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| author | Sebastien Jodogne <s.jodogne@gmail.com> |
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| date | Thu, 05 Nov 2020 07:36:18 +0100 |
| parents | 50bdd7c7e9b9 |
| children | 4f3a6145ae34 |
| rev | line source |
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| 0 | 1 .. _dicom-guide: |
| 2 | |
| 3 Understanding DICOM with Orthanc | |
| 4 ================================ | |
| 5 | |
| 6 .. contents:: | |
| 7 | |
| 8 | |
| 9 This section of the Orthanc Book provides a **gentle, informal, | |
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10 high-level introduction to DICOM**. We will try and map DICOM's own |
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11 concepts to a modern terminology that should hopefully be more easy to |
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12 understand for software engineers diving for the first time into |
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13 DICOM. Once the concepts of this page are understood, interested |
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14 readers are invited to read a more formal textbook about DICOM, such |
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15 as the so-called "`Practical introduction and survival guide |
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16 <https://www.springer.com/us/book/9783642108495>`__", or to read the |
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17 full `DICOM specification |
| 25 | 18 <http://dicom.nema.org/medical/dicom/current/output/html/>`__. |
| 0 | 19 |
| 20 All the DICOM concepts that are defined in this introduction are | |
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21 illustrated with `Orthanc <https://www.orthanc-server.com/>`__, a |
| 0 | 22 lightweight, yet powerful standalone DICOM server for healthcare and |
| 23 medical research. As Orthanc is free and open-source software, it is a | |
| 24 good companion to learn DICOM. | |
| 25 | |
| 26 | |
| 27 .. _dicom-format: | |
| 28 | |
| 29 DICOM file format | |
| 30 ----------------- | |
| 31 | |
| 32 The DICOM standard can be very roughly divided in 2 parts: | |
| 33 | |
| 34 1. The part specifying the DICOM **file format**. | |
| 35 2. The part specifying the DICOM **network protocol**. | |
| 36 | |
| 37 The DICOM file format is the topic of the present section. It is | |
| 38 inherently similar to well-known formats such as JPEG, PNG or | |
| 39 TIFF. However, besides the so-called "**pixel data**" that encodes the | |
| 40 medical image itself, a DICOM file also embeds medical information. | |
| 41 | |
| 42 | |
| 43 .. _dicom-tags: | |
| 44 | |
| 45 DICOM tags | |
| 46 ^^^^^^^^^^ | |
| 47 | |
| 48 The medical information encoded by a DICOM file is called a **data | |
| 49 set** and takes the form of a `key-value associative array | |
| 50 <https://en.wikipedia.org/wiki/Associative_array>`__. Each value can | |
| 51 itself be a list of data sets (called a **sequence**), leading to a | |
| 52 hierarchical data structure that is much like a XML or JSON file. | |
| 53 | |
| 54 In the DICOM terminology, each key is called a **DICOM tag**. The list | |
| 55 of the standard DICOM tags are normalized by an official dictionary, | |
| 56 each tag being identified by two 16-bit hexadecimal numbers. For | |
| 57 instance, the birth date of a patient is associated with the DICOM tag | |
| 58 ``(0x0010, 0x0030)``. Note that it is common to drop the "``0x``" | |
| 59 prefix and to simply write ``0010,0030``. For better readability, it | |
| 60 is also common to nickname these DICOM tags with a `camel case | |
| 61 <https://en.wikipedia.org/wiki/CamelCase>`__ English name (such as | |
| 62 "*PatientName*" or "*StudyDescription*"). The standard associates each | |
| 63 DICOM tag with a data type (a string, a date, a floating-point | |
| 64 number...), that is known as its **value representation**. | |
| 65 | |
| 66 Here is how :ref:`Orthanc Explorer <orthanc-explorer>` displays the | |
| 67 DICOM tags stored in a DICOM file (note that the file contains a | |
| 68 sequence identified by the tag ``ProcedureCodeSequence (0x0008, | |
| 69 0x1032)``): | |
| 70 | |
| 71 .. image:: images/DicomTags.png | |
| 72 :align: center | |
| 73 :width: 450px | |
| 74 | |
| 75 | | |
| 76 | |
| 77 The DICOM file format also specifies the set of DICOM tags that are | |
| 78 mandatory or optional for each kind of imaging modality (CT, MR, NM, | |
| 79 CBCT, PET...). Such a specification is called a **storage | |
| 80 service-object pair** (storage SOP). Mandatory tags are called "*type | |
| 81 1*", mandatory tags that can have a null value are called "*type 2*", | |
| 82 and optional tags are called "*type 3*". | |
| 83 | |
| 84 The DICOM standard also allows vendors to introduce non-standard, | |
| 85 **proprietary tags** for their own use. Proprietary tags can be | |
| 86 identified by the fact that their first hexadecimal number is odd | |
| 87 (e.g. ``(0x0009, 0x0010)``). Obviously, such proprietary tags should | |
| 88 be avoided for maximal interoperability. | |
| 89 | |
| 90 | |
| 91 .. _dicom-pixel-data: | |
| 92 | |
