Nuclear Medicine Resources Manual
Подождите немного. Документ загружается.
CHAPTER 4. INSTRUMENTATION
150
the sensitivity of the collimator. Shielding may be either integral in the
design of the probe or in the form of removable collars of a heavy
attenuating material. It should only be used when necessary. It adds to
the weight of the detectors and reduces their manoeuvrability. It is
advisable to use collimators when there is adjacent activity next to the
sentinel node. The thickness of the collimator depends on the energy of
the radionuclide used.
(b) Count rate should be recorded by both a digital display at time intervals
in the range of 1–10 s and by an audible sound, the intensity of which is
proportional to the count rate.
(c) The probe should have a long cable between the detector at the surgical
field and the associated electronics in order to allow sufficient space
between the surgeon and her/his associates.
(d) The response of the detector should be constant over its useful life and
can be checked using a calibration source (dependent on the radionuclide
used). For
99m
Tc applications
57
Co can be used.
(e) The probe should be battery operated for several hours and should have
a rapid charging system.
(f) Sterilization is easily achieved by covering the detector with a sterile,
disposable plastic sheath.
4.5.3. Well counters
Well counters are used for low activity, high efficiency counting of in vitro
samples, and are available either as manually operated single sample (or
limited number of samples) devices or as fully automatic, multiple sample
counters.
All well counters use large volume NaI detectors in the form of a well,
where the sample is virtually surrounded by the detector. Ideally, they should
have the following capabilities:
—Automatic photon spectrum calibration, with continuous correction for
drift;
—Ability to select and count multiple radionuclides;
—Automatic radioactive decay correction for the selected radionuclide(s);
—Variable counting time;
—Sample identification;
—A printed report for each sample including sample identification,
counting time, energy selected and counts;
—An indication of errors in the electronics or mechanical sample changer.
4.6. COMPUTERS AND NETWORKING
151
Manual systems are satisfactory for small numbers of samples, such as for
glomerular filtration rate (GFR) measurement, but automatic sample changing
systems will be needed where in vitro testing is widely used.
4.6. COMPUTERS AND NETWORKING
4.6.1. Purchasing a nuclear medicine computer
Computers have been central to the practice of nuclear medicine for
many years, particularly as the extraction of functional information commonly
necessitates image analysis. Computers form an integral part of imaging
equipment, providing on-line acquisition and data correction to improve
instrument performance, essential functions such as tomographic recon
-
struction and flexible display of images. As computer speed increases exponen-
tially, and with memory and disk capacity showing similar growth, the capacity
of the computer to tackle more complex and challenging tasks in a clinically
acceptable time increases. Patient throughput and efficiency of operation are
greatly aided by the computer tools available.
In recent years most vendors of nuclear medicine imaging equipment
have moved away from proprietary computer designs towards general purpose
computer platforms such as the IBM PC, the Macintosh and various desktop
workstations running the UNIX operating system. By adopting these relatively
highly developed and widely used computer systems, for which numerous
hardware and software options are available, the vendors are now able to offer
support for industry standards in several important areas, including
networking.
When purchasing nuclear medicine equipment it is usual to include a
computer supplied from the same manufacturer, although there are instances
where computers may be purchased separately. Choice of equipment should be
based on the criteria outlined in earlier sections, with choice of computer being
secondary to general considerations such as the amount of support available.
Since computers are increasing in performance so rapidly, the main problem
with them is that they have a much shorter life than that of the associated
imaging equipment. The limitation therefore is in the ability to upgrade
systems so that software and new features are available. Continuation of a
software support contract is advisable as this will normally include any
improvements, fixes of any bugs and new releases for a reasonable time.
However, at some stage, hardware will also need to be upgraded at the
customer’s cost to permit operation of the current software. The adoption of
industry standards by manufacturers has reduced the problem of hardware
CHAPTER 4. INSTRUMENTATION
152
support, although most suppliers will still utilize customized hardware for
special functions (e.g. data acquisition).
