Goal

Understand DICOM concepts well enough to navigate pacs_system's API: SOP classes, transfer syntaxes, UIDs, and the four-level hierarchy (Patient/Study/Series/Image).

What is DICOM?

DICOM (Digital Imaging and Communications in Medicine) is the standard for medical imaging data exchange. Unlike JPEG or PNG, a DICOM file bundles the pixels with rich metadata: patient identity, modality (CT, MR, US, etc.), acquisition parameters, and administrative information.

pacs_system implements the DICOM standard from scratch with no external DICOM library — every byte of the wire format and file format is handled by code you can read and modify.

SOP Classes

A SOP (Service-Object Pair) class combines an object type with a service operation. For example:

CT Image Storage — CT scan pixel data + C-STORE service = "I can send/receive CT images"
Patient Root Query/Retrieve — Patient hierarchy + C-FIND/C-MOVE = "I can search and fetch by patient"

Each SOP class is identified by a unique UID (e.g., 1.2.840.10008.5.1.4.1.1.2 for CT Image Storage). pacs_system supports 27 SOP classes covering CT, MR, US, XA, NM, PET, RT, SR, SEG, MG, CR, SC storage plus Query/Retrieve, MWL, MPPS, Storage Commitment, Print, and UPS.

Transfer Syntaxes

A transfer syntax specifies how data is encoded on the wire: byte order, VR representation, and compression.

Implicit VR Little Endian (1.2.840.10008.1.2) — default, uncompressed
Explicit VR Little Endian (1.2.840.10008.1.2.1) — more robust, uncompressed
JPEG Baseline — lossy compression for 8-bit images
JPEG Lossless — lossless compression
JPEG 2000 — modern lossless/lossy
HTJ2K — high-throughput JPEG 2000
RLE Lossless — simple lossless

The receiver advertises which syntaxes it accepts during association negotiation; the sender picks one of the accepted syntaxes for each presentation context.

UIDs

A UID (Unique Identifier) is a globally unique dotted-numeric string. DICOM uses UIDs for:

SOP Class UIDs — identify what kind of object/service
Transfer Syntax UIDs — identify the encoding
Study/Series/SOP Instance UIDs — identify specific patient data

pacs_system provides uid_generator to create valid UIDs rooted at your organization's prefix.

Patient/Study/Series/Image Hierarchy

Every DICOM object belongs to a four-level hierarchy:

Patient (John Smith)
  └── Study (2026-04-01 abdominal CT)
        └── Series (axial, 3mm slices)
              └── Image (slice 1 of 64)
              └── Image (slice 2 of 64)
              └── ...

Query/Retrieve services operate at any level: find all studies for a patient, retrieve all images in a series, etc.

Read your first DICOM file

#include <pacs/core/dataset.h>
#include <pacs/core/part10_reader.h>
 
int main(int argc, char** argv) {
    pacs::core::part10_reader reader;
    auto result = reader.read(argv[1]);
    if (result.is_error()) {
        std::cerr << "Read failed: " << result.error().message << std::endl;
        return 1;
    }
 
    auto dataset = result.value();
    auto patient_name = dataset.get_string(pacs::tags::patient_name);
    auto modality = dataset.get_string(pacs::tags::modality);
 
    std::cout << "Patient: " << patient_name.value_or("UNKNOWN") << "\n"
              << "Modality: " << modality.value_or("UNKNOWN") << std::endl;
}

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