Modality Performed Procedure Step

Introduction

After the post on Modality Worklist, I felt that it wouldn’t be a complete without explanation on Modality Performed Procedure Step. MWL without MPPS is like a task list without checkboxes, and after all, striking a checkbox on a completed task is great fun. Talking of which, I once red this article about productivity and task lists and since then I’m using a circular checkbox on my paper to do notes because it’s 4 times faster. Instead of 4 lines you only need one. Think of it.

IHE Comes to Rescue

Though the DICOM standard states that it doesn’t go into the details of the implementation and what should be the implications of MPPS on workflow it is very clear from reading the details of the standard that an MPPS is the checkmark of MWL. The gap is closed by IHE radiology technical framework that does a great job and details exactly what should be the workflow and how the implementation should look like. If you are not familiar with IHE, I strongly recommend navigating to their web site and start digging. Getting familiar with the IHE Technical Frameworks can save a lot of expensive software architect hours and more important, save you from implementing things wrong. The IHE TF is high quality software specification document that you can use almost as is for your healthcare IT software projects.


Anyway, if you don’t have time to dig inside the long documents of IHE and DICOM and HL7, here’s a short data and program flow summary:

1. The modality makes a MWL Query. Each result is a requested procedure object with one or more Scheduled Procedure Steps (SPS).
2. The user picks one SPS to perform.  
3. The modality creates a new Modality Performed Procedure Step (MPPS) that references the Study, the requested procedure, and the SPS.  This is done using the N-CREATE command. 
4. There’s a state machine for MPPS with three states:
1. In Progress (A dot at the center of the circular checkbox)
2. Completed (A dash on the checkbox)
3. Discontinued (Back to the beginning)
5. After the images acquisition is done the modality sends an updated status for the MPPS using N-SET command. The N-SET must include a performed series sequence with at least one series in it, even if the procedure was aborted (in which case the series will have no images).
6. At this point the Scheduler should dash the checkbox to mark the task as completed (or discontinued).
7. Though usually you would have a 1-to-1 relationship between a scheduled procedure and a performed procedure, the DICOM data model has a n-to-m relationship between SPS and MPPS. The connection is made by the MPPS that references the SPS that it was performed for.

The DIMSE-N Protocol

Unlike all the other command that we’ve discussed so far in this tutorial namely C-ECHO, C-STORE, C-FIND and C-MOVE that are DIMSE-C commands, MPPS uses the normalized, DIMSE-N protocol commands N-CREATE and N-SET to create and update the Modality Performed Procedure Step normalized  information entity. We’ve discussed the normalized data model (aka DICOM Model of the Real World) briefly in chapter 4 when discussing DICOM Objects and stating that image objects are composites of modules from different information entities.

Like, in MWL before, here’s where MPPS fits into the DICOM Data Model:

A Study is comprised of one or more Modality Performed Procedure Steps.

A Modality Performed Procedure Step includes one or more Series.

(Series in turn contains one or more composite objects such as images).

The complete data model can be found at the beginning of chapter 3 of the DICOM standard.

What’s important to remember out of this ERD is that the MPPS is a child of the Study and parent of Series. Note that Series is still a child of a study; it is just that it can have a MPPS parent as well.

 If we look at workflow the sequencing is:

1. An order generates a Study (thru HL7 usually).

2. The Modality gets the Study Instance UID (by making a MWL query) and creates a MPPS.

3. The Modality creates new Series and updates the MPPS.

Practically, the Study Instance UID is created by the PACS/RIS and the Series Instance UID by the modality.

Programming a MPPS Client

The code for the following MPPS client can be found in MODALIZER-SDK Example projects. Try it out.

Let’s do some coding now and see how it all combines together. There are already two examples of MPPS in the DICOM example applications of MODALIZER-SDK, one in C++ and one in C# that you can download. These examples are for unsolicited study because the MPPS is created with no referenced SPS. In this post I want to continue the example of last post on MWL and add MPPS to it so we will start with the MWL client example.

As a start we’ll add three buttons to create complete and discontinue the MPPS. We will create a MPPS for the selected SPS in the lower grid the user will do the following:

1. Fill the MWL Query filters

2. Click MWL Query Button

3. Select an item from the upper grid holding the requested procedure

4. Select an item from the lower grid holding the SPS

5. Click the MPPS Start button. This will add a row to the MPPS grid at the bottom

6. Select a row from the MPPS grid

7. Click either Complete or Abort

Here’s a screenshot of the new MWL  test application after adding MPPS.

