How a viewer opens a studyfrom the worklist to the screens
When a radiologist opens a study, the viewer does more than fetch images. It can ask for a hanging protocol, retrieve the presentation states, and read the monitor setup before it lays anything out. This diagram follows those DICOM services. The order is how a viewer can use them, because the standard does not fix one.
- DICOM
- Internal action
- Dashed = reply
Tip: click an arrow or box in the diagram to jump to that step. Time flows top to bottom.
The radiologist opens a study
Core stepWhat happens
The radiologist selects a study in the reading worklist. The viewer knows the study and its patient, and now has to find everything needed to show it.
Why it is needed
The study is more than its images. The layout and the way each image looks come from other objects.
Technical details
- StudyInstanceUID (0020,000D)
- PatientID (0010,0020)
- Modality and procedure code of the study
The viewer finds the study
Core stepWhat happens
The viewer asks the PACS what the study holds, and what earlier studies exist for the patient, using Query/Retrieve on the Study Root model.
Why it is needed
The number of series and priors decides which hanging protocol fits.
Technical details
- SOP Class = Study Root Query/Retrieve Model - FIND
- QueryRetrieveLevel (0008,0052) = STUDY, then SERIES
- Return keys: Modality, Study Description, series counts
The viewer asks for a hanging protocol
Key detailWhat happens
If the viewer supports hanging protocols it queries the Hanging Protocol Information Model with the keys of the study. The PACS answers with one response for every matching protocol.
Why it is needed
The protocol decides what goes where on the screens. A match on modality, anatomic region, laterality, procedure code and priors means the protocol was made for this kind of study.
Technical details
- SOP Class 1.2.840.10008.5.1.4.38.2
- Hanging Protocol Definition Sequence (0072,000C): Modality, Anatomic Region, Laterality, Procedure Code
- Number of Priors Referenced (0072,0014), Number of Screens (0072,0100)
- The standard does not say how to choose between several matches
The viewer retrieves the protocol
Key detailWhat happens
The viewer picks one protocol and retrieves it by its SOP Instance UID. The protocol arrives by a C-STORE sub-operation under Hanging Protocol Storage.
Why it is needed
The query only returns the keys. The protocol itself, with its layout, is the stored object.
Technical details
- SOP Class 1.2.840.10008.5.1.4.38.3 (MOVE) or .38.4 (GET)
- SOP Instance UID (0008,0018) = one UID or a list
- QueryRetrieveLevel (0008,0052) must not be present
- Sub-operation: Hanging Protocol Storage 1.2.840.10008.5.1.4.38.1
The viewer retrieves images and presentation states
Core stepWhat happens
The viewer retrieves the images of the study and the presentation states stored with them. Presentation states are composite objects, so they are retrieved like images.
Why it is needed
Without the states the viewer shows the images as stored, which can look different from what the radiologist saw before.
Technical details
- Image Storage SOP Classes of the study
- Grayscale Softcopy Presentation State 1.2.840.10008.5.1.4.1.1.11.1
- A state refers to images of the same study and one Image SOP Class
- Both sides must have the SOP Classes in their conformance statements
The viewer reads the display setup
Key detailWhat happens
When supported, the viewer reads the properties of the display subsystem with an N-GET on the well-known Display System instance. It names the attributes it needs.
Why it is needed
A hanging protocol is made for a number of screens of a given size. Knowing the real monitors lets the viewer decide how well the protocol fits.
Technical details
- SOP Class 1.2.840.10008.5.1.1.40
- SOP Instance UID 1.2.840.10008.5.1.1.40.1 (well known)
- Number of Display Subsystems (0028,7001), Display Subsystem Sequence (0028,7023)
- Optional: not every viewer or display system supports it
The display system answers
ConfirmationWhat happens
The display system returns the values for the attributes that were asked for, with a status.
Why it is needed
The viewer can now compare the monitors with the screen definition of the protocol.
Technical details
- Status 0000 = Success
- Manufacturer, model, serial number, station name
- Calibration and ambient light details, if the SCP provides them
The viewer lays out and applies
Core stepWhat happens
The viewer arranges the images following the hanging protocol and applies the transformations of the presentation states: contrast, shutter, rotation, annotations. If the protocol or a state is missing, it uses its own defaults.
Why it is needed
This is where the radiologist finally sees the study as it was meant to be seen.