| 93 Pixel data | |
| 94 ^^^^^^^^^^ | |
| 95 | |
| 96 The image itself is associated with the DICOM tag ``PixelData (0x7fe0, | |
| 97 0x0010)``. The content of image can be compressed using many image | |
| 98 formats, such as JPEG, `Lossless JPEG | |
| 99 <https://en.wikipedia.org/wiki/Lossless_JPEG>`__ or `JPEG 2000 | |
| 100 <https://en.wikipedia.org/wiki/JPEG_2000>`__. Obviously, | |
| 101 non-destructive (lossless) compression should always be favored to | |
| 102 avoid any loss of medical information. Note that a DICOM file can also | |
| 103 act as a wrapper around a video encoded using `MPEG-2 | |
| 104 <https://en.wikipedia.org/wiki/MPEG-2>`__ or `H.264 | |
| 105 <https://en.wikipedia.org/wiki/H.264/MPEG-4_AVC>`__. | |
| 106 | |
| 107 The image compression algorithm can be identified by inspecting the | |
| 108 **transfer syntax** that is associated with the DICOM file in its | |
| 109 header. | |
| 110 | |
| 111 In practice, few imaging devices in hospitals (besides the `PACS | |
| 112 <https://en.wikipedia.org/wiki/Picture_archiving_and_communication_system>`__ | |
| 113 itself) support image compression. As a consequence, to ensure best | |
| 114 portability, the pixel data of most DICOM files circulating in | |
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115 hospitals is generally **uncompressed**. In other words, the image is |
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116 encoded as a raw buffer, with a given width, height, pixel type |
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117 (integer or float), `color depth |
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118 <https://en.wikipedia.org/wiki/Color_depth>`__ (most often 8, 10, 12 |
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119 bpp - *bits per pixel*) and photometric interpretation (most often |
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120 grayscale or RGB). The transfer syntax that is associated with such |
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121 uncompressed images can either be `little endian |
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122 <https://fr.wikipedia.org/wiki/Endianness>`__ (the most common case) |
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123 or big endian (retired in recent versions of the DICOM standard). |
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124 |
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125 The process of converting one DICOM instance from some transfer syntax |
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126 to another one is referred to as **transcoding**. The topic of |
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127 transcoding is covered in a :ref:`separate FAQ entry <transcoding>`. |
| 0 | 128 |
| 129 A DICOM image can be **multi-frame**, meaning that it encodes an array | |
| 130 of different image frames. This is for instance used to encode | |
| 131 uncompressed video sequences, that are often referred to as **cine** | |
| 132 or **2D+t** images (e.g. for `ultrasound imaging | |
| 133 <https://en.wikipedia.org/wiki/Medical_ultrasound>`__). | |
| 134 | |
| 135 `As written in its DICOM conformance statement | |
| 449 | 136 <https://hg.orthanc-server.com/orthanc/file/default/OrthancServer/Resources/DicomConformanceStatement.txt>`__, |
| 0 | 137 the Orthanc software can receive, store and send any kind of DICOM |
| 138 images (i.e. all the standard transfer syntaxes are | |
| 139 supported). Furthermore, Orthanc can :ref:`convert most uncompressed | |
| 140 images <supported-images>` to PNG images. The `PNG format | |
| 141 <https://en.wikipedia.org/wiki/Portable_Network_Graphics>`__ was | |
| 142 chosen by Orthanc as it is lossless, is natively supported by many | |
| 143 browsers, software or programming frameworks, and is able to encode up | |
| 144 to 16bpp integer pixels. This on-the-fly conversion to PNG images is | |
| 145 what happens when previewing a DICOM image within :ref:`Orthanc | |
| 146 Explorer <orthanc-explorer>`: | |
| 147 | |
| 148 .. image:: images/PreviewInstance.png | |
| 149 :align: center | |
| 150 :width: 400px | |
| 151 | |
| 152 | |
| 28 | 153 .. _model-world: |
| 154 | |
| 0 | 155 Model of the real world |
| 156 ^^^^^^^^^^^^^^^^^^^^^^^ | |
| 157 | |
| 158 This concludes our overview of the DICOM file format itself. It is now | |
| 159 important to give an overview of the so-called "**DICOM model of the | |
| 160 real world**" (`source | |
| 25 | 161 <http://dicom.nema.org/medical/dicom/current/output/html/part04.html#sect_C.6.1.1>`__): |
| 0 | 162 |
| 163 .. image:: images/PS3.4_C.6-1.svg | |
| 164 :align: center | |
| 165 :height: 400px | |
| 166 | |
| 167 This UML diagram shows that a given **patient** benefits during her | |
| 168 life from a set of medical imaging **studies**. Each study is made of | |
| 169 a set of **series**. Each series is in turn a set of **instances**, | |
| 170 the latter being a synonym for a single DICOM file. In Orthanc's | |
| 171 vocabulary, a **DICOM resource** is an umbrella term to talk either | |
| 172 about a patient, a study, a series or an instance. | |
| 173 | |
| 174 Any imaging study can be associated with multiple series of | |