The major changes are at the lower part of the form. The start button, the new grid with the MPPS that we already sent and the two buttons to Complete or Abort the selected MPPS.
This is example has relatively a lot of code but most of it has nothing to do with DICOM. This code is rather related to the data grids and their associated data set and handling all the UI of the application. Because of that, I’ve took out all the MPPS code into a separate class (oddly named MPPS) and we’re going to go over this class now. It is really very simple and it has only two public methods, one for the N-CREATe and one for the N-SET.
In the N-CREATE we create an object with the information from the RP and SPS and then send it using a N-CREATE command. If all goes well we get back from the Server the SOP Instance UID of the newly created MPPS instance.
Here’s the code with lots of greens. This method create the object we are going to send with the minimum set of elements that the standard requires:

private DCXOBJ BuildNCreateObject()

        {

            DCXOBJ ssas = new DCXOBJ();

            DCXELM e = new DCXELM();

            DCXUID uid = new DCXUID();

            // Scheduled Step Attributes Sequence

            // This element hold the list of ID's that identify the SPS and RP that we

            // created this MPPS for

            // Get the STUDT INSTANCE UID from the RP we got from the MWL

            // or create a new one if not found

            e = TryGetElement(rp, DICOM_TAGS_ENUM.studyInstanceUID);

            if (e != null)

                ssas.insertElement(e);

            else

            {

                // Create a new UID

                e.Init((int)DICOM_TAGS_ENUM.studyInstanceUID);

                e.Value = uid.CreateUID(UID_TYPE.UID_TYPE_STUDY);

                ssas.insertElement(e);

            }

            e.Init((int)DICOM_TAGS_ENUM.ReferencedStudySequence);

            ssas.insertElement(e); /// Type 2 (0 length is OK)

            // Get the accession number from the RP. It should always be there

            e = TryGetElement(rp, DICOM_TAGS_ENUM.AccessionNumber);

            if (e != null)

                ssas.insertElement(e);

            // Get the RP ID and add it

            e = TryGetElement(rp, DICOM_TAGS_ENUM.RequestedProcedureID);

            if (e != null)

                ssas.insertElement(e);

            // Get the RP description

            e = TryGetElement(rp, DICOM_TAGS_ENUM.RequestedProcedureDescription);

            if (e != null)

                ssas.insertElement(e);

            // Get the SPS ID from the SPS object we got from the MWL

            e = TryGetElement(sps, DICOM_TAGS_ENUM.ScheduledProcedureStepID);

            if (e != null)

                ssas.insertElement(e);

            // SPS description

            e = TryGetElement(sps, DICOM_TAGS_ENUM.ScheduledProcedureStepDescription);

            if (e != null)

                ssas.insertElement(e);

            // If we have codes, not only text description

            e.Init((int)DICOM_TAGS_ENUM.ScheduledProtocolCodeSequence);

            ssas.insertElement(e); /// Type 2 (0 length is OK)

            // Add the Scheduled Step item to a sequence

            DCXOBJIterator sq = new DCXOBJIterator();

            sq.Insert(ssas);

            e.Init((int)DICOM_TAGS_ENUM.ScheduledStepAttributesSequence);

            e.Value = sq;

            ///

            /// Performed Procedure Step Object

            ///

            DCXOBJ pps = new DCXOBJ();

            // Add the Scheduled Step sequence to the MPPS object

            pps.insertElement(e);

            // Add Patient name

            e = TryGetElement(rp, DICOM_TAGS_ENUM.PatientsName);

            if (e != null)

                pps.insertElement(e);

            // Add Patient ID

            e = TryGetElement(rp, DICOM_TAGS_ENUM.patientID);

            if (e != null)

                pps.insertElement(e);

            // Add birth date null

            e.Init((int)DICOM_TAGS_ENUM.PatientsBirthDate);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            // Add sex null

            e.Init((int)DICOM_TAGS_ENUM.PatientsSex);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            // Referenced Patient Seq.

            e.Init((int)DICOM_TAGS_ENUM.ReferencedPatientSequence);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            // MPPS ID has not logic on it. It can be anything

            // The SCU have to create it but it doesn't have to be unique and the SCP

            // should not relay on its uniqueness

            // Here we use a timestamp

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepID);

            e.Value = DateTime.Now.ToString("yyyymmddhhmmssttt");

            pps.insertElement(e);

            // Performed station AE title - this identify the modality that

            // did the work

            e.Init((int)DICOM_TAGS_ENUM.PerformedStationAETitle);

            e.Value = aeTitle;

            pps.insertElement(e);

            // A logical name of the station

            e.Init((int)DICOM_TAGS_ENUM.PerformedStationName);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            // The location

            e.Init((int)DICOM_TAGS_ENUM.PerformedLocation);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            // Start date and time - let's use 'Now'

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepStartDate);

            e.Value = DateTime.Now;

            pps.insertElement(e);

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepStartTime);

            e.Value = DateTime.Now;

            pps.insertElement(e);

            // This is important! The initial state is "IN PROGRESS"

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepStatus);

            e.Value = "IN PROGRESS";

            pps.insertElement(e);

           

            // Description, we can set it later as well in the N-SET

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepDescription);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            // Some more type 2 elements ...

