Physikalische Grundlagen der Nuklearmedizin/ PACS und fortgeschrittene Methoden der Bildverarbeitung

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Einleitung[Bearbeiten]

Dies ist das zwölfte Kapitel des Wikibooks Basics Physics of Nuclear Medicine.

Durch die phänomenale Entwicklung der Computertechnologie in der letzten Zeit ist es möglich geworden medizinische Bilder in digitaler Form zu speichern und zu übertragen. PACS Systeme basieren im allgemeinen auf einem dafür vorgesehenen Computer der auf die von den verschiedenen Bildaufnahmesystemen gespeicherten Daten zugreifen und mit hoher Geschwindigkeit zu entfernten Rechnern übertragen kann auf denen die Bilder betrachtet werden als auch zu archivarischen Speichermedien als auch zu anderen Computersystemen entweder inerhalb des Krankenhauses oder an entfernten Orten.

Grundelemente einer PACS Lösung für ein Lehrkrankenhaus. Die Bilder werden von den Bildaufnahmeeinrichtugen gleichzeitig zur kurzzeitigen Speicherung an RAID Speicher und zu archivierungen in Langzeitspeicher versendet. Archivierte Bilder können auch wieder abgerufen werden falls sie benötigt werden (siehe ausgefüllte Pfeile) Zugriff auf die Bilddaten wird durch ein hochgeschwindigkeits LAN Netzwerk ermöglicht welches die Bilder an die Rechner der klinischen Diagnostik einen Webserver und Teleradilogie Server verteilt, der die Daten über ein WAN (Wide Area Network) an entfernte orte weiterleitet. Access to images is enabled Glossar HIS Hospital Information System RIS Radiology Information System LAN Local Area Network RAID Redundant array of independent disks

Die erfolgreiche Implementierung von PACS hängt kritisch von verschiedenen Faktoren ab. Diese schliessen die standardisierung der Bildformate, HIS und RIS Integratration, Bildanzeigegeräte, Bildübertragungsraten und Speicherkapazitäten mit ein. Diese Punkte werden wir nun einzeln diskutieren. Die Standardisierung der Art und Weise wie Bilddaten zwischen verschiedenen Medizinischen Bildgebenden Geräten ausgetauscht werden. Der Digital Imaging & Communications in Medictine (DICOM) Standard wurde durch die meisten Gerätehersteller aufgenommen und entwickelt um dies zu erreichen. Neben einer Spezifikation für die Bilddaten schliesst dieser Standard untersuchungsergebnisse am Patienten mit ein die in die Bilddateien eingebettet werden. Diese zweite Eigenschaft ist sehr wichtig in der medizinischen Bildgebung, weil so zum Beispiel verhindert werden kann, dass Verwechslungen auftreten und Daten unterschiedlicher Patienten vermischt werden. Ferner kann man annehmen dass für jedes Bild oder jede Bildreihe ein Geburtszertifikat erzeugt werden. Ein Beispiel einer solchen DICOM Headerinformation ist in den folgenden vier Abbildungen dargestellt. (Der Header der üblicherweise ein Kontinuierliches Dokument darstellt wurde in vier teile unterteilt um die Diskussion zu vereinfachen):

DICOMOBJECT
MetaElementGroupLength 0002,0000 166
PNG
Lossless
Header information
TIFF
Lossless/Lossy
Large file sizes
GIF
Lossy
Graphical data

Das Joint Photographic Expert Group (JPEG) Format wir häufig verwendet um Bilder durch das Internet zu übertragen, da die Bildgrösse durch Bildkompressionsmethoden reduziert wird und die Bilder somit relativ schnell übertragen werden können. Die von diesem Format verwendete Kompressionstechnik führt zum verlust von Bilddaten die nicht exakt wiederhergestellt werden können. Daher der Hinweis auf die verlustbehaftete Kompression in der Tabelle. Das Format wird daher nicht primär zur Diagnose eingesetzt, es ist jedoch für Lehrzwecke usw. gut zu gebrauchen.

Das Portable Network Graphics (PNG) Format ist eines der neuere Bilddatenformate und bietet verlustfreie Kompression, Plattform unabhängige darstellungs und kompressions methoden und bietet Möglichkeiten Daten über den Patienten und die durchführte Untersuchung mit den Bildinformationen zusammen in einer Datei abzulegen.

Das Tagged Image File Format (TIFF) wird in von Verlagen und Druckereien häufig verwendet und biete möglichkeit für verlustfrei wie auch für verlustbehaftete Kompressionsverfahren. Die Verlustfreie Kompression führt jedoch zu relativ grossen Bilddateien.

Schliesslich wird das Graphics Interchange Format (GIF) häufig verwendet graphische Bilder (Graphiken, Tabellen, Flussdiagramme usw.) durch das Internet zu übertragen.

