background:
The DICOM-RT series of blog posts looks at the complementary standard in DICOM 3.0 to the field of radiotherapy, namely DICOM-RT. In order to facilitate the understanding of the relevant concepts of IOD and SOP in DICOM-RT, the column has recently made the popularization of knowledge points related to radiotherapy.
PS: Of course, I am not a radiotherapy professional. The concepts in the text are mostly from foreign literature books, google, and wiki. inaccuracies in the text. Email exchanges are welcome.
Related images in the field of radiotherapy
The previous blog posts have introduced the relevant processes of radiotherapy and the relevant participating roles, and also mentioned the formulation of radiotherapy plans, including target delineation, geometric planning and dose planning. There will be all kinds of images in there. Here's one by one:
1. CT from CT SIM
In the blog post DICOM-RT: Radiation Therapy Process and Participating Roles when introducing the radiation therapy process. It is mentioned that the patient first needs to be scanned for normal state data. This refers to conventional diagnostic CT scans (of course, additional MRI and PET modal data may be taken). Modal data such as kv CBCT and MV CT may also be introduced, so CT here is limited to diagnostic-grade 3D images of CT simulators.
The CT simulator is a diagnostic CT equipped in the oncology department, which is the same as the CT spiral CT equipment in the traditional radiology department. The data collected is mainly used for oncologists to delineate the target volume and to formulate radiotherapy plans by physicists (dosimeters). (Conventional CT can truly reflect the density of matter and can be used for dose calculations, which is different from CBCT images that will be described below).
The following figure is excerpted from a set of target delineation data in 3DSlicer ( ie RT Structure Set in DICOM-RT ):
2. EPID（Electronic Portal Imaging Device）
EPID is commonly known as electron portal imaging, and its energy belongs to the megavolt level ( that is, imaging with therapeutic-grade X-rays during tumor treatment. It is higher than the X-ray energy level used in conventional CT ). It is mainly used for image acquisition, position calibration and image guidance during treatment.
Its inherent shortcomings are " low contrast of megavolt-level radiographic images and limited image acquisition range ", and most of the existing radiotherapy information systems cannot integrate EPID images . EPID is matched with radiotherapy accelerator. The details are as follows:
Images of EPID were acquired prior to patient treatment. A flat 2D map of different viewing angles obtained from several angles. Target alignment is achieved by registering with the two-dimensional image (DRR, Digitally Reconstructed Radiographs. This is the RT Image in DICOM-RT that may be introduced in detail) after the above-mentioned planned CT image reconstruction .
3. kV-CBCT ( including plain 2D, volume 3D and real-time fluoroscopic images )
KV-level CBCT, endows the traditional medical linear accelerator with the function of volumetric imaging. CBCT image acquisition and reconstruction is performed while the patient is already in the treatment position before treatment begins, providing high-resolution volumetric imaging of 3D and soft tissue. Achieve more precise target alignment.
4. MV-CBCT ( including plain 2D, volume 3D and real-time fluoroscopic images )
Megavolt CBCT, which directly utilizes a linear accelerator as the imaging radiation source , has good imaging quality for tissues with high contrast (such as bone and lung) or materials with high atomic numbers (such as dentures or hip prostheses). However, compared with kV-CBCT, the image quality of soft tissue is very difficult to achieve the accuracy required for image guidance.
5. MV CT
Illumination is performed by an MV linear accelerator on a ring gantry combined with the slip-ring technology of diagnostic helical CT, similar in structure to a conventional helical CT scanner, and the linear accelerator rotates continuously around the patient. The multi-leaf grating-modulated fan beam can deliver slice-by-slice intensity-modulated radiation therapy to the target volume in the patient from any angle. However, MV CT imaging has its limitations, including inherently low soft tissue contrast and inefficient detectors.
【Remarks】:
Various imaging modes are mentioned above. Among them, the amorphous silicon detectors (AMFPIs) used in CBCT are cheaper to manufacture and more . Therefore, kV-CBCT and MV-CBCT can directly use linear accelerators as X-ray sources.
Standard diagnostic imaging utilizes an X-ray tube as the photon source, typically with a peak voltage of 70-120kVp. Compared with MV-level photons generated by linear accelerators, kV-level photons have a higher probability of interacting with the illuminated target, resulting in higher-contrast images with lower radiation doses.
Because the focal size of the X-ray tube is usually smaller than that of the linac, the spatial resolution of kV images is higher than that of MV images.
By integrating kV-X-ray tube and AMFPIs amorphous silicon detector, it can be used for CBCT imaging, and at the same time, it can perform fluoroscopy and flat film imaging.
Summarize
Updates to the various technologies mentioned above. Promote the development of the field of radiotherapy, from the initial conventional two-dimensional radiotherapy to three-dimensional conformal radiotherapy (3D-CRT). To intensity modulated radiotherapy (IMRT), to image-guided radiotherapy (IGRT), to finally adaptive radiotherapy (ART). The only purpose is "faster, better and cheaper". Ensure accurate target positioning, alignment and image guidance, reducing the uncertainty in radiation therapy that can occur between fractional treatments. It can also occur with every treatment.
The error between different treatments can be controlled through another setup or another plan; the positional changes of the tissue structure in the body during the treatment can be monitored through treatment images. Ability to correct errors within each treatment. The images described above can be classified from the perspectives of planning and actual radiotherapy, and can classify the diagnostic CT data of the CT simulator into the radiotherapy planning process. CT, etc.) are used for target alignment and treatment monitoring during treatment.
Author: [email protected]
Time: 2016-04-13