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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_605_Библиотеки_им_академика_М_И_Перельмана.pdf
111Diagnostic Procedures
Figure 5.16 The opposite magnetic fields and the FFP at the origin. From Goodwill
PW, Saritas EU, Croft LR, Kim TN, Krishnan KM, Schaffer DV, et al. X-space MPI: magnetic
nanoparticles for safe medical imaging. Adv Mater 2012;24(28):38707. Modified by D.
Ostler.
Table 5.1 Key facts on MPI
Typical
applications
Strengths and
weaknesses
Recent
developments
Research
potential and
future trends
Intracanalicular No line-of-sight
restrictions
Integrable into every
tool
Superior spatial
resolution
High contrast
No attenuation
Real-time
MPI and
projection
MPI
Less accur acy
Field distortion
by electromagnetic
objects
The unsaturated ferromagnetic particles produce an MPI signal in the
receiver coil, while those that are not saturated do not. This is further
affected by the fact that the receiver coil can only detect time-varying
magnetization. By rapidly shifting the FFP across the field-of-view, an
image is acquired and a voltage is induced in the receiver coil. The raw
MPI signal from the ferromagnetic nanoparticles is then reconstructed.
Currently, there are two principal methods for reconstructing an MPI
image: harmonic-space MPI and x-space MPI (
Table 5.1).
112 Biomedical Engineering in Gastrointestinal Surgery
Human tissue does not generate an MPI signal. This means MPI images
have near-perfect contrast, with no obscuring background tissue. In addition, there is a zero depth attenuation even with low-frequency magnetic
fields, which means that the MPI scan is quantitative at any depth. The
outstanding contrast in MPI images offers a clear advantage over today’s
standard angiography techniques, such as X-ray, CT, and MRI angiography. In addition, MPI does not require the use of toxic CA.
Current research efforts in MPI focus on real-time MPI and projection MPI.
Future directions for MPI are uncertain, but the number of procedures that could be carried out using this technique is significant. Next
steps will involve developing this technique into a medical imaging
modality, and ultimately, introducing new imaging technologies and new
hardware components. MPI can also be a useful method for cancer detection and angiography in fur ther clinical applications.
5.4 DIAGNOSTIC ULTRASOUND
According to the World Health Organization, ultrasound (US) is defined
as “sound of frequencies above 20,000 hertz (Hz), beyond the range of
human hearing.” Frequencies of 130 megahertz (MHz) are typical for
diagnostic purposes.
Ultrasonic waves are produced by devices containing transducer elements that convert electrical energy into ultrasonic energy and vice versa.
An US probe (also referred to as a transducer) emits ultrasonic pulses,
which propagate through tissues in the body and return an ultrasonic
echo at each tissue interface that is encountered.
5.4.1 History
The piezoelectric effect, which is the base of US generation and reception, was detected in 1880 by the brothers Jacques and Pierre Curie. It
was technically applied first by Paul Langevin in 1916 as a depth finder
(sonar) for the French naval forces. The first medical application came
from Karl Dussik who used US to examine the cerebral ventricles in
1942 (“Hyperphonographic”). W. D. Keidel introduced A-mode examinations into cardiology in 1950. In 1952, the first B-mode-like images of
the abdomen were produced by J. J. Wild and J. H. Holmes.
Diagnostic US imaging depends on the computerized analysis of
reflected US waves, which noninvasively build up (highly detailed) images
Diagnostic Procedures
113
of internal body structures. The resolution is higher with shorter wavelengths, with the wavelength being inversely proportional to the frequency. However, the use of high frequencies is limited by their greater
attenuation (loss of signal strength) in tissue and thus shorter depth of
penetration. For this reason, different ranges of frequency are used for the
examination of different parts of the body: 35 MHz for abdominal
areas, 510 MHz for small and superficial parts, and 1030 MHz for the
skin or the eyes.
There are different basic imaging modes involved in the visualization
of US image data (
Fig. 5.17).
A-mode (amplitude mode) is the original type of US as it was used
for depth measurement. The transducer sends a beam through the anatomy with the echoes plotted on screen as a function of depth.
