CT (Computed Tomography)

Rotating X-ray source and detector array reconstruct cross-sectional images from hundreds of projections per rotation. The detector-row count drives clinical capability — "16-slice", "64-slice", "320-slice" refers to the number of parallel detector rows. The highest-volume cross-sectional modality in modern radiology, and the workhorse of every emergency department in the developed world.

Physics

X-ray tube and detector rotate around the patient on a slip-ring gantry. The patient couch translates through the gantry during helical acquisition. Reconstruction algorithms (filtered back projection historically; iterative reconstruction since the 2010s; deep-learning reconstruction since the late 2010s) convert projection data into cross-sectional images expressed in Hounsfield units. Photon-counting CT (NAEOTOM Alpha) replaces the energy-integrating detector with direct photon counting, enabling spectral-by-default imaging and finer spatial resolution.

History

1972 — Godfrey Hounsfield (EMI) builds the first commercial CT (EMI Mark I).
1979 — Hounsfield + Cormack share the Nobel Prize in Physiology or Medicine.
1989 — single-slice helical (spiral) CT.
1998 — 4-slice CT (multi-detector era begins).
2004 — 64-slice CT becomes the clinical standard (GE LightSpeed VCT, Siemens SOMATOM Sensation 64).
2006 — Siemens SOMATOM Definition dual-source CT.
2007 — Canon (Toshiba) Aquilion ONE ships 320-slice (160 mm z-axis).
2013+ — iterative reconstruction (ASiR, ASiR-V, SAFIRE, ADMIRE, iDose, AIDR 3D) becomes standard.
2018+ — deep-learning reconstruction (TrueFidelity / DLIR, AiCE) reaches clinical scale.
2021 — Siemens NAEOTOM Alpha — first commercial photon-counting CT.

Key specs buyers evaluate

Slice count — 16 / 32 / 64 / 128 / 256 / 320, plus dual-source and photon-counting tiers.
Rotation time — 0.25–0.35 s on cardiac-tier; 0.5–0.7 s mid-tier; 1+ s entry.
Gantry aperture — 70 cm standard, 78–82 cm wide-bore (oncology simulation, bariatric, trauma).
Tube — heat capacity (MHU), focal spot size, scan-count history. See Straton, Vectron, Gemstone Clarity, MegaCool, Philips MRC.
Dose-reduction software tier — ASiR-V / TrueFidelity (GE), ADMIRE (Siemens), iDose / AI-suite (Philips), AIDR 3D / AiCE (Canon).
Cardiac capability — gating, multi-segment recon, dual-source temporal advantage, motion-correction (SnapShot Freeze).
Dual-energy / spectral — TwinBeam (Siemens), GSI (GE), Spectral Detector (Philips dual-layer, IQon).

Systems

GE — LightSpeed family (legacy workhorse), Discovery CT750 HD, Revolution CT family (HD / Apex).
Siemens — SOMATOM Sensation (legacy), Definition family (AS / Flash / Force), go.Top, X.cite, NAEOTOM Alpha (photon-counting).
Philips — Brilliance / Ingenuity / iCT / IQon Spectral CT / Incisive.
Canon — Aquilion ONE family, Aquilion Prime, Aquilion Precision, Aquilion Lightning.

Clinical applications

CT Trauma (pan-scan)
Stroke Thrombectomy (workup)
Cardiac CTA, pulmonary embolism, oncologic staging, CT colonography, dual-energy material decomposition

Service and refurb reality

The X-ray tube is the dominant cost of ownership. A premium-tier tube replacement is a substantial line item in the scanner's lifetime capital plan. Duty cycle determines tube lifespan more than calendar age — chronically pushing mA limits shortens tube life.
Plan capital around tube replacement cycles, not scanner lifespan.
Detector module replacement is a parallel lifecycle event for premium platforms; module-level swap markets exist.
Iterative + DL recon licensing is the largest software refurb price-determining variable. A scanner without current-tier recon licensing performs at the older-platform dose / image-quality curve regardless of chassis age.
Slip-ring wear correlates with rotation speed and duty cycle; brush replacement is a scheduled event on heavy-use sites.
See LightSpeed VCT Field Guide, Revolution CT Field Guide, and SOMATOM Definition Field Guide.

Regulatory

ACR CT accreditation for reimbursement of advanced indications.
Dose monitoring — CTDI / DLP tracked and reported via DICOM structured dose reports under federal and state dose-tracking regulations.
State X-ray-machine registration for the equipment itself.