Hochwertige Bildanzeigegerät. Monitore mit weniger 1600 Punkten pro Zeilen können für diagnostische Zwecke, wie ComputerRadiographie (CR) bzw. digitale Radiographie (DR),nicht hinreichend geeignet sein auch wenn sie für die meisten anderen Zwecke ausreichen. Der Kontrast und die Helligkeit müssen auch hinreichend gut sein und die Luminanz anerkannten Standards entsprechen. Weiterhin sollte eine Bilddarstellungsrechner eine benutzerfreundliche bedienoberfläche bieten. Das bedeutet das die Anzeige Manipulation Analyse, Speicherung und Verteilung der Bilddaten intuitiv im medizinischen Kontext seien müssen, wenn die Technologie vom klinischen und Radiologischen Personal angenommen werden sollen.

Weiterhin wurde die effiziente Verteilung der Bilder über grosse Krankenhausgelände und hinzugehörige Kliniken durch die Verfügbarkeit freier Webbrowsingsoftware verbessert, so dass die Verteilung nun zu einem Bruchteil der Kosten, die vor deren Einführung anfielen, möglich ist.

Bilddatentrasferzeiten sollten in jedem PACS System aus offensichtlichen Gründen möglichst kurz gehalten werden. Idealerweise sollte das Bild zwei Sekunden nachdem es angefordert wurde auf dem Monitor erscheinen. Die höherere Verfügbarkeit von hochgeschwindigkeitsnetzen sorgt dafür das diese Bedingung leichter zu erfüllen ist. Ein Vergleich der Transferraten für einige gängiger Netzwerke ist in der folgenden Tabelle angegeben.

Schliesslich sollte eine PACS Umgebung zugang zu einem relativ preiswerten Archvieungsmedium für einige TBytes (also einige Millionen MBytes) und sie muss in der Lage sein auf archivierte Bilder in einer vernünftigen Zeit - sagen wir mal ein bis zwei Minuten - zugreifen können. Heutige lösungen sind zub Beispiel. Robotische digitale Bandarchive und optische Juke Boxen.

Das Internet und das World Wide Web[Bearbeiten]

Das Intenet ist ein globaler Zusammenschluss von Computernetzwerken und seine Anwendungen sind in den letzten Jahren explodiert. Durch die phänomenale Entwicklung der Computertechnologie in der letzten Zeit ist es möglich geworden medizinische Bilder in digitaler Form zu speichern und zu übertragen. PACS Systeme basieren im allgemeinen auf einem dafür vorgesehenen Computer der auf die von den veschiedenen Bildaufnahmesystemen gespeicherten Daten zugreifen und mit hoher Geschwindigkeit zu entfernten Rechnern übertragen kann auf denen die Bilder betrachtet werden als auch zu archivarischen Speichermedien als auch zu anderen Computersystemen entweder innerhalb des Krankenhauses oder an entfernten Orten.

Picture Archival & Communication Systems (PACS)[Bearbeiten]

With the phenomenal development of computer technology in recent times has come the possibility of storing and communicating medical images in digital format. PACS systems are generally based on a dedicated computer which can access data stored in the digital image processors of different imaging modalities and transfer this data at high speeds to remote viewing consoles, to archival storage media and to other computer systems either within the hospital or at external locations - see the figure below:

Basic elements of a generic PACS solution for a teaching hospital. Images from the modalities are sent to short term RAID storage and archived simultaneously. Archived images may also be retrieved as required [solid arrows]. Access to images is enabled through the high speed LAN network which distributes the images to clinical and diagnostic workstations, to a Web server and to a teleradiology server for dispatch to remote sites via a Wide Area Network (WAN).
Glossary - HIS: Hospital Information System; RIS: Radiology Information System; LAN: Local Area Network; RAID: Redundant Array of Independent Disks.

Successful implementation of PACS is critically dependent on several factors which include image format standardization, HIS and RIS integration, image display devices, image transfer rates and storage capacity. These features are discussed below.

Standardization of the manner in which the image data is interchanged between different medical imaging devices. The Digital Imaging & Communications in Medicine (DICOM) standard has been embraced by most equipment manufacturers to facilitate this. Besides specifying the format of digital image data, this information interchange standard also covers patient and examination details which are embedded within the image file. This latter feature is particularly important in medical imaging so that patient studies do not get mixed up, for example, and can be regarded as generating a birth certificate for each acquired image, or set of images. An example of such DICOM header information is shown in the following four figures (the header, which is typically one continuous document, has been broken into four parts here to assist with this discussion):

The first part of a DICOM file header.

Notice that the data provide patient details as well as the image type, the date and time of the study, the modality, the scanner manufacturer and image processing workstation used. The second part of this header is shown below:

The second part of a DICOM file header.

