Medical Images: Creation, Storage, and Access

• Medical images are created, stored, accessed, and processed in a hospital environment.
• The semantic of an image is related to why it was created.
• Archiving and transferring of images are governed by HL7 and DICOM standards.
• The first problem in processing digital medical images is accessing the image within a hospital's information systems framework.
• It is hard to develop software modules for postprocessing without grasping how images are archived/transferred.

Differences Between Medical Images and Others

• Medical images get meaning from the context they were created in.
• Context includes demographic information, image acquisition system, or examination reasons.
• Data integrity and security require context to be associated with the medical image.
• Usage of medical images is tightly controlled and regulated.
• Medical images are mappings of measurements into pictorial representations.
• Image acquisition requires storing details about different image acquisition techniques.
• Efficiently keeping meta-information about the image acquisition system leads to different image formats compared to conventional formats.

Information systems in a hospital

• Images may be generated from patients either admitted or sent in for outpatient examination.
• Patient information needs to be accessible at various locations, for administrative needs, performing examinations, or interpreting images.
• Clinics include orthopedics and neurosurgery; Diagnostics include Radiology and Nuclear Medicine; Teaching and Research include Anatomy.

Different Information Systems

• Patient administration data is kept in the Hospital Information System (HIS).
• HIS maps the hospital's internal structure.
• Modern hospitals must have network infrastructure, storage devices, and access points.
• HIS evolved to create paperless administration of inpatients and outpatients.
• Data about radiological examinations are kept in the Radiology Information System (RIS).
• The complexity of the information structure and information flow are why there are independent subsystems within the hospital.
• Modern hospitals must have network infrastructure, storage devices, and access points.
• The RIS and PACS organize, transfer, and archive image data and image meta data.
• RISs is built to manage information about services connected to a radiological examination.
• Information in RISs includes examinations ordered, patient scheduling, images created as part of an examination and reporting.
• Images themselves are not included in an RIS due to historical/practical reasons.
• Images were linked to the RIS through an identification code for pictures kept on film.
• Most images created in radiology in the 1970s were analogue x-ray images.
• With x-ray CT's advent in the 70s and MRI in the 80s, the primary image representation became digital.
• A PACS should archive and share pictures with related information in radiology and departments that ordered the images.
• Information includes patient demographics, reports, and technical information.
• RIS and PACS separation is much less distinct than between HIS and RIS.
• PACS and RIS will probably combine in the future into a single departmental information system.

HL7 and DICOM

• HL7 standardizes communication between clinical information systems, connecting HIS, RIS, and PACS systems.
• DICOM standardizes image communications, particularly within PACS components.
• HL7 is a standard developing organization accredited by ANSI.
• HL7 refers to the application layer (seventh layer) of the OSI.
• HL7 describes messages syntax and semantics on this layer.
• HL7 messages are based on HL7's Reference Information Model (RIM).
• The RIM specifies six stereotypes, four relating to nodes and two referring to the relations between nodes.
• The relations are entity, act, role, participation; and role-relationship and act-relationship.
• HL7 messages in versions 2.x use ASCII characters delimited by carriage return symbols.
• HL7 V3 Standard XML Tags structure the messages.

DICOM Standard

• Information to store: Patient, examination, and technical information, (auxiliary) reporting information and the image or image sequence.
• DICOM is a full specification of the OSI model's application layer.
• It simplifies connectivity between HIS and imaging components and replaces other file communication protocols (FTP) for medical images.
• It standardizes communication between imaging systems, printers, archives, and workstations.
• DICOM aids component sharing in a multivendor and ensures system compatibility when adding new components.
• DICOM specifies a protocol for communicating objects between devices.
• Two object types, composite and normalized, may be exchanged.
• An image is a typical composite object with: various text entities, numerical entities, and the image itself.
• Normalized objects consist only of a single entity like a report.

Composite and Normalised Services

• Four services, C-STORE, C-FIND, C-GET, and C-MOVE are designed to exchange images.
• A service for updating an image is not provided to prevent the intentional or unintentional alteration of an acquired image; will add a new image to a study.
• Four general services, N-CREATE, N-DELETE, N-SET, N-GET, and two specialized services, N-ACTION and N-EVENT-NOTIFY were defined.
• Service classes describe services that may be rendered to representations of information entities.
• Classes of information entities are called information object description (IOD) templates in the DICOM world.
• A service-object pair combines an IOD with services called DIMSE (DICOM Message Service Elements).
• It invokes an operation or notification across the network.
• A DIMSE service group is a collection of DIMSEs applicable to an IOD.
• An information object description and the set of services operating on it are a service object pair (SOP) class.
• An SOP class using composite services is a composite SOP class; an SOP class using normalized services is a normalized SOP class.
• DICOM classes are static, meaning information entities (data structures) and services (methods) are provided as a template.
• Communication follows the client-server paradigm, with servers called SCP and clients called SCU.
• A communication session between two entities called application entities (AE) begins with establishing a DICOM association.
• Supported Applications of DICOM Service Classes: Network Image, network image interpretation, and network print management, imaging procedure, offline storage media Management.
• In push mode images are transferred without a request, while pull mode only transfers requested images.
• Instead of listing data by file names, which are UIDs and difficult to interpret.
• The query's result may structure image data by patient name, patient ID, and study ID.
• DICOM network management services are specified in the service, query/retrieve, and storage commitment service classes and defined for composite objects only.
• The storage service class specifies the C-STORE service for pushing data to a client. The query/retrieve service class specifies C-MOVE, C-FIND, and C-GET services.

