PACS and DICOM Basics for Radiologic Technologists

Why PACS and DICOM Matter to Every Rad Tech

As a radiologic technologist, your primary focus is on positioning, exposure factors, and patient care. But every image you produce passes through a sophisticated digital ecosystem long before it reaches the radiologist's reading station. Understanding how PACS and DICOM work isn't just an IT concern — it is increasingly tested on the ARRT exam and essential for troubleshooting your daily workflow.

Whether you work in a large hospital system with a fully integrated enterprise PACS or a small clinic with a mini-PACS attached to a single DR unit, the principles are the same. Your images must be acquired, formatted, transmitted, stored, retrieved, and displayed correctly. When a study fails to appear on the radiologist's worklist, the technologist who understands the system can often solve the problem before it requires an IT ticket.

This guide covers the PACS and DICOM fundamentals that every rad tech needs, from understanding image tags and compression to navigating RIS-PACS integration and troubleshooting common connectivity problems. Let this be your go-to reference for the digital side of radiography.

What Is PACS? The Three Core Components

PACS stands for Picture Archiving and Communication System. It is the hardware and software infrastructure that enables digital medical images to be acquired from modalities, stored on servers, transmitted across networks, and displayed on workstations — all without film.

Every PACS has three essential subsystems:

These three components are connected by a DICOM network that handles communication between all devices. Most modern PACS also include a web-based viewer (VNA or zero-footprint viewer) that allows clinicians to access images from any hospital computer without installing special software.

Types of PACS by Scale

Not all PACS are created equal. You will encounter different configurations depending on the size of your facility:

• Enterprise PACS: A hospital-wide system connecting all modalities, all departments, and multiple sites. Usually features vendor-neutral archiving (VNA), advanced reporting tools, and deep RIS/EMR integration. Found in large hospitals and health systems.
• Mini-PACS: A modality-specific or department-scale system, such as a CT mini-PACS or ultrasound mini-PACS. Often used in specialty clinics or as a temporary solution in smaller departments.
• Cloud-based PACS: Images are stored on remote servers and accessed via the internet. Growing in popularity for teleradiology, outpatient imaging centers, and multi-site practices. Requires reliable, high-bandwidth internet.
• Vendor-Neutral Archive (VNA): A storage layer that accepts images from any DICOM-compliant PACS or modality, regardless of vendor. Allows health systems to switch PACS vendors without migrating image data.

What Is DICOM? The Language of Medical Imaging

DICOM (Digital Imaging and Communications in Medicine) is the international standard (ISO 12052) that defines how medical images are formatted, stored, transmitted, and displayed. Without DICOM, a CT scanner from GE could not send images to a PACS from Philips or a workstation from Siemens. DICOM is the universal language that makes multi-vendor interoperability possible.

Every DICOM-compliant device — whether it is a CT scanner, DR detector, CR reader, ultrasound machine, MRI scanner, or nuclear medicine gamma camera — implements a specific set of DICOM services. The most important ones for rad techs are:

• DICOM Storage (C-STORE): The service that sends images from a modality to a PACS archive (or between PACS systems). This is what happens when you press "Send to PACS" on the acquisition console.
• DICOM Query/Retrieve (C-FIND and C-MOVE): Allows a workstation or modality to search for and retrieve studies from a remote PACS archive. Used when re-pulling prior exams.
• Modality Worklist (MWL; C-FIND): Allows the modality to query the RIS for scheduled exams and pre-populate patient demographics — eliminating manual data entry and reducing errors.
• DICOM Print (C-PRINT): Sends images to a DICOM-compliant printer (still used in some departments for hard-copy film).
• Modality Performed Procedure Step (MPPS): Sends status updates from the modality to the RIS (e.g., "exam started," "exam completed").

DICOM Information Model: Patient → Study → Series → Image

DICOM organizes image data in a hierarchical model with four levels. Understanding this hierarchy is critical for navigating any PACS:

Each of these levels is identified by a globally unique UID (Unique Identifier). The Study UID, Series UID, and SOP Instance UID (for each image) ensure that every image in the world can be uniquely referenced — critical for multi-site health information exchanges and teleradiology.

DICOM Tags: The Metadata Behind Every Image

Every DICOM file contains two parts: the pixel data (the actual image) and metadata stored as DICOM tags. A tag is a structured key-value pair identified by a group number and element number in hexadecimal format. For example, the patient's name is stored at tag (0010,0010), and the modality type is at tag (0008,0060).

Here are the most common DICOM tags you should know for the ARRT exam and clinical practice:

RIS-PACS Integration: How Images and Data Flow Together

In most imaging departments, the PACS and RIS (Radiology Information System) work together seamlessly. The RIS handles the administrative and workflow side: patient scheduling, exam ordering, billing, report transcription, and result distribution. The PACS handles the image side. The two systems communicate through HL7 (Health Level 7) for text-based data and DICOM for image data.