| 175 images. This is especially visible with nuclear medicine, as any | |
| 176 `PET-CT-scan <https://en.wikipedia.org/wiki/PET-CT>`__ study will | |
| 177 contain at least two separate series: the CT series and the PET | |
| 178 series. But any kind of imaging study will usually generate a set of | |
| 179 separate series. In general, a series can be thought of as either a | |
| 180 single 2D image (as in standard `digital radiography | |
| 181 <https://en.wikipedia.org/wiki/Digital_radiography>`__), a single 3D | |
| 182 volume (as in a `CT-scan <https://en.wikipedia.org/wiki/CT_scan>`__), | |
| 183 or a 2D+t cine sequence. But a series might also encode a single PDF | |
| 184 report, a `structured report | |
| 185 <http://www.dclunie.com/pixelmed/DICOMSR.book.pdf>`__, a 3D+t image | |
| 186 (i.e. a temporal sequence of 3D images)... | |
| 187 | |
| 188 In any case, the actual pixel data of a given series is spread across | |
| 189 multiple DICOM instances. This allows to split a single huge image | |
| 190 (medical imaging commonly deals with 4GB images) into hundreds of | |
| 191 small files of several megabytes, each of which can entirely fit in | |
| 192 the computer memory, at the price of a severe redundancy of the | |
| 193 medical information that is embedded within these files. | |
| 194 | |
| 195 For each of these 4 kinds of DICOM resources, the DICOM standard | |
| 196 specifies a **module** as a set of DICOM tags that describe these | |
| 197 resources. For instance, the DICOM tag *PatientName* is part of the | |
| 198 patient module, whereas *SeriesDescription* is part of the series | |
| 199 module. Any storage service-object pair (as :ref:`defined above | |
| 200 <dicom-tags>`) can be decomposed into a set of modules that make sense | |
| 201 for its associated type of modality, and whose conjunction forms | |
| 202 encodes all the medical information. | |
| 203 | |
| 204 According to this model of the real world, the default Web interface | |
| 205 of Orthanc allows to browse from the patient level to the instance | |
| 206 level. Here is how :ref:`Orthanc Explorer <orthanc-explorer>` displays | |
| 207 a series: | |
| 208 | |
| 209 .. image:: images/RealWorld.png | |
| 210 :align: center | |
| 211 :width: 450px | |
| 212 | |
| 213 | | |
| 214 | |
| 215 On the left side of the interface, an overview of the patient module, | |
| 216 the study module and the series module is displayed. On the right side, | |
| 217 the multiple instances of the series are accessible. | |
| 218 | |
| 219 | |
| 220 .. _dicom-identifiers: | |
| 221 | |
| 222 DICOM identifiers | |
| 223 ^^^^^^^^^^^^^^^^^ | |
| 224 | |
| 225 Very importantly, the DICOM standard specifies DICOM tags that allow | |
| 226 to index each single DICOM resource: | |
| 227 | |
| 228 * Patients are indexed with ``PatientID (0x0010, 0x0020)`` (part of the patient module). | |
| 229 * Studies are indexed with ``StudyInstanceUID (0x0020, 0x000d)`` (part of the study module). | |
| 230 * Series are indexed with ``SeriesInstanceUID (0x0020, 0x000e)`` (part of the series module). | |
| 231 * Instances are indexed with ``SOPInstanceUID (0x0008, 0x0018)`` (part of the SOP module). | |
| 232 | |
| 233 The DICOM standard orders *StudyInstanceUID*, *SeriesInstanceUID* and | |
| 234 *SOPInstanceUID* to be `globally unique | |
| 235 <https://en.wikipedia.org/wiki/Globally_unique_identifier>`__. In | |
| 236 other words, it is mandatory for two different imaging devices to | |
| 237 never generate the same identifiers, even if they are manufactured by | |
| 238 different vendors. Orthanc exploits this rule to derive its :ref:`own | |
| 239 unique identifiers <orthanc-ids>`. | |
| 240 | |
| 241 Importantly, even if the *PatientID* must be unique inside a given | |
| 242 hospital, it is not guaranteed to be globally unique. This means that | |
| 243 different patients imaged in different hospitals might share the same | |
| 244 *PatientID*. For this reason, you should always browse from the study | |
| 245 level (and not from the patient level) as soon as you deal with an | |
| 246 application that handles patients from different hospitals. | |
| 247 | |
| 166 | 248 *Note:* Note that since Orthanc 1.4.0, the :ref:`Orthanc Explorer |
| 249 <orthanc-explorer>` interface provides a new tab entitled "Studies" | |
| 250 for study-level access. | |
| 0 | 251 |
| 252 Finally, the patient module is not always fully meaningful. This is | |
| 253 for instance the case in emergency imaging, where the `radiology | |
| 254 information system | |
| 255 <https://en.wikipedia.org/wiki/Radiology_information_system>`__ might | |
| 256 not have previously encountered the imaged patient. In such a case, an | |
| 257 unique ``AccessionNumber (0x0008, 0x0050)`` is associated with the | |
| 258 imaging study by the imaging device of the emergency room: The patient | |
| 259 module will be injected later on in the PACS once the administrative | |
| 260 information is available, as part of a reconciliation process. | |
| 261 | |
| 262 In any case, the core engine Orthanc keeps an index of all these DICOM | |