The B-mode (brightness modulation) uses a linear array (LA) of transducers to obtain a planar (2D) set of echo data, oriented parallel to the
longitudinal axis. The B-mode denotes the acquisition of 2D echo data at
a fixed depth while the transducer is moved in the 2D plane to sample
Figure 5.17 Current imaging modes in medical US application: A-mode: Mere depth
measurement (original image W. D. Keidel); B-mode: Using a LA of multiple transducers, a planar set of echo data generates a 2D image. 3D images are achieved by filtering together 2D images of different direction to get a volume data set. The fourth
dimension is time. Courtesy: PD Dr. B. Kuschel, Klinikum rechts der Isar.
114 Biomedical Engineering in Gastrointestinal Surgery
the entire region at this fixed depth. The M-mode provides real-time
visualization of organ motion.
Volumetric imaging refers to the rendering of 3D image data by
acquisition of multiple adjacent cross-sectional images. Dynamic volumetric imaging provides the ability to display motion. The modification of
US imaging to acquire volumetric data is based to a great extent on positioning technologies and computerized image processing.
An additional mode is the Doppler principle, which measures the shift
in the frequency of the returning signals, providing information about the
velocity motion, e.g., of blood flow or muscle (see
Section 5.4.2:
Transducer Arrays).
In general, there are four main topics in US imaging modifications.
All US applications are based on transducer array technology. Doppler
US and volumetric imaging are modifications involving visualization.
Elastography, especially acoustic radiation force impulse (ARFI) imaging
and shear wave elastography (SWE), which uses US for the displacement
of tissue, is a different application principle for US. Hybrid US systems
are of increasing relevance, combining different imaging technolog ies
with US imaging.
Recent Developments and Current Research
There is continuous technical advancement in US. Developments
include portable scanners, miniature pocket-size scanners, and highfrequency scanners. A reduction in physical size has been made possible by
incorporating application-specific integrated circuits into the imaging system. A couple of pocket-sized scanners have been introduced onto the
market recently
[40]. High-frequency (above 20 MHz) scanners have been
developed for eye, skin, small animal, and intravascular imaging. They have
improved spatial resolution at the expense of penetration depth.
5.4.2 Transducer Arrays
A transducer is a component in US devices that translates electrical
impulses into sound waves and sound waves into electrical impulses
detected by using the pulse-echo method. A transducer usually applies
elements of silicon or piezoelectric crystals to code sound waves into
electric signals. They are also named transceivers because they both transmit and receive US waves. There are multiple different transducers, ranging from a single element to broadband transducer ar rays of hundreds of
different elements. Different probes applied in different medical imaging
Diagnostic Procedures
115
procedures generally have to satisfy varying criteria in terms of shape,
depth of penetration, resolution, and other application-driven requirements (
Fig. 5.18) [41]. Therefore, different transducers are installed in
order to achieve the performances necessary in each particular procedure.
The main parameters for the perfor mance of transducers are element
pitches, numbers, apertures, gaps, and width. The arrays employed can be
classified by the pattern in which the elements are embedded on the array,
as either 1D or 2D arrays. Another way to distinguish transducers is by
the underlying procedure of image acquisition, separated into mechanical
sector (MS) transducers and phased array (PA) transducers
[42].
MS probes use a single or a group of single crystal transducers. The
probes employing MS transducers have to be moved physically for the
beam to scan a field, usually via an electric motor that rotates the crystals.
Figure 5.18 A selection of typical transducers. Left: Sector scanner, most often used
in cardiology; middle: convex scanner, standard in abdominal US; right: linear scanner. In addition, a large number of purpose-designed probes (transanal, transvaginal,
transesophageal, etc.) are available. Courtesy: PD Dr. M. Kranzfelder, Klinikum rechts
der Isar.
116 Biomedical Engineering in Gastrointestinal Surgery
Mechanical transducers have two advantages over electrically driven modifications. First, the single element design requires less sophisticated electronics; secondly, visual aspects unique to electrical steering and artifacts
caused by improper reflections are decreased. The main disadvantages of
MS probes include a fixed-beam focus that requires the whole probe to
be replaced in order to change the focus. Furthermore, the frame rate
depends on the speed of the transducer rotation which may lead to the
problem of frame rates occasionally dropping too low when a wider FOV
is necessary.
