10.4 CT Systems, PACS, and Information Systems
Key Takeaways
- On hybrid cameras, CT may be low-dose for attenuation correction/localization or higher-technique diagnostic CT—know which protocol is ordered and justified
- Co-registration of CT and emission data is required for accurate AC and fusion; motion between acquisitions breaks registration
- CT QC includes noise, uniformity, spatial/contrast resolution, and artifact review; follow manufacturer and accreditation schedules
- Matrix size, pixel size, and monitor quality affect what can be seen; soft-copy reading environments need appropriate luminance and ambient light control
- PACS, RIS, and EMR integration demand correct patient ID, accession matching, and data integrity—wrong ID is a critical safety event
10.4 CT Systems, PACS, and Information Systems
Quick Answer: Hybrid CT = AC + localization (± diagnostic). Mind dose, registration, and CT QC. On the IT side: correct patient ID, PACS/RIS workflow, and monitors fit for the matrix you acquire.
Nuclear medicine technologists increasingly operate hybrid SPECT/CT and PET/CT systems. Domain IV expects fluency not only with emission detectors but also with CT roles, basic CT QC, digital image parameters, and information systems that move studies to interpretation.
CT for Attenuation Correction vs Diagnostic CT
| Role | Typical intent | Technique / dose concept |
|---|---|---|
| AC / localization CT | μ-map + anatomic landmarks | Often lower dose; may be non-diagnostic by policy |
| Diagnostic CT | Primary CT interpretation (with appropriate supervision/credentialing) | Higher technique, possible IV contrast, full diagnostic protocols |
Never assume a localization CT is a full diagnostic study. Conversely, when a diagnostic CT is ordered with PET/CT or SPECT/CT, technique factors, breath-hold, and contrast timing follow CT—not emission—rules. Document which CT mode was performed.
Co-Registration
Emission and CT volumes must share spatial alignment. Systems use calibrated bed positions and software fusion. Patient motion, different arm positions, or table sag between CT and SPECT/PET shifts the μ-map relative to activity → false defects, hot rims, or wrong organ localization. If motion is obvious, repeat the affected portion when feasible and annotate the study.
CT Quality Control (Exam-Level)
Hybrid CT subsystems need QC analogous to radiology CT, scaled to departmental and manufacturer requirements.
| Test concept | What it checks |
|---|---|
| Noise | Pixel standard deviation in a uniform water phantom ROI—too high noise may mean technique or detector issues |
| Uniformity | Mean CT numbers across phantom positions should be consistent (and near expected HU for water/air) |
| Spatial resolution | Ability to see high-contrast line pairs or beads |
| Contrast resolution / low-contrast detectability | Seeing subtle density differences (often phantom inserts) |
| Artifact review | Rings, streaks, bad detector channels |
Failed CT QC can invalidate attenuation maps even if the gamma or PET detectors pass floods/blanks. After tube replacement or major service, complete required CT calibrations before hybrid clinical use.
CT Safety Alerts and Pre-Scan Notification
CT adds x-ray ionizing radiation and, for diagnostic protocols, possible IV contrast risks.
| Safety focus | Technologist actions |
|---|---|
| Pregnancy / screening | Department screening questions before CT |
| Cumulative / appropriate use | Correct protocol selection; avoid unnecessary diagnostic-dose CT when only AC is ordered |
| Contrast | Allergy/renal history per policy when contrast used; emergency response readiness |
| Implants / devices | Note metal; some devices need CT consideration |
| Pre-scan notification / alerts | Respect dose alert/notification thresholds on modern CT consoles; do not casually override without justification and documentation per policy |
Hybrid rooms still enforce controlled area practices: doors, warning lights, and ensuring non-essential persons leave during CT exposure.
Matrix Sizes, Pixels, and Monitors
| Concept | Why it matters |
|---|---|
| Matrix size (e.g., 64², 128², 256², 512²) | Larger matrices can support finer sampling but need enough counts (NM) or dose (CT) |
| Pixel size | FOV / matrix; determines mm per pixel |
| Display monitors | Primary interpretation monitors need adequate resolution, luminance, and calibration; casual consumer screens can hide pathology |
| Window/level | CT and NM grayscale mapping; wrong window hides disease |
Technologists should acquire at protocol matrix/zoom and not “save counts” by unauthorized matrix changes that violate quantitative or interpretive standards.
PACS, RIS, and Networking
| System | Role |
|---|---|
| RIS (Radiology Information System) | Orders, scheduling, accession numbers, reports workflow |
| PACS (Picture Archiving and Communication System) | Stores and distributes images (DICOM) |
| EMR / HIS | Enterprise medical record; may receive reports and some images |
| Modality worklist | Pulls correct patient demographics to the camera/console |
DICOM is the standard for medical image communication. Studies move from modality → PACS (sometimes via a gateway). Network outages require downtime procedures: do not invent temporary IDs that never get merged.
Data Integrity and Patient Identification
Wrong-patient imaging is a critical safety event in hybrid and pure NM workflows.
| Practice | Purpose |
|---|---|
| Two identifiers (e.g., name + DOB/MRN) | Verify identity at every step |
| Modality worklist selection | Avoid manual mistyping when possible |
| Accession / order match | Right procedure, laterality, and radiopharmaceutical |
| Image check before push | Correct patient, orientation, markers |
| QC of demographics | Fix errors before PACS finalization per policy |
If images are sent under the wrong name, follow institutional error disclosure and correction procedures immediately—do not silently “overwrite” without documentation.
Computers and Acquisition Consoles
The acquisition computer stores raw projections (SPECT), list-mode or sinogram data (PET), CT series, and reconstructed volumes. Protect data integrity by:
- Completing studies under the correct worklist entry before reconstruction.
- Avoiding unapproved software “experiments” on clinical databases.
- Knowing downtime procedures (local archive path, paper orders, later reconciliation).
- Applying HIPAA minimum-necessary access: lock consoles, no shared passwords, no PHI on portable media without encryption per policy.
Secondary captures and screenshots used for teaching must be de-identified. Quantitative screenshots (SUV tables) should match the series sent to PACS so the report and images agree.
Network troubleshooting (exam level): if the worklist is empty, check modality connectivity and RIS interface before manually entering demographics—manual entry is the highest-risk path for ID errors. If PACS send fails, do not delete local data until successful archive is confirmed.
Memory aid: Hybrid excellence = right patient, right CT mode, registered volumes, passing CT+NM QC, clean PACS send.
On a PET/CT system, a low-dose CT acquired solely to create an attenuation map and localize uptake is best classified as:
Patient motion between the CT and SPECT portions of a SPECT/CT exam most importantly risks:
Which practice best protects data integrity when sending hybrid images to PACS?