| 263 identifiers (*PatientID*, *AccessionNumber*, *StudyInstanceUID*, | |
| 264 *SeriesInstanceUID* and *SOPInstanceUID*) and is able to quickly maps | |
| 265 them to its own :ref:`internal identifiers <orthanc-ids>`. This lookup | |
| 266 is implemented by the ``/tools/lookup`` URI of the :ref:`REST API of | |
| 267 Orthanc <rest>`. | |
| 268 | |
| 269 | |
| 238 | 270 .. _dicom-protocol: |
| 271 | |
| 0 | 272 DICOM network protocol |
| 273 ---------------------- | |
| 274 | |
| 275 This concludes our overview of the DICOM file format. :ref:`As written | |
| 276 above <dicom-format>`, we now describe the second main part of the | |
| 277 DICOM standard, i.e. the **DICOM network protocol**. | |
| 278 | |
| 279 | |
| 280 .. _dicom-protocol-overview: | |
| 281 | |
| 282 Overview | |
| 283 ^^^^^^^^ | |
| 284 | |
| 285 The DICOM protocol is actually one of the earliest example of `Web | |
| 286 services <https://en.wikipedia.org/wiki/Web_service>`__, long before | |
| 287 the introduction of `SOAP <https://en.wikipedia.org/wiki/SOAP>`__ or | |
| 288 `REST | |
| 289 <https://en.wikipedia.org/wiki/Representational_state_transfer>`__. It | |
| 290 allows to: | |
| 291 | |
| 292 1. **Test the connection** between two devices (:ref:`C-Echo | |
| 293 <dicom-echo>`). | |
| 294 2. **Send images** from the local imaging device to a remote device | |
| 295 (:ref:`C-Store <dicom-store>`). | |
| 296 3. **Search the content** of a remote device (:ref:`C-Find <dicom-find>`). | |
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297 4. **Retrieve images** from a remote device (:ref:`C-Move |
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298 <dicom-move>` or :ref:`C-Get <dicom-get>`). |
| 0 | 299 |
| 300 Here is a picture that summarizes some key concepts: | |
| 301 | |
| 302 .. image:: images/Protocol.svg | |
| 303 :height: 200px | |
| 304 :align: center | |
| 305 | |
| 306 In the DICOM terminology, the client of a DICOM service is called a | |
| 307 **service class user** (SCU), and the server that handles the requests | |
| 308 is called a **service class provider** (SCP). The client sends a | |
| 309 request that is encoded as a DICOM file (the **command**), and the | |
| 310 server answers with a DICOM file. | |
| 311 | |
| 312 The connection of a DICOM client to a DICOM server is called an | |
| 313 **association**. Such an association starts with a handshake where the | |
| 314 client and the server agree on which commands can be exchanged between | |
| 315 them, and on which :ref:`transfer syntaxes <dicom-pixel-data>` are | |
| 316 supported. The result of this negotiation is called the **presentation | |
| 317 context**. Once the association is negotiated, this communication | |
| 318 channel can be used to successively send multiple, independent | |
| 319 commands. | |
| 320 | |
| 321 | |
| 322 Parameters of a DICOM server | |
| 323 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |
| 324 | |
| 325 Historically, the DICOM protocol was designed to work over | |
| 326 `point-to-point links | |
| 327 <https://en.wikipedia.org/wiki/Point-to-point_(telecommunications)>`__. | |
| 328 Nowadays, the DICOM protocol is used over the `TCP/IP stack | |
| 329 <https://en.wikipedia.org/wiki/Internet_protocol_suite>`__. This | |
| 330 implies that a DICOM server can be identified by specifying the | |
| 331 parameters of its `network socket | |
| 332 <https://en.wikipedia.org/wiki/Network_socket>`__: | |
| 333 | |
| 334 1. Its **IP address** (or, equivalently, its symbolic DNS hostname). | |
| 335 2. Its **TCP port** (the standard DICOM port is 104, but Orthanc uses | |
| 336 the non-priviliged port 4242 by default). | |
| 337 | |
| 338 Furthermore, each imaging device (may it be a client or a server) must | |
| 339 be associated with a symbolic name that is called the **application | |
| 340 entity title (AET)**. The AET is assumed to be unique inside the | |
| 341 Intranet of the hospital. For best compatibility between vendors, the | |
| 342 AET should be only made of alphanumeric characters in upper case (plus | |
| 343 the "``-``" and "``_``" characters), and its length must be below 16 | |
| 344 characters. | |
| 345 | |
| 346 Taken together, the IP address, the TCP port and the AET describe all | |
| 347 the parameters of a DICOM server. The administrators of a medical | |
| 348 imaging network should carefully keep track of these parameters for | |
| 349 each imaging device, and should define an hospital-wide policy to | |
| 350 assign AETs to new devices. | |
| 351 | |
| 352 | |
| 353 Configuring Orthanc | |
| 354 ^^^^^^^^^^^^^^^^^^^ | |
| 355 | |
| 356 Orthanc can act both as a DICOM client and as a DICOM server, | |
| 357 depending on the parameters in its :ref:`configuration file | |
| 358 <configuration>`. To configure the **DICOM server** of Orthanc, the | |
| 359 following options are especially important: | |
| 360 | |
| 361 1. ``DicomServerEnabled`` must be set to ``true``. | |
| 362 2. ``DicomAet`` must be set to the AET that is reserved to Orthanc. | |
| 363 3. ``DicomPort`` specifies the TCP port of the DICOM server. | |
| 364 | |
| 365 To configure Orthanc as a **DICOM client**, you must list the remote DICOM | |
