technologiesTechnologyStandard of care

SPECT/CT

A gamma camera with a CT scanner bolted on, so a hot spot on a bone scan or a sentinel-node scan is pinned to the exact bone or lymph node, and the dose from a radioactive drug can be measured after treatment.

SPECT/CT acquires a rotating gamma-camera study and a CT scan on one gantry. The CT corrects the SPECT for attenuation and places each focus of tracer anatomically, turning an ambiguous spot on a planar bone scan into a rib fracture or a metastasis and mapping a sentinel node to the correct basin before surgery. In oncology it is used for technetium bone scans in prostate and breast cancer, sentinel lymph node mapping in melanoma and breast cancer, radioiodine scans in thyroid cancer, MIBG in neuroblastoma, somatostatin-receptor scans where PET is not available and, increasingly, for quantitative imaging of lutetium-177 after radioligand therapy so the absorbed dose to tumour and kidneys can be calculated. Newer cameras replace sodium iodide crystals with cadmium zinc telluride detectors arranged in a ring (GE StarGuide, Siemens Symbia Pro.specta with digital detectors), which are faster and more quantitative.

SPECT is cheaper and far more widely available than PET, and the tracers are generator-produced without a cyclotron, but its resolution and sensitivity are lower and scans are slower.

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How it works

Rotating gamma cameras record single photons from technetium-99m, iodine-123 or lutetium-177 tracers; a co-registered CT provides attenuation correction and anatomical localisation for a quantitative three-dimensional map.

Strengths

Widely available and cheaper than PET
Tracers from generators, no cyclotron needed
Dosimetry after lutetium-177 therapy

Limitations

Lower resolution and sensitivity than PET
Slower acquisitions
Fewer targeted tracers than PET

Since

1999

Dependencies

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Needed by (1)

Quantitative SPECT

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Products

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ApprovedSPECT tracer

Gallium citrate Ga-67

One of the oldest cancer-imaging scans still on