Establishing DICOM connectivity

• DICOM needs to be highly modularized to accommodate many information objects exchanged in hospitals and the wide range of services asked.
• Equipment conforming to DICOM provides a DICOM conformance statement.
• A DICOM conformance statement consists of: A problem, application entity, communication profiles, and specialization statement.
• The vendor states the purpose of communication for his piece of equipment via a problem statement.
• The major part of the conformance statement is the application entity specification.
• An application entity is a software module of the equipment implementing an application.
• The communication profile describes services communicated.
• The specialization part of the conformance statement relates to extensions and vendor-specific specializations.
• Conformance has been tested thoroughly and fails in the specific environment or information is not present or optional, and is thus, impossible communication.

DICOM File format

• The file format should support storing many information objects and should be highly variable, the effort for reading a file should be minimal.
• The two needs are met by choosing a tagged format.
• Each tag is related to some data element of an information object.
• An object's description is found in a data dictionary.
• DICOM file's name is its UID.
• Content has a header of a fixed length, then a sequence of tagged data elements.
• The contents of the Header: 128-byte preamble, meant to support specific DICOM implementations, otherwise, it should be filled with zeroes; four byte identification by the ASCII codes of the letters “D,” “I,” “C,” and “M.”; a mandatory set of data elements containing meta-information of the file.
• Data Elements: The tag consists of a-4-byte group number and a 4-byte element number; Tags are listed by their group and element number in the data dictionary.
• The length of the data element specified by the tag has a 4-byte element following the tag.
• Shadow Groups: Odd group numbers allow vendor-specific adaptations or specializations, are also called shadow groups, used to represent non-mandatory data elements (an efficient way to hide info).
• Data elements which tag definition is missing may be skipped by DICOM reader.

Technical Properties of Medical Images

• 2D images may be slices of the human body like an ultrasound image or CT slice, or projections of a 3D scene like an x-ray or scintigram.
• 3D images are volumes of the human body, like a computed tomography sequence, or time sequences of 2D images.
• 4D images are 3D volumes over time.
• The DICOM file format treats 2D images as an information unit, even if part of a 3D/4D sequence.
• A 3D data set is the sequence, and a 4D data set is a study of several sequences.
• Pixel values of medical images are quantized, differs from digital photos, and range may exceed 256 values when the information acquired justifies a bigger quantization range.
• Transferring Digital Image Files: MR image has correct endianity (scaled from its original range 0. . 4000 to 0. . . 255) and the same image with endianity reversed.

Displays and Workstations

• Many medical images are viewed as hard copies.
• For reading an image as a radiologist, the film is placed in front of a light box.
• Backlit light boxes create good perceived contrast.
• Modern light box systems can easily carry several films.
• PACS Displays Images on Monitors Instead of Light Boxes.
• Advantages of Displaying by PACS: Digital data can be Copied and transferred to any location reachable on the network; easy localizing images; Transfer can be extended to long distances in a short time (teleradiology); Image enhancement may support image interpretation.
• Disadvantages of Displaying by PACS: The first problem is related to image transfer.
• Faster networks and solutions similar to a reading session can reduce this problem.
• Limited spatial and contrast resolution of a monitor.
• Professional reading systems(16 megapixels) instead of 2 megapixels control such influence.
• Using monitors is only a partial replacement since there is never enough monitor space to display as many images.
• Suitable user interfaces for switching have to compensate for this deficiency.
• Display and Display Software Recommendations: The luminance of a grayscale monitor should be >= 50 footlamberts; Controlled lighting eliminate monitor lights; Low ambient light level; A means to select image sequences to be provided; The software should associate a patient with a study.
• Window and level adjustment, pan and zoom, and rotating/flipping images; able to calculate linear measurements and pixel values; able to display prior image compression ratio, processing, or cropping; have the matrix size, the bit depth, and the total number of images acquired in the study.
• Software Groups: Image display, enhancement, annotation, and analysis, 3D imaging and specialized interpretation or planning.
• DICOM Viewer Functionality: Implementation of a query to the database of a DICOM sender (C-FIND); Implementation of the C-GET to retrieve images; An interface to displaying a query's result and images, a rendering module to display retrieved images, a storage module.

Compression of Medical Images

• DICOM supports data compression with images compressed lossless or lossy.
• Lossy compression is a difficult issue.
• The JPEG and JPEG2000 standards were adopted by the DICOM standardization committee.
• The DICOM committee made no decision on lossless or lossy methods of compression.

Abbreviations

• HIS: Hospital Information System
• RIS: Radiology Information System
• PACS: Picture Archiving and Communication System
• HL7: Health Level 7
• RIM: HL7 Reference Information Model
• DICOM: Digital Image Communication in Medicine
• IOD: Information Object Description
• DIMSE: DICOM Message Service Elements
• SOP: Service Object Pair
• SCP: Service Class Provider
• SCU: Service Class User
• AE: Application Entities