| 366 servers that are known to Orthanc into the ``DicomModalities`` option. | |
| 367 For each remote server, you must specify in the following order: | |
| 368 | |
| 369 1. An user-friendly, symbolic name for the server that will be | |
| 370 displayed by :ref:`Orthanc Explorer <orthanc-explorer>` (possibly | |
| 371 its AET). | |
| 372 2. The AET of the remote server. | |
| 373 3. Its IP address or its hostname. | |
| 374 4. Its DICOM port (most probably 104, or 4242 if the remote server is | |
| 375 another instance of Orthanc). | |
| 376 | |
| 377 Of course, after any change to the configuration of Orthanc, the | |
| 378 software must be restarted to take the new parameters into account. | |
| 379 | |
| 380 | |
| 381 .. _dicom-echo: | |
| 382 | |
| 383 C-Echo: Testing connectivity | |
| 384 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |
| 385 | |
| 386 Routers might block the DICOM protocol between separate `subnets | |
| 387 <https://en.wikipedia.org/wiki/Subnetwork>`__ (often, only the HTTP, | |
| 388 HTTPS and SSH protocols are enabled by default). Furthermore, the | |
| 389 firewalls that are installed on the clients or on the servers might | |
| 390 also block the DICOM protocol. This is especially true for the | |
| 391 `Microsoft Windows firewall | |
| 392 <https://en.wikipedia.org/wiki/Windows_Firewall>`__ and for | |
| 42 | 393 :ref:`RedHat-based GNU/Linux boxes <redhat>`. As a consequence, after |
| 394 any change in the DICOM topology of an hospital (notably when | |
| 395 connecting two imaging devices), one should always check whether the | |
| 396 DICOM protocol can travel from end to end, i.e. between the DICOM | |
| 397 client and the DICOM server. | |
| 0 | 398 |
| 399 This is where the **DICOM Echo service** comes into play. This service | |
| 400 is triggered when the client sends a so-called ``C-Echo`` command to | |
| 401 the server as its DICOM query. The server answers with an empty DICOM | |
| 402 answer. In practice, to test the connectivity between two devices, you | |
| 403 should: | |
| 404 | |
| 405 1. Use the standard command-line tool ``ping`` to test the **TCP-level | |
| 406 connectivity** between the client and the server, then | |
| 407 2. Issue the **C-Echo** from the client to the server to test the | |
| 408 DICOM-level connectivity. | |
| 409 | |
| 410 The second step can be done directly from :ref:`Orthanc Explorer | |
| 411 <orthanc-explorer>`, the embedded administrative interface of Orthanc, | |
| 412 in the "*Query/Retrieve*" panel, as depicted below: | |
| 413 | |
| 414 .. image:: images/Echo.png | |
| 415 :align: center | |
| 416 :width: 450px | |
| 417 | |
| 418 | | |
| 419 | |
| 420 In this screenshot, ``sample`` corresponds to the symbolic name of a | |
| 421 DICOM server that is declared in the ``DicomModalities`` | |
| 422 :ref:`configuration option <configuration>`. Once DICOM Echo succeeds, | |
| 423 the client should be able to send images to the server, and to | |
| 424 initiate a query/retrieve. This is the topic of the following | |
| 425 sections. | |
| 426 | |
| 427 | |
| 428 .. _dicom-store: | |
| 429 | |
| 430 C-Store: Sending images to a server | |
| 431 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |
| 432 | |
| 433 The **DICOM Store Service** is used to send DICOM instances to a | |
| 434 remote imaging device. This service is triggered when the client sends | |
| 435 to the server a so-called ``C-Store`` command together with the | |
| 436 content of a DICOM instance. Schematically, C-Store works as follows: | |
| 437 | |
| 438 .. image:: images/CStore.svg | |
| 439 :align: center | |
| 440 :width: 500px | |
| 441 | |
| 442 Orthanc can act both as a C-Store client (SCU) or as a C-Store server | |
| 443 (SCP). In other words, it can either send or receive DICOM files. | |
| 444 | |
| 445 In the :ref:`Orthanc Explorer <orthanc-explorer>` interface, each | |
| 446 DICOM resource (patient, study, series or instance) comes with a | |
| 447 button entitled "*Send to remote modality*". Clicking on this button | |
| 448 allows to send the image to any DICOM server that is declared in the | |
| 449 ``DicomModalities`` :ref:`configuration option <configuration>`. In | |
| 450 the following screenshot, all the instances of one patient will be | |
| 451 sent to the device whose symbolic name is ``sample``: | |
| 452 | |
| 453 .. image:: images/CStoreGui.png | |
| 454 :align: center | |
| 455 :width: 450px | |
| 456 | |
| 457 | |
| 458 .. _dicom-find: | |
| 459 | |
| 460 C-Find: Browsing the content of a server | |
| 461 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |
| 462 | |
| 463 The **DICOM Find Service** is used to **search** a list of DICOM | |
| 464 resources that are hosted by some remote DICOM server. The kind of | |
| 465 resource that is looked for (patients, studies or series) must be | |
| 466 specified, leading to the **query level** of the query. Besides the | |
| 467 query level, the query contains a set of filters on DICOM tags of | |
| 468 interest: These filters are essentially `wildcards | |
| 469 <https://en.wikipedia.org/wiki/Glob_(programming)>`__ describing the | |