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureTypeDescription);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            e.Init((int)DICOM_TAGS_ENUM.ProcedureCodeSequence);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepEndDate);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepEndTime);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            // Modality - it's a type 1

            e.Init((int)DICOM_TAGS_ENUM.Modality);

            if (modality != null && modality.Length > 0)

                e.Value = modality;

            else

                e.Value = "OT";

            pps.insertElement(e);

            // More type 2 elements

            e.Init((int)DICOM_TAGS_ENUM.StudyID);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            e.Init((int)DICOM_TAGS_ENUM.PerformedProtocolCodeSequence);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            e.Init((int)DICOM_TAGS_ENUM.PerformedSeriesSequence);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            return pps;

        }

After creating the object we can send it using DCXREQ.MPPS_Create and get the SOP Instance UID back. Note the standard also allows the SCU to create this UID. IHE set  this as the SCP responsibility for MPPS.

public void Create(NetConnectionInfo connInfo)

        {

            DCXOBJ createObj = BuildNCreateObject();

            ///

            /// Send the N-CREATE command

            ///

            DCXREQ req = new DCXREQ();

            this.SOPInstanceUID =

                req.MPPS_Create(

                connInfo.CallingAETitle,

                connInfo.CalledETitle,

                connInfo.Host,

                connInfo.Port,

                createObj);

        }


After this is done, we add an item to the MPPS grid that also safe keep the MPPS instance for us in this application.
The next phase is done after the acquisition is done.

private DCXOBJ BuildNSetObject(bool completed)

        {

            DCXOBJ pps = new DCXOBJ();

            DCXELM e = new DCXELM();

            ///

            /// Performed Procedure Step

            ///

            // These are the elements we can update in the N-SET

            // Set the status to "COMPLETED" or "DISCONTINUED"

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepStatus);

            e.Value = completed ? "COMPLETED" : "DISCONTINUED";

            pps.insertElement(e);

            // End date and time

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepEndDate);

            e.Value = DateTime.Now;

            pps.insertElement(e);

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepEndTime);

            e.Value = DateTime.Now;

            pps.insertElement(e);

            // More type 2 elements that we are allowed to change in the N-SET

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureStepDescription);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            e.Init((int)DICOM_TAGS_ENUM.PerformedProcedureTypeDescription);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            e.Init((int)DICOM_TAGS_ENUM.ProcedureCodeSequence);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            e.Init((int)DICOM_TAGS_ENUM.PerformedProtocolCodeSequence);

            pps.insertElement(e); /// Type 2 (0 length is OK)

            ///

            /// Performed Series Sequence

            ///  - Must have at least one item even if discontinued!

            ///

            DCXOBJ series_item = new DCXOBJ();

            e.Init((int)DICOM_TAGS_ENUM.PerformingPhysiciansName);

            series_item.insertElement(e);

            e.Init((int)DICOM_TAGS_ENUM.ProtocolName);

            e.Value = "SOME PROTOCOL";

            series_item.insertElement(e);

            e.Init((int)DICOM_TAGS_ENUM.OperatorsName);

            series_item.insertElement(e);

            e.Init((int)DICOM_TAGS_ENUM.seriesInstanceUID);

            e.Value = "1.2.3.4.5.6";

            series_item.insertElement(e);

            e.Init((int)DICOM_TAGS_ENUM.SeriesDescription);

            series_item.insertElement(e);

            e.Init((int)DICOM_TAGS_ENUM.RetrieveAETitle);

            series_item.insertElement(e);

            e.Init((int)DICOM_TAGS_ENUM.ReferencedImageSequence);

            series_item.insertElement(e);

               e.Init((int)DICOM_TAGS_ENUM.ReferencedNonImageCompositeSOPInstanceSequence);

            series_item.insertElement(e);

            DCXOBJIterator series_sq = new DCXOBJIterator();

            series_sq.Insert(series_item);

            e.Init((int)DICOM_TAGS_ENUM.PerformedSeriesSequence);

            e.Value = series_sq;

            pps.insertElement(e);

            return pps;

        }

To send the N-SET we have to keep the SOP Instance UID from the N-CREATE and use it. The SOP Instance UID of the MPPS is not part of the object. It is send using the Affected SOP Instance UID of the N-SET command. In the N-CREATE it is returned from the SCP in the N-CREATE response in the same manner.

public void Set(bool completed, NetConnectionInfo connInfo)

        {

            DCXOBJ ppsSet = BuildNSetObject(completed);

            ///

            /// Send the N-SET

            ///

            DCXREQ req = new DCXREQ();

            req.MPPS_Set(

                connInfo.CallingAETitle,

                connInfo.CalledETitle,

                connInfo.Host,

                connInfo.Port,

                ppsSet,

                SOPInstanceUID);

            State = completed ? PPSState.COMPLETED : PPSState.DICOUNTINUED;

        }

This reminds me, I didn't mention the SOP Class UID of the MPPS N-SET and N-CREATE command. Can you guess what it is? Surprise! It’s the MWL SOP Class UID: 1.2.840.10008.3.1.2.3.3
That’s it for today. 

As always, comments and questions are most welcome