PA modi fications are a more sophisticated form of transducer technology compared to MS transducers. They provide electrical steering
through sequenced pulses of aggregated beams, causing backscatter that
makes different interferences detectable on a time scheme. The detected
interferences give information that make it possible to determine the
angle at which the beam should be aimed. Consequently, electr ical
steering makes a physical movement of the probe unnecess ary a nd
provides better resolutions. As the maximum angle is decrease d the
FOV decreases in turn, resulting in higher frame rates. With a PA, the
probe can be small and facilitate imaging of otherwise inaccessible body
areas. For example, in cardiology PAs are favored because they easily
fit between the ribs and allow simultaneous Doppler- and M-mode
imaging
[5].
As already mentioned, the term “1D-transducers” refers to the 1D
arrangement of the elements, and includes linear array (LA), annular array
(AA), and curved array (CA) transducers.
A LA consists of a line of transducer elements that emit an array of
parallel beams, resulting in a rectangular image. The beams can be aimed
sequentially and steered electronically so that no movement of the probe
is required. A probe using an LA is capable of producing good near-field
images, which makes it favored for imaging fetuses and the lower abdomen. On the other hand, it comes with the disadvantage of creating
blurred images as a result of insufficient flush contact. Due to that, the
body surface often has to be compressed to meet the transducer’s footprint. LA transducers are generally inexpensive, but their imaging quality
is inferior to other systems.
In AA transducer configurations, the transducer elements are arranged
in concentric circles to create a cylindrical or conical wave front.
CA transducers only differ from other 1D configurations of elements in
being arranged in a curved- or fan-pattern. The convex form helps produce
a sector-shaped image with a larger FOV compared to flat LAs. Both AA and
117Diagnostic Procedures
CA transducers can employ PA technology to provide electrical beamsteering.
The 2D transducers basically consist of elements formed in a pattern
along two axes. The field of 2D arrays is subdivided into two different
types: Matrix arrays (MAs) and segmented AA.
In a MA, the transducers are arranged in a matrix pattern. Probes
with MA transducers are capable of simultaneously acquiring two orthogonal spatial planes
[43].
In a segmented AA, the transducer elements are arranged in nested
concentric rings. One important advantage of this design is that focusing
can be achieved in two dimensions and that the focal zone can be altered
without changing the probe. When PA technology is employed, beamsteering in 3D space becomes feasible. The use of 2D transducers maximizes the advantages of PA technology
[42].
Application-fitted transducers include endocavity transducers, intraoperative transducers, and transesophageal transducers. There are technical
difficulties in fitting a high number of elements and cables on the surface
of a transducer. It is, therefore, the aim to reduce the number of elements
without a loss of imaging quality
[44].
Recent developments and current research
Evolution in microelectromechanical systems (MEMS), especially in
complementary metal oxide semiconductors (CMOS), has vastly contributed to recent growth in all performance parameters, allowing frequencies of
more than 50 MHz
[45]. Those developments will continue making medical
applications more portable due to weight reductions, as well as improving
live volumetric imaging catheters
[46] through further decreases in size. A
change from piezoelectric to silicone-based modalities has led to more sensitivity in detection and is currently helping to minimize transducer elements,
facilitating larger transducer arrays with better imaging qualities. Moreover,
optical sensors have matured over the past decades and found their way to
clinical applicability
[47]. In medical US, they have a promising potential,
especially in the field of high-frequency applications, as they allow higher
sensitivity at increased frequencies compared to piezoelectric modalities
[48].
5.4.3 US Application in Visceral Medicine
US offers a long row of advantages: The examination is noninvasive,
quick and cheap, easily repeatable, and extremely informative. As the US
units are portable, the examination can be performed bedside. US is not
confined to one medical specialty (e.g., as X-ray is confined to the
118
Biomedical Engineering in Gastrointestinal Surgery
radiologist) but is utilized by practically all clinical disciplines. However, it
is examiner dependent and objective documentation is difficult. For visceral medicine, another drawback is more decisive: the limitations of the
method because of air and bone. The examination of the abdomen is disturbed by both of them (
Fig. 5.19).