| 470 resources that are looked for. This service is triggered when the | |
| 471 client sends to the server a so-called ``C-Find`` command that encodes | |
| 472 the query level and the filters. Schematically, C-Find works as | |
| 473 follows: | |
| 474 | |
| 475 .. image:: images/CFind.svg | |
| 476 :align: center | |
| 477 :width: 500px | |
| 478 | |
| 479 Orthanc can act both as a C-Find client (SCU) or as a C-Find server | |
| 480 (SCP). In other words, it can be used to search the content of a | |
| 481 remote server, and conversely it can inform a remote client about its | |
| 482 own content. | |
| 483 | |
| 484 In the :ref:`Orthanc Explorer <orthanc-explorer>` interface, it | |
| 485 is possible to initiate a search at the study level. This feature | |
| 486 is available in the "*Query/Retrieve*" panel, as depicted below: | |
| 487 | |
| 488 .. image:: images/CFindGui1.png | |
| 489 :align: center | |
| 490 :width: 500px | |
| 491 | |
| 492 | | |
| 493 | |
| 494 This screenshot shows that we are looking for a study whose associated | |
| 495 patient has a name that starts with "*Brain*", and that is of MR | |
| 496 modality. Orthanc Explorer then lists the matching studies: | |
| 497 | |
| 498 .. image:: images/CFindGui2.png | |
| 499 :align: center | |
| 500 :width: 500px | |
| 501 | |
| 502 | | |
| 503 | |
| 504 It is then possible to click on some matching study, to list the | |
| 505 individual series it is made of: | |
| 506 | |
| 507 .. image:: images/CFindGui3.png | |
| 508 :align: center | |
| 509 :width: 500px | |
| 510 | |
| 511 | |
| 512 .. _dicom-move: | |
| 513 | |
| 514 C-Move: Query/retrieve | |
| 515 ^^^^^^^^^^^^^^^^^^^^^^ | |
| 516 | |
| 517 This brings us to the last important component of the DICOM network | |
| 518 protocol, the **DICOM Move Service**. This service is notably used to | |
| 519 locally retrieve DICOM files from a remote server, given the results | |
| 520 of a :ref:`C-Find query <dicom-find>`. Taken together, ``C-Find`` and | |
| 521 ``C-Move`` give rise to the **query/retrieve** mechanism at is at the | |
| 522 core of the exchange of DICOM files within the hospital. | |
| 523 | |
| 524 C-Move is probably the part of the DICOM standard that is the less | |
| 525 intuitive, which leads to many configuration problems in medical | |
| 526 imaging networks. This stems from the fact that C-Move is actually not | |
| 527 limited to query/retrieve. It is a more generic protocol that can | |
| 528 involve up to **3 distinct imaging devices**, as depicted below: | |
| 529 | |
| 530 .. image:: images/CMove.svg | |
| 531 :align: center | |
| 532 :width: 500px | |
| 533 | |
| 534 Whenever an imaging device (called the *issuer* above) initiates a | |
| 535 C-Move command, it asks a DICOM server (called the *source* above) to | |
| 536 send some of its images to another DICOM server (called the *target* | |
| 537 above). Accordingly, a C-Move command encodes the :ref:`identifiers | |
| 538 <dicom-identifiers>` of the DICOM resources to be sent, together with | |
| 539 the AET of the target server. To put it in other words, a C-Move | |
| 540 command drives a **C-Store between two remote DICOM servers** (the | |
| 541 *source* and the *target*). | |
| 542 | |
| 543 The query/retrieve mechanism corresponds to the special case of a | |
| 544 C-Move command where the target and the issuer are the same imaging | |
| 545 device. The most common configuration problem for query/retrieve is | |
| 546 therefore to forget to declare the AET of the "*issuer/target*" device | |
| 547 into the configuration of the "*source*" device. | |
| 548 | |
| 549 Orthanc can act both as a C-Move client (SCU) or as a C-Move server | |
| 550 (SCP). This implies that Orthanc can either initiate a query/retrieve, | |
| 551 or be driven as part of a query/retrieve request. You might want to | |
| 552 give a look at the ":ref:`query-retrieve`" section of this book to | |
| 553 learn how to concretely configure Orthanc for query/retrieve. | |
| 554 | |
| 555 A C-Move session can be initiated from the :ref:`Orthanc Explorer | |
| 556 <orthanc-explorer>` interface, after a :ref:`C-Find query | |
| 557 <dicom-find>` is complete. It is sufficient to click on the download | |
| 558 button that is to the right of the study or series of interest: | |
| 559 | |
| 560 .. image:: images/CMoveGui1.png | |
| 561 :align: center | |
| 562 :width: 500px | |
| 563 | |
| 564 | | |
| 565 | |
| 566 A dialog box then pops up, asking the AET of the *target* modality. | |
| 567 By default, this field is pre-filled with the AET of Orthanc, which | |
| 568 corresponds to the initiation of a query/retrieve: | |
| 569 | |
| 570 .. image:: images/CMoveGui2.png | |
| 571 :align: center | |
| 572 :width: 500px | |
| 573 | |
| 574 | | |
| 575 | |
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576 *Note 1:* Even if C-Move may seem counter-intuitive, this is the most |
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577 popular and widespread way to initiate a query/retrieve against a PACS |
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578 server. The DICOM standard features an alternative mechanism called |