Notice that this data covers the slice thickness and spacing used in this SPECT study, image sampling and quantization information, the number of images and the photon energy window used by the scaanner. The third part of this header is shown next:

The third part of a DICOM file header.

Notice that this data covers details of the scanner movement used to acquire the study. The fourth and final part of this header is shown below:

The fourth and final part of a DICOM file header.

Notice that this final part details the patient and scanner orientation as well as the actual image data.

Other image formats are in common use in medicine, for purposes other than primary diagnosis. These formats can be useful for teaching, multimedia and publication purposes. Examples of these formats are included in the following table.

Format
Compression
Comment
JPEG
Lossy
Small file sizes
PNG
Lossless
Header information
TIFF
Lossless/Lossy
Large file sizes
GIF
Lossy
Graphical data

The Joint Photographic Expert Group (JPEG) format is widely used for image transfer using the World Wide Web because the image data can be reduced in size using image compression techniques and hence can be transferred relatively quickly. The compression technique used by this format results in the loss of image data which cannot be exactly recovered. Hence the reference to Lossy compression in the Table. The format, as you might appreciate, is not used for primary diagnosis but is nevertheless useful for teaching and related applications.

The Portable Network Graphics (PNG) format is the most recent of these formats and has advantages in terms of Lossless compression, platform independent image display and compression features and the ability to embed patient and study identification information.

The Tagged Image File Format (TIFF) is widely used in the publication industry and provides the capability for both lossless and lossy compression. Its lossless compression however results in large image file sizes.

Finally the Graphics Interchange Format (GIF) is widely used for transferring graphical images (e.g. graphs, diagrams, flowcharts etc.) via the World Wide Web.

High quality image display devices. Display monitors with less than 1600 scan lines may not be adequate for diagnosis from Computed Radiography (CR) and Digital Radiography (DR) images although they can be adequate for most other applications. The available contrast and brightness must also be high, with the luminance conforming with recognised standards.

In addition, the image display workstation interface must be user-friendly. That is, the display, manipulation, analysis, storage and distribution of images must be intuitive in a medical context if the technology is to be accepted by radiological and clinical staff.

Connection
Speed
Time to Transfer
a 5 Mbyte Image File
Telephone Modem
56 kbit/s
about 12 minutes
ISDN
128 kbit/s
about 5 minutes
DSL
384 kbit/s
1.8 minutes
Ethernet
10 Mbit/s
4 seconds
Fast Ethernet
100 Mbit/s
0.4 seconds

Further, the efficient distribution of images throughout large hospital campuses and associated clinics has been enhanced by the availability of free web browsing software and has meant that distribution of images can be achieved at a fraction of what the cost was prior to their introduction.

Image transfer times should be short in any PACS system for obvious reasons. Ideally an image should appear on the monitor within 2 seconds of the request for the image. The increasing availability of high speed networks are allowing this requirement to be met more readily. A comparison of transfer speeds for a number of common network connections are shown in the following table.

Finally, PACS environments should have access to relatively cheap archival storage up to a few Tbytes (i.e. a few million Mbytes) of image data and must provide retrieval of non-current image files in a reasonable time - say, less than a minute or two. Current solutions include robotic digital tape archives and optical disk Juke Boxes.

The Internet & The World Wide Web[Bearbeiten]

The Internet is a global assemblage of computer networks and an explosion in its use has occurred in recent years. Its origins can be traced to activities associated with connecting US university, military and research networks about 30 years ago by the Advanced Research Projects Agency and the Inter-Networking Working Group, to the US National Science Foundation network in 1986, through the release of public-domain software by groups at the European Laboratory for Particle Physics (CERN) in 1991 and at the University of Illinois National Center for Supercomputing Applications (NCSA) in 1993 to the recent generation of substantial global interest.

The system facilitates the transfer of data, computer programs and electronic mail and allows for discussion of specialised topics among newsgroups as well as other features such as telnet, internet relay chat and peer-to-peer file sharing. Irrespective of the application, however, the system essentially allows for the convenient exchange of information between computers on a global basis. This section gives a very brief overview of the Internet from a general perspective of electronic communication protocols and the World Wide Web.

All forms of communication, be they based on electronic or other means, are reliant on some form of protocol. A common protocol when someone answers a telephone, for instance, is to say hello, to give a greeting or to announce the location/telephone number of the receiver. Communication between computers connected to the Internet uses a protocol called the Transmission Control Protocol/Internet Protocol (TCP/IP). This approach is an amalgam of two protocols, the details of which are of no great relevance to this discussion, other than to note that they together provide an electronic communication protocol which allows two computers to connect via the Internet. One feature of TCP/IP to note however is that it can be used to communicate between different types of computers, i.e. it is platform independent. An IBM-compatible personal computer can therefore communicate with, for example, an Apple-compatible computer or UNIX workstation. Related protocols which are used when computers communicate over a telephone line are the Serial Line Internet Protocol (SLIP) and the Point-to-Point Protocol (PPP). Once communication has been established between two computers, an additional protocol is needed to exchange computer files. A common protocol used for this purpose is called the File Transfer Protocol (FTP). The types of files which can be transferred are typically computer programs as well as data such as word processed documents, spreadsheets, database files and images.