When specifying requirements it is important to define the expected
functionality rather than the technical specifications. Acceptance testing can
then be based on the capability to provide results for a specified clinical
analysis in an acceptably short time. Comparison of the time for processing
various clinical studies is a useful indicator of system performance,
independent of underlying technical specifications. Many manufacturers now
offer package software that is common across multiple manufacturers (e.g.
cardiac SPECT analysis). The software supplied sometimes requires an exact
acquisition protocol, otherwise its application may be invalid. Every effort
should be made to ensure that the software purchased is validated in-house.
Software phantoms and test data sets available via the Internet may assist in
the validation of some programs.
When purchasing computers the following factors should be carefully
considered:
(a) Advice should be sought regarding the version of the operating system
used. System software tends to be fairly stable but major changes
occasionally occur that may limit the availability of future releases. Care
should be taken to avoid manufacturers whose system software lags well
behind the current release (as available directly from the system software
supplier, e.g. Sun or Microsoft).
(b) Although most systems are supplied with all the clinical functionality that
can be envisaged, it is important that there is some ability to customize
protocols to match individual requirements. In particular, the ability to
easily add simple programs is important to permit flexibility in use. In
most cases some level of programmability is available suited to a non-
expert user.
(c) Training in the use of the computer is as important as training in the use
of the equipment itself. Ensure that adequate provision for training is
included with the equipment purchase.
(d) The connectivity of the system to other equipment in the department or
institution is an important consideration which may require additional
hardware and software to be purchased. This is further addressed in the
next section.
(e) Choice of system may be based on personal preference for a particular
user interface and speed of response rather than choice of options.
4.6. COMPUTERS AND NETWORKING
153
4.6.2. An introduction to networking
An important development in computing has been the ability to connect
computers via a network so that there can be communication and data transfer
between systems. A network that extends over a limited geographical area (e.g.
a single hospital department) is called a local area network (LAN). However,
networking is not limited to a department but permits communication between
computers in different institutions, even if these are located in different
countries, via the Internet and the World Wide Web. A brief overview of the
components of a typical network is provided below in order to ensure
familiarity with some of the jargon used. The most important applications of
networks in nuclear medicine are:
(a) To permit interconnection of imaging equipment in a department or
hospital;
(b) To permit transfer of images for reporting or provision of an opinion
remote from the site of data acquisition;
(c) To permit access to educational information, technical advice or software.
Networking is made possible by the adoption of a set of standards that
define how information can be sent via electrical signals in a cable and
deciphered by a computer interfaced to the cable. The underlying standard
model for networking as defined by the International Standards Organisation
(ISO) is the Open Systems Interconnect Model (OSI). Networking therefore
uses specialized hardware involved in interconnecting computers and the
software used to interpret or translate the information transferred.
By far the most common means of connecting computers in LANs is the
Ethernet, a standard that defines both the protocol for data transfer and the
cable used. It uses a method called carrier-sense multiple access with collision
detect (CSMA/CD) to share a common cable among several computers.
CSMA/CD operates as follows. When it is desired to transmit data from one
computer to another computer on the network, the computer first ‘listens’ to
determine whether the cable is in use. If the cable is in use, the computer waits
for a random period of time before listening again. If the cable is not in use, the
computer transmits, and at the same time listens to ensure that another
computer did not commence transmitting at the same time. When two or more
computers attempt to transmit at the same time, a ‘collision’ is said to occur. All
the computers involved detect the collision and each waits for a random period
of time before trying again. Until recently Ethernet networks operated at a
transmission rate of 10 megabits per second (Mb/s). However, the fast Ethernet
protocol, which runs at 100 Mb/s on standard, high quality (CAT5) Ethernet
CHAPTER 4. INSTRUMENTATION
154
cables, is gaining rapid acceptance, and hardware also exists for gigabit transfer
rates, usually via optical fibres. Alternative approaches to the Ethernet exist for
transfer via optical fibre (fibre distributed data interface (FDDI)) and for
transfer (usually fast) via other media (e.g. asynchronous transfer mode (ATM)
for use with microwaves). However, these are beyond the scope of this
overview.