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579 :ref:`DICOM C-Get <dicom-get>` that has been introduced in Orthanc |
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580 1.7.0 (see below). |
| 0 | 581 |
| 582 *Note 2:* As :ref:`written above <dicom-pixel-data>`, the Orthanc | |
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583 engine is quite generic and is compatible with many image compression |
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584 algorithms (aka. transfer syntax). In particular, during the |
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585 :ref:`negotiation of a presentation context |
| 0 | 586 <dicom-protocol-overview>`, Orthanc reports by default that it is |
| 587 compatible with the JPEG 2000 encoding. This leads some PACS engines | |
| 588 to compress their images before sending them to Orthanc, so as to | |
| 589 reduce the network bandwidth. Unfortunately, because many medical | |
| 590 image analysis software are not compatible with such an image | |
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591 compression, the JPEG 2000 image that is received by Orthanc might not |
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592 be usable by such software. Check out the :ref:`FAQ about DICOM |
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593 transcoding <transcoding>` for more information about converting |
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594 between transfer syntaxes over the DICOM protocol. |
| 0 | 595 |
| 596 | |
| 412 | 597 .. _dicom-get: |
| 598 | |
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599 C-Get: Retrieve with one single SCP |
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600 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
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601 |
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602 Orthanc 1.7.0 introduces support for DICOM C-Get SCP. DICOM C-Get |
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603 provides a simpler alternative to DICOM C-Move, if the *issuer* and |
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604 the *target* (as depicted in the section about :ref:`C-Move |
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605 <dicom-move>`) correspond to the same modality: |
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606 |
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607 .. image:: images/CGet.svg |
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608 :align: center |
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609 :width: 500px |
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610 |
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611 In the case of C-Get, contrarily to C-Move, the *target* modality |
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612 doesn't need to act as a C-Store SCP: The requested data is sent |
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613 directly back to the client over the same DICOM association the C-Get |
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614 request is made on. Therefore C-Get request does not depend on |
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615 separate associations being established to move DICOM from a remove |
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616 source to a remove target. Instead C-Get "turns around" the same |
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617 connection on which the request is made, and re-uses the connection to |
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618 receive the inbound C-Store operation. |
| 412 | 619 |
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620 This has advantage of simplifying the design of the SCU (only one |
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621 DICOM server is needed in the *source* modality), and to ease the |
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622 network configuration (instead of being bidirectional as in |
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623 query/retrieve through C-Move, C-Get is unidirectional from *issuer* |
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624 to *source*, which simplifies firewall rules). Nevertheless, C-Get is |
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625 less generic than C-Move and less often encountered in clinical PACS |
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626 workflow. It is more often used by DICOM viewers. Also note that |
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627 :ref:`DICOMweb WADO-RS <dicomweb>` is designed for the same kind of |
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628 use cases than C-Get. |
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629 |
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630 For a complete description of the C-Get DICOM Request and a review of |
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631 the advantages of C-Get over C-Move, have a look at `David Clunie's |
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632 dedicated blog post |
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633 <http://dclunie.blogspot.be/2016/05/to-c-move-is-human-to-c-get-divine.html>`__. |