Technical details
- Protocol: image boxes per screen, sorting and selection
- State: Modality LUT, VOI LUT, Presentation LUT, shutter, displayed area, annotations
- The state replaces the image's own transformations
- If several states apply, the choice is up to the viewer
All steps at a glance
Glossary
- Viewer
- The reading workstation software that shows images to the radiologist.
- Hanging protocol
- A stored object that says which images go where on the screens, for a kind of study.
- Presentation state
- A stored object that says how an image is displayed: contrast, rotation, annotations.
- Display system
- The part of the workstation that drives the monitors, which can describe itself with an N-GET.
- SOP Instance UID
- The unique ID of one stored object. A hanging protocol is retrieved by it.
- Prior
- An earlier study of the same patient, shown for comparison.
Summary
- The viewer first finds the study with
C-FIND, then asks for a hanging protocol with aC-FINDon1.2.840.10008.5.1.4.38.2. - A protocol is retrieved with
C-MOVEorC-GETby SOP Instance UID. Images and presentation states are retrieved like any composite object. - The Display System
N-GETis optional and tells the viewer about the monitors. - The viewer lays out the screens from the protocol and applies the presentation states, whose transformations replace the images' own.
- The standard defines the services. The order of the steps and the choice between several matching protocols or states are up to the viewer.
The 8 steps in plain text
The radiologist opens a study
The radiologist selects a study in the reading worklist. The viewer knows the study and its patient, and now has to find everything needed to show it.
Why: The study is more than its images. The layout and the way each image looks come from other objects.
The viewer finds the study
The viewer asks the PACS what the study holds, and what earlier studies exist for the patient, using Query/Retrieve on the Study Root model.
Why: The number of series and priors decides which hanging protocol fits.
The viewer asks for a hanging protocol
If the viewer supports hanging protocols it queries the Hanging Protocol Information Model with the keys of the study. The PACS answers with one response for every matching protocol.
Why: The protocol decides what goes where on the screens. A match on modality, anatomic region, laterality, procedure code and priors means the protocol was made for this kind of study.
The viewer retrieves the protocol
The viewer picks one protocol and retrieves it by its SOP Instance UID. The protocol arrives by a C-STORE sub-operation under Hanging Protocol Storage.
Why: The query only returns the keys. The protocol itself, with its layout, is the stored object.
The viewer retrieves images and presentation states
The viewer retrieves the images of the study and the presentation states stored with them. Presentation states are composite objects, so they are retrieved like images.
Why: Without the states the viewer shows the images as stored, which can look different from what the radiologist saw before.
The viewer reads the display setup
When supported, the viewer reads the properties of the display subsystem with an N-GET on the well-known Display System instance. It names the attributes it needs.
Why: A hanging protocol is made for a number of screens of a given size. Knowing the real monitors lets the viewer decide how well the protocol fits.
The display system answers
The display system returns the values for the attributes that were asked for, with a status.
Why: The viewer can now compare the monitors with the screen definition of the protocol.
The viewer lays out and applies
The viewer arranges the images following the hanging protocol and applies the transformations of the presentation states: contrast, shutter, rotation, annotations. If the protocol or a state is missing, it uses its own defaults.
Why: This is where the radiologist finally sees the study as it was meant to be seen.
Common questions
What does a PACS viewer do when a study is opened?
To open a study a viewer first finds it with C-FIND, then, if it supports hanging protocols, queries the Hanging Protocol Information Model with the keys of the study and retrieves the protocol it picks. It retrieves the images, and any presentation states stored with them, with C-MOVE or C-GET. It can read the monitor setup with an N-GET on the Display System. Then it lays out the images following the hanging protocol and applies the presentation state transformations. The standard defines the services; the order and the choice between several matches are up to the viewer.
Is this the order that every viewer follows?
No. DICOM defines the services, not the order. Many viewers keep their own layouts and view settings in their own database and do not use hanging protocol objects or presentation states, and some fetch the images first. The diagram shows how the standard's services can be combined.
Why does the viewer retrieve presentation states with the images?
A presentation state is a composite object stored in the same study as the images it refers to, so it is retrieved the same way. It tells the viewer how the radiologist wants the images shown.
What if no hanging protocol matches?
The viewer falls back on its own default layout. The standard does not say what that default is.
Does a viewer need the display system N-GET?
No. It is optional and product specific. Without it a viewer finds out about the monitors from the operating system.