The stomach and, in particular, the colon usually contains gas which
makes an examination of deeper laying structures impossible. Whenever
feasible, the examination should accordingly be done after a longer fasting
period (8 hours) of the patient. The liver, usually not covered by other
organs, is not camouflaged by air, but the ribs impede visibility.
Nonetheless, an experienced examiner is able to achieve a high diagnostic
yield. US is, e.g., the standard diagnostic procedure for diseases of the
gallbladder (
Fig. 5.20).
Figure 5.19 US speed in various tissues and media: the significant difference in
impedance of air and bone makes them to the classical foes of the US examiner.
From MITI.
Figure 5.20 US check of the gallbladder. (A) Normal gallbladder; (B) multiple gallstones in an otherwise normal gallbladder. The quality of the image is low due to suboptimal selection of US parameters. Nonetheless, at least two stones with their typical
shadow are visible (1). Note the total reflection by air in the adjacent colon (2); (C)
severely inflammated gallbladder: “3 layer wall” (3, light, 4, dark, 5, light). All from MITI.
119Diagnostic Procedures
Another classical indication for US is the checkup of the liver
(
Fig. 5.21).
Moreover, other organs or organ systems can be assessed, although the
diagnostic impact may vary (
Figure 5.21 US of the liver. (A) Normal aspect of the liver; (B) typical image of an
asymptomatic liver cyst; (C) liver metastases. All from MITI.
Table 5.2 The role of US in visceral medicine
Indications
and organs
Potential diagnostic
yield
Table 5.2).
Practicability/
importance
Free
Ascites, blood 111
intraabdominal
fluid
Gallbladder Inflammation, stones 111
Liver Size, consistency, bile duct
111
obstruction, primary and
secondary tumors
Spleen Size, traumatic or atraumatic
111
lesions
Kidney Tumors, cysts, stones, dilatation
111
of the pelvis
Thyroid gland 111
Aorta/caval vein Aneurysms 111
Pancreas Cysts, tumors, dilatation of the
duct
11 often hidden
behind stomach/
colon
Appendix Enlargement, inflammation 11
Ureter Stones, dilatation 11
Stomach Major tumors 1
Colon (1)
120
Biomedical Engineering in Gastrointestinal Surgery
5.4.4 Doppler Imaging
The Doppler-effect enables US to be used to detect motion. US Doppler
systems display the Doppler frequency shift produced by moving objects
in an US beam (
of the blood flow but also in detecting the velocity of structure movements, such as the heartbeat.
There are three main types of Doppler systems: continuous wave,
pulsed wave, and power Doppler. They differ in transducer design and
operating features, signal processing procedures, and in the types of information provided. Additionally, there are two main display modes used in
Doppler systems: Spectral Doppler measurements display the spectrum of
flow velocities graphically on the y-axis and time on the x-axis. In color
Doppler mode, velocities are measured in points within a B-mode plane
and represented by a color-coded image that is coregistered with a
B-mode scan. Power Doppler imaging (PDI) is a technique for evaluating
the vascular system. It uses special processing to display the amplitude or
strength of the Doppler signal, rather than velocity and directional information as in conventional color Doppler. This allows a much greater sensitivity in detecting small vessels and slow-moving blood. Currently, PDI
is being used in conjunction with color Doppler, but it has proven valuable in many applications. This approach makes an image very similar to
an X-ray angiogram, which is easy to interpret. In addition, it avoids the
aliasing problem of conventional color Doppler
In general, Doppler US is used in any application that may need to
evaluate the blood flow. In this manner, it is possible to find blood clots
and blocked or narrowed blood vessels in almost any part of the body
(
Fig. 5.23).
Fig. 5.22). Commonly, it is used to measure the velocity
[40].
Figure 5.22 (A, B) Principle of Doppler sonography. Modified by Dr. A. Schneider.