Here is how a typical exam flows from order to report:

1. Order Entry (RIS): A physician orders a "CT chest with contrast" in the EMR. The order arrives in the RIS with an Accession Number — the unique identifier for this exam.
2. Modality Worklist Query (DICOM MWL): The CT technologist selects "Worklist" on the scanner console. The CT scanner queries the RIS via DICOM MWL and retrieves the scheduled exam demographics. The patient's name, ID, Accession Number, and exam details are auto-populated — no manual typing required.
3. Exam Performed (Modality): The technologist performs the scan. Image data is generated along with DICOM tags containing the patient and study information.
4. Image Send (DICOM C-STORE): The CT scanner sends images to the PACS archive via DICOM Storage service. The PACS receives the images and stores them.
5. Exam Status Update (HL7 or DICOM MPPS): The modality sends a status update back to the RIS: "Exam completed." The RIS marks the exam as performed and billing can proceed.
6. Radiologist Reads (PACS): A radiologist opens the study on a diagnostic workstation, interprets the images, and dictates a report, which is transcribed back into the RIS.
7. Report Distribution (HL7): The final report is sent from the RIS to the EMR, where the referring physician can view both the report and the images (via a PACS web viewer link).

Image Compression: Lossless vs Lossy

Medical images consume enormous amounts of storage space. A single CT chest study with 1 mm slices can contain 300–500 images at 512×512 pixels each — roughly 150–250 MB of uncompressed data. A busy CT department performing 40–50 scans per day generates 6–12 GB of new data daily. Compression is essential for managing storage costs and network bandwidth.

DICOM supports two types of compression:

JPEG Lossless (DICOM Transfer Syntax 1.2.840.10008.1.2.4.70) is the most common lossless compression in PACS. JPEG 2000 offers both lossless and lossy modes with improved compression ratios. The DICOM standard mandates that every modality and PACS must support at least one lossless compression scheme. Lossy compression should never be applied to images that will be used for primary diagnosis — it is reserved for long-term archives or non-clinical purposes.

Troubleshooting Common PACS Connectivity Issues

Even the most robust PACS encounters problems. Here are the most common issues rad techs face and how to resolve them:

"Send Failed" on the Modality Console

This is the most frequent PACS problem. The modality attempted to send images via DICOM C-STORE but the PACS did not acknowledge receipt. Checklist: (1) Is the network cable connected? (2) Is the PACS destination IP address and port correct in the modality configuration? (3) Is the PACS archive online and accepting connections? (4) Does the PACS have sufficient storage space? Most modalities have a "Send Queue" or "Retry" function — use it before re-scanning the patient.

Study Missing from Radiologist's Worklist

The images were sent successfully but did not arrive at the reading workstation. Checklist: (1) Verify in the PACS archive that the study was received (most PACS have a "Study Status" search). (2) Check whether the study was "split" or "merged" incorrectly. (3) Ensure the study status is not "Hold" or "Unverified." (4) Verify that the reading radiologist's worklist filter is correct — sometimes the filter excludes certain exam types or time ranges.

Wrong Patient Information Visible

Another patient's data appears on your study. This is a serious safety event and must be reported immediately. Most PACS and modalities allow demographic correction — but only by authorized personnel. Never modify patient data on the modality without following your facility's correction policy. If images were sent with incorrect demographics, the study must be deleted from PACS and re-sent with the corrected data.

Slow Image Loading

Images load slowly on the workstation, especially for large CT or MR studies. Check: (1) Network bandwidth — large studies can saturate a 100 Mbps connection. Hospital IT departments should prioritize imaging traffic via QoS (Quality of Service). (2) Pre-fetching settings — the PACS should pre-fetch prior exams for comparison before the radiologist opens a study. (3) Workstation hardware — diagnostic monitors require dedicated graphics cards with sufficient VRAM. (4) Archive type — studies on slower spinning-disk archives take longer to retrieve than those on solid-state storage.

Key Terms for the ARRT Exam

These terms are the most likely to appear on your registry exam related to PACS and DICOM:

• DICOM: Digital Imaging and Communications in Medicine — the standard for medical image exchange.
• PACS: Picture Archiving and Communication System — the complete system for storing, retrieving, and displaying images.
• RIS: Radiology Information System — manages scheduling, billing, and reports.
• HL7: Health Level 7 — the standard for exchanging text-based health information between systems.
• Modality Worklist (MWL): DICOM service that downloads scheduled exams from the RIS to the scanner.
• Accession Number: The unique identifier assigned to a radiology order/exam.
• UID (Unique Identifier): A globally unique string that identifies a study, series, or image.
• Lossless Compression: Compression with zero data loss (required for diagnostic images).
• Lossy Compression: Compression that discards non-essential data (for archival/review only).
• Query/Retrieve: DICOM service for searching and pulling studies from a remote PACS archive.
• VNA (Vendor-Neutral Archive): A storage platform that accepts DICOM objects from any vendor's PACS or modality.
• MPPS (Modality Performed Procedure Step): DICOM service that sends exam status updates (in progress, completed, discontinued) to the RIS.