Hardware linking computers involves a network interface card (NIC) on
each connected device and various ‘boxes’ that interconnect cables or control
the flow of traffic, limiting connection to specific Internet addresses. Lack of
this traffic control would result in all computers worldwide receiving communi
-
cation from all others, clearly an impossible situation. Examples of network
devices are hubs that simply connect cables without any attempt to alter traffic
flow, switches that permit interconnection between cables with transfer rate
maintained and routers that simply direct or filter traffic. The simplest network
in nuclear medicine involves direct connection between two machines, with
appropriate software handling the network communication (usually with one
machine acting as a server that effectively takes control of the network). Where
more devices and interconnection to larger networks are involved, additional
devices are necessary.
Another important networking standard which, like the Ethernet, has
been implemented on all commonly used computer platforms is Transmission
Control Protocol/Internet Protocol (TCP/IP). Although other protocols exist
(e.g. Internetwork Packet Exchange (IPX) and Netbios Extended User
Interface (NetBEUI)), TCP/IP is by far the most widely used and forms the
basis for worldwide communication via the Internet. The term Internet dates
back to the earliest days of TCP/IP in the early 1980s, when it was used to
describe any network built using IP. Since that time, the power and flexibility of
TCP/IP have resulted in the creation and explosive growth of the Internet, a
worldwide network of networks linked by TCP/IP.
TCP/IP is a set of protocols designed to facilitate the interconnection of
dissimilar computer systems, and it makes no assumptions about the nature of
the connection between the computers. TCP/IP provides computer users with a
number of useful services, and new services are being added regularly. Until the
early 1990s, most Internet usage involved three TCP/IP protocols — Simple
Mail Transfer Protocol (SMTP), Terminal Emulation Program for TCP/IP
Networks (TELNET) and File Transfer Protocol (FTP). SMTP handles
electronic mail delivery. With TELNET, an interactive session can be
established with another computer connected to the Internet. FTP facilitates
file transfer between computers. The network file system (NFS) is another
useful TCP/IP protocol, which allows computers connected to the same LAN
to share files.
4.6. COMPUTERS AND NETWORKING
155
The advent of the TCP/IP protocol and, later in the early 1990s, the
HyperText Transfer Protocol (HTTP), brought about the creation of the World
Wide Web. The World Wide Web is made up of millions of documents
containing many sources of information (including text, graphics, sound and
video) that are stored on computers called web servers. A web browser
program such as Netscape is needed to fetch documents from a web server and
display them with links appearing as highlighted text (hypertext). Clicking on a
link fetches and displays the hypertext document addressed by the link. Web
browsers provide users with a much friendlier, more intuitive interface to
Internet resources than do FTP or TELNET.
4.6.3. Connecting imaging equipment via computer networks
Section 4.6.2 refers to general networking details that are common to all
networks and that are not specific to nuclear medicine. In medical imaging, the
transfer of image data between computers is common, either within nuclear
medicine or between different imaging modalities. The networking of
computers in this environment, with the possibility of circulating images for
reporting purposes and review and central archiving, is commonly referred to
as a Picture Archiving and Communication System (PACS). If fully imple
-
mented, this will link closely with both a Radiology Information System (RIS)
and a Hospital Information System (HIS), both of which handle the adminis
-
trative aspects of diagnostic imaging. Interconnection of remotely sited
imaging modalities via the Internet (rather than a LAN) is usually referred to
as teleradiology or telenuclear medicine. In this case the objective is usually to
transfer images for remote reporting.
It should be recognized that medical images usually occupy fairly large
files and therefore the speed of transfer remains a major concern, even with
modern technology, particularly if there are many file transfers occurring. In a
PACS system, direct high speed cabling can be arranged between critical
components, usually dedicated to this purpose. In contrast, teleradiology is
dependent on the slowest link between centres, which may be a modem
connected to a telephone. Table 4.4 illustrates some typical figures for files
encountered in diagnostic imaging and the time required for transfer via
different media; clearly, nuclear medicine poses no problem compared with
some other modalities.
