance issues. In the future, the
relevance of these systems is expected to rise as a result of technical
improvements and mass-market adoption.
5.4.7 Ultrasound Computed Tomography
Ultrasound CT (USCT) is a new digital imaging technique that creates
reconstructed 3D images of inhomogeneous media, such as soft tissue. It
attempts to solve the problem of the inv erse-scattering field. A few research
groups have developed such systems to test its usability. One of them built a
USCT that consists of a water-filled cylinder and contains 1920 transducers—
384 sending and 1536 receiving transducers—which are grouped in three
rings on the cylinder surface. The water-filled cylinder can be moved in six
different positions via an electric motor. The advantage of this method is the
high spatial resolution and the high tissue contrast. There are different algorithms currently under research for evaluating the emerged data sets, such as
129Diagnostic Procedures
the synthetic aperture focusing technique (SAFT) algorithm developed and
optimized by the Forschungszentrum Karlsruhe/Germany, the multifrequency nonlinear 3D inverse-scattering algorithm, or new concepts like the
3D refraction corrected 360 degrees compounded reflection algorithm
[60,61]. The early diagnosis of breast cancer is still a major challenge in spite
of recent developments in research
[62].Thecurrentdiagnosticprocedure
only detects cancer that is already in a developed state. In order to diagnose
early-stage cancer within the breast, an application using USCT is currently
under development.
Future developments will focus on introducing this technique to clinical
use and to reduce the time needed to evaluate the examined data sets
[63].
5.5 NUCLEAR IMAGING SYSTEMS
Nuclear imaging systems use gamma rays, which, like X-rays, are a form
of electromagnetic, indirectly ionizing radiation, but possess more energy
due to higher frequencies
Images generated in nuclear medicine are a result of the selection and
injection of a suitable radioactive tracer, the detection of the radiation,
the use of tomographic reconstruction algorithms, and finally the conduction of a series of corrections
for the image acquisition. A radioactive biologically active substance is
chosen in such a way that its spatial and temporal distribution in the body
reflects a particular body function or metabolism. To avoid disturbances
of vital functions while studying the distributed radiation, only small
amounts of the tracers are administered to the patient. The gamma rays of
positrons are emitted as the tracer decays. The distribution of the radioactive tracer is inferred from the detected radiation and mapped as a function of time and/or space
Medical nuclear imaging assesses the functional aspects of organs,
whereas other techniques describe their anatomical structure. With its
current techniques, medical nuclear imaging offers high-resolution multidimensional images of organs in order to analyze complex structures and
physiologic functions for—among other things—computer-assisted
diagnosis, evaluation of treatments, and interventions
Nuclear imaging systems can be divided into three main categories:
positron emission tomography (PET), single-photon emission tomography, and the hybrid systems (see
[64].
[65]. The tracer principle works as a basis
[65,66].
[65].
Section 5.8: Hybrid Systems).
130 Biomedical Engineering in Gastrointestinal Surgery
5.5.1 Gamma Camera
The gamma camera, also called scintillation camera, is the most commonly used imaging device in nuclear medicine. It simultaneously detects
radiation from the entire FOV and enables the acquisition of dynamic as
well as static images of the area of interest in the human body
general, the gamma camera consists of a collimator, a scintillation crystal,
and photomultiplier tubes (PMTs).
Recent Developments and Current Research
Recent developments in gamma ray detection have addressed its
implementation within multimodal or hybrid systems. The fusion of PET
and single-photon emission computed tomography (SPECT) with MRI
systems (see
Section 5.8: Hybrid Systems) has been a particular challenge
because PMTs and nuclear imaging electronic hardware are sensitive
about magnetic fields. To overcome the problem of incompatibility, two
approaches have been developed.
The first approach uses optical fibers to couple the scintillation crystals
inside the magnet to either PMTs and electronics outside the fringe of
the magnetic field, or to solid-state photosensors situated at the end of
the magnet bore. However, such a connection tends to lose light signals
and limits the axial extent of the PET detector array due to difficulties in
connecting the fiber bundles to the scintillation crystals and then to routing them out of the magnet
[65,68].
The other approach substitutes the PMT with avalanche photodiodes,
which are magnetic field-insensitive, solid-state photon detectors. These
photodiodes are directly coupled to the scintillation element
[67].In
[69].
5.5.2 Positron Emission Tomography
PET is a noninvasive nuclear imaging technique that can help detect
anatomic, functional, and biochemical abnormalities in organs.
Furthermore, measurements of body functions such as blood flow,
oxygen usage, glucose metabolism, and tissue perfusion are possible.
This wide range of a pplica tions makes PET highly attractive for
diagnostic and interventional purposes i n cardiology, ne urology, and
oncology
multiple rings of detectors. Similar to gamma cameras, the PET detectors comprise of scintillation cr ystals with coupled PMTs. The ring
design utilizes the concept that two photons detected in close
[70].
A PET scanner consists of a dedicated camera system, including