A refinement to FTP is the Hypertext Transfer Protocol (HTTP) which allows the transfer of documents which contain data in the form of different media-types, and is widely used for webpage display. Examples of media-types are text, images and sound. Finally, two protocols relevant to electronic mail are the Post Office Protocol (POP) and the Simple Mail Transfer Protocol (SMTP), and a protocol in use for newsgroups is the Network News Transfer Protocol (NNTP).

Illustration of a client-server connection on the WWW.

The World Wide Web (WWW) is a conceptual interpretation of the Internet when it is used to transfer documents using the HTTP. These documents are generally called web pages and are written using an editing language called Hypertext Markup Language (HTML). This format provides control over, for instance, the size and colour of text, the use of tables and, possibly most importantly, the facility to link the document to documents which exist elsewhere on the WWW. HTML also allows the insertion of various media-types into documents. Images can be inserted, for instance, in formats such as the Graphical Interchange Format (GIF), the Joint Photographical Experts Group (JPEG) format or the Portable Network Graphics (PNG) format, as discussed earlier, and image sequences can be displayed using one of the Moving Picture Experts Group (MPEG) formats. This latter functionality is useful, for instance for display of dynamic nuclear medicine studies.

The transfer of HTML documents is illustrated in the following figure. The user's computer (referred to as the client) is equipped with software (called a web browser) which allows it to interpret HTML documents and to communicate via the Internet using TCP/IP. The computer is also equipped with hardware which allows it to physically connect to the Internet, for example, a modem for connecting via a telephone line to an Internet Service Provider, and Local Area Network (LAN) hardware for connecting via an institutional network, such as an Ethernet connection. At the other end of the connection is a computer containing a document or set of documents of interest to the user. This second computer is called a server and contains the documents in HTML format. The sequence of events is typically as follows:

  • The user establishes contact between the client and server computers by directing the browser at the Uniform Resource Location (URL) of the server and requests a given HTML document. The direction is typically of the form:
http: //www.server.type.code/doc.html

      where:

http:// the transfer protocol to be used
server the name of the server computer
type shorthand for the environment where that computer resides, e.g. com: company and edu: educational institution
code shorthand for the country where the server is located, e.g. au: Australia and ie: Ireland
doc the name of the document
.html identifies the format of the document
  • The server receives the request, gets the requested document from its storage device and sends the document using HTTP to the client.
  • The client receives the document and the browser interprets the HTML so that text, links and media-types are presented appropriately on the display device.

Many WWW browsers also provide the ability for the user to download files using the file transfer protocol (FTP), to send and receive e-mail messages and to contribute to newsgroups. For example, the process for downloading files using FTP is similar to that illustrated in the last figure except that the user directs the browser at a URL of the form:

ftp: //ftp.server.type.code/doc.xxx

Sophisticated WWW browsers, such as Netscape Navigator or Internet Explorer, also provide the ability to generate more than basic web-pages at the client's computer. One implementation is the ability to interpret client-side scripts. These are small programmes which are downloaded as part of the HTML document and are executed using the client computer's resources. By this means, for instance, the script can read the date and time from the client computer or use its arithmetic functions to make calculations, and embed this information in downloaded webpages. Client-side scripts can be written using languages such as JavaScript.

Another implementation is the ability to execute small applications (called applets) which are downloaded with the HTML document and run on the client computer. Such applets can be generated using languages such as Java (not to be confused with JavaScript!). Applets are well developed for graphics applications, such as animations and scrolling banners. One exciting development in this field is the ability to download image processing software along with an image, so that the user can manipulate the image without the need for a special image processing program.

Finally, a refinement to HTTP server software allows interaction from the client so that information can be returned to the server for executing specific tasks, such as searching a database, entering information into a database or automatically correcting and giving feedback on multiple choice exam questions. Additional software is required for this server-side processing - a common form of which uses the Common Gateway Interface (CGI) protocol. Small CGI programs are generally referred to as scripts and are written in a language such as Perl.

As might be anticipated, the field of electronic communication introduces a vast range of additional concepts to those discussed above - and, like PACS, are just as unrelated to medical concepts! However, further treatment of the subject is beyond our scope here since our interest is confined to the distribution of medical images.


Advanced Image Processing[Bearbeiten]

  • registration
  • fusion
  • volume rendering
  • surface rendering
  • mip
  • mpr
  • OsiriX