In order to optimize speed for medical image transfer, images can be
compressed by methods that may lose some information (lossy or irreversible
methods) or preferably by lossless or reversible methods. Reversible methods
are capable of retrieving exactly the same image data as originally compressed
and are essential for most diagnostic reporting, whereas irreversible
methods
CHAPTER 4. INSTRUMENTATION
156
may still be acceptable in some non-diagnostic situations. The most popular
currently used approach was developed by the Joint Photographic Experts
Group (JPEG), with lossless or lossy compression achieved depending on the
degree of compression used. A popular alternative involves wavelet
compression. For lossless compression, reduction in file length, savings in
storage space or improvement in transmission speed by a factor of 2–3 is
possible; for lossy compression the factor can be much higher (e.g. 10–20),
depending on the modality.
Even with established networks that permit image transfer between
systems, there remains a further obstacle to establishing a workable system.
Most suppliers of medical imaging equipment have developed their own
database structure for images, with a proprietary format for the structure of
image files. After transferring a file to a second system, translation between file
formats is necessary and is not a trivial step. Certain file formats have become
useful as intermediaries between individual manufacturer’s equipment.
Examples are Interfile, which was developed specifically for nuclear medicine
and includes a readable header that is easily edited, or the file structure
developed initially by the American College of Radiologists and NEMA
(ACR–NEMA). Fortunately, more sophisticated standards have been
established that define not only the file format but also the method to be used
to establish communication between systems. The result is DICOM (Digital
Imaging Communication in Medicine) which extends to ACR–NEMA but
adheres to the more general networking standards summarized earlier in this
section. Even with this standard available to link computers there are
frequently incompatibilities in different manufacturers’ implementations of
DICOM conversion, resulting in missing administrative data or, in the worst
case, inability to transfer data. Most suppliers are actively improving their
DICOM software as they now recognize the importance of connectivity. Useful
tools for checking DICOM conformance are readily available.
TABLE 4.4. SOME TYPICAL FIGURES OF DIFFERENT DIAGNOSTIC
IMAGING FILES AND RATES OF TRANSFER VIA DIFFERENT MEDIA
Study Matrix size
Number
of images
File size
(Mb)
Time via
100 Mb/s
Time via 32 kb/s
Mammography 4096 × 5120 × 12 4 125 10 s 31250 s = 8.7 h
Functional MRI 256 × 256 × 12 300 30 2.4 s 7500 s = 2.1 h
CT 512 × 512 × 12 30 12 1 s 3000 s = 50 min
Nuclear medicine 128 × 128 × 8 24 0.4 0.03 s 25 s
4.7. GLOSSARY OF TECHNICAL TERMS
157
For useful information on issues related to medical image transfer, the
reader is referred to the web site maintained by D. Clunie (http://
www.dclunie.com/medical-image-faq/html).
4.7. GLOSSARY OF TECHNICAL TERMS
analog-to-digital converter (ADC). These devices convert continuous electrical
voltages to discrete integer numbers in a defined range. When a digital
image is acquired from an analog gamma camera an ADC converts the
electrical signal that represents the x and y positions for a detected
photon to a matrix location in the ranges of, for example, 1–64 or 1–128.
annihilation photons. When a positron is emitted it travels a short distance in
tissue, losing energy. It eventually combines with an electron and the two
annihilate (disappear), with the mass being converted into energy in the
form of two gamma rays (511 keV) that travel in opposite directions.
asymmetric energy window. Normally, the energy window is centred on the
main peak(s) of the radionuclide being imaged. To reduce scatter, an off-
centre energy window, shifted up on the peak, is sometimes used. This is
referred to as an asymmetric window.
back-projection. This is the process used in reconstruction, which allocates
counts in the reconstructed image at each voxel proportional to the
number of recorded counts on the projection, defined by the geometry of
detection. In the simplest case assuming a parallel hole collimator, each
voxel will be allocated counts from a projection pixel, defined by a line
drawn at right angles to the projection that passes through the voxel.
bar phantom. These are phantoms consisting of lead bars of varying widths and
separations set in Perspex or another plastic material. The bars can be
arranged either parallel to each other across the entire phantom, or more
usually into four quadrants of parallel bars. Bar separation and width is
then different for each quadrant. Bar phantoms are primarily used for
measuring resolution.
bismuth germanate oxide. This is a detector material commonly used in PET
cameras. It has a higher density than NaI and is therefore well suited to
detection of the high energy (511 keV) annihilation photons.
CHAPTER 4. INSTRUMENTATION
158
centre of rotation. This defines the point that should correspond to the exact
centre around which the detectors rotate; it should correspond exactly to
the centre of the projections recorded at all angles. Any error in this point
will lead to loss of resolution.
coincidence detection. In order to detect the two gamma rays emitted from a
positron annihilation event, two detectors are used and a valid event is
recorded when both detectors record an interaction at the same time (or
within a very short time of each other). The detectors operate in
electronic coincidence. This term is used with detectors in dedicated PET
systems as well as in gamma camera based PET systems.
collimator angulation. A parallel collimator should normally be constructed so
that the holes and septa are exactly at right angles to the detector. Any
difference in this angle is referred to as a collimator angulation error. Any
such error will lead to some artefacts due to acquired data not being
correctly treated during reconstruction.
converging collimator. These collimators are similar to parallel hole colli-
mators, except that the holes are angled to converge to a focal point at
some distance in front of the collimator. These collimators can be used to
obtain magnified images of small organs.
convolution. Convolution is the filtering procedure undertaken in the spatial
domain. It involves replacing each pixel value by a weighted sum of the
neighbouring values and the value itself. The result will depend on the
weighting values, usually resulting in a smoother image (e.g. nine point
smooth).
cut-off/critical frequency. The shape of a filter is defined by some mathe-
matical function, with the value 1 at zero frequency and lower values at
progressively higher frequencies. The cut-off or critical frequency is a
parameter that defines the shape of the function, a lower cut-off
frequency defining a curve that drops to zero faster, resulting in a
smoother result. In the case of the Butterworth filter the cut-off
frequency defines the point when the amplitude reaches half the
maximum value.
DICOM. This is a relatively new standard that defines both the image format
and the communication protocol to permit transfer of image data
4.7. GLOSSARY OF TECHNICAL TERMS
159
between medical imaging modalities. On the basis of earlier approaches
developed by ACR–NEMA, the method is now widely available.
diverging collimator. These are similar to parallel hole collimators, except that
the holes diverge outwards. They can be used to fit a large organ onto a
small FOV gamma camera, i.e. they can be used to reduce image size.
electronic collimation. Since annihilation photons travel in opposite directions,
the origin of the annihilation can be defined by the straight line joining
the points of detection of the two photons without the need for conven
-
tional collimation.
energy spectrum. A plot of the number of gamma photons detected as a
function of the energy of the gamma rays. Such spectra are useful for
setting energy windows with the pulse height analyser and for observing
the amount of scatter present.
energy window. Setting a lower and upper energy threshold, the energy window
determines which gamma ray energies are accepted and displayed.
Ethernet. This is a standard that defines the type of cable and interface, as well
as the protocol, for data transfer between computers.
extrinsic. This usually refers to measurements performed with a collimator
attached to the detector.
fanbeam collimator. Most collimators are parallel as the holes are straight and
parallel to each other. However, the fanbeam collimator holes are
focused in one direction only and are parallel in the other direction (fan
shaped). As a result the collimator focuses on a line at the focal distance.
This collimator results in magnification in one direction (normally in the
plane corresponding to projections) with improved sensitivity and
improved resolution compared with a parallel hole collimator.
filter kernel. The filter kernel defines the relative weights to be applied to
neighbouring pixels during a convolution process; the result is normally
divided by the sum of the weights. As an example in a nine point smooth
fit the central kernel value is 4, the nearest neighbours 2 and the corners
or next nearest neighbours 1; the result is divided by 16.