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634 |
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635 .. highlight:: json |
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636 |
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637 In practice, to retrieve DICOM instances using C-Get, you must provide |
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638 one ore more of the unique key attributes (``PatientID``, |
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639 ``StudyInstanceUID``, ``SeriesInstanceUID`` or |
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640 ``SOPInstanceUID``). This information can for instance be retrieved |
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641 through a :ref:`C-Find request <dicom-find>`. As an example, let us |
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642 consider the following minimalist :ref:`Orthanc configuration |
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643 <configuration>`:: |
| 412 | 644 |
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645 { |
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646 "DicomModalities" : { |
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647 "getscu" : [ "GETSCU", "localhost", 2000 ] |
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648 } |
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649 } |
| 412 | 650 |
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651 .. highlight:: text |
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652 |
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653 Given this configuration, here is a sample command-line to call the |
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654 C-Get SCP of Orthanc using the `DCMTK |
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655 <https://support.dcmtk.org/docs/dcmconv.html>`__ toolkit, given some |
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656 known ``StudyInstanceUID``:: |
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658 $ getscu -v localhost 4242 -aec ORTHANC -k "0008,0052=STUDY" -k "0020,000d=1.2.840.113543.6.6.4.7.64067529866380271256212683512383713111129" |
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| 528 | 660 *Note:* As of Orthanc 1.8.0, Orthanc implements C-Get as a service |
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661 class provider (SCP). Using C-Get as a service class user (SCU) is not |
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662 currently supported in Orthanc. |
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| 664 | |
| 0 | 665 |
| 666 Using HTTP instead of the DICOM protocol | |
| 667 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |
| 668 | |
| 669 We conclude this brief overview of DICOM by insisting on the fact that | |
| 670 the **DICOM protocol inherently targets the Intranet** of a single | |
| 671 hospital, not the Internet or the cloud. This protocol might be | |
| 672 blocked by outbound firewalls, and there is no convention ensuring | |
| 673 that the AETs are globally unique across all the | |
| 674 hospitals. Furthermore, even though the DICOM protocol supports TLS | |
| 675 encryption, this feature is rarely enabled. | |
| 676 | |
| 677 Depending on your application, you might therefore want to leverage | |
| 678 the **HTTP protocol** in the context of DICOM. Such Web-based | |
| 679 protocols are probably more familiar to physicians/physicists/software | |
| 680 engineers, are easier to work with, can be transparently encrypted | |
| 681 (**HTTPS**), and are compatible with multiple-hospital scenarios. To | |
| 682 this end, you have two possibilities: | |
| 683 | |
| 684 1. Resort to the :ref:`Orthanc peer <peers>` mechanism. Because each | |
| 685 Orthanc server comes with its built-in :ref:`REST API <rest>`, | |
| 686 remote systems can obtain a full programmatic control over the | |
| 687 content of Orthanc. This access can be secured by HTTP | |
| 688 authentication and :ref:`SSL encryption <https>`. The :ref:`Orthanc | |
| 689 Explorer <orthanc-explorer>` interface can transparently use this | |
| 690 peer mechanism to send DICOM files over HTTP(S). | |
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691 2. Resort to the `DICOMweb standard |
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692 <https://www.dicomstandard.org/dicomweb/>`__, that is an extension |
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693 to the DICOM standard specifying how to access the content of a |
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694 remote DICOM server through HTTP(S). Because the peer mechanism is |
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695 bound to Orthanc, DICOMweb offers a less expressive, but more |
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696 generic access to remote servers. Importantly, a `DICOMweb plugin |
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697 to Orthanc |
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698 <https://www.orthanc-server.com/static.php?page=dicomweb>`__ is |
| 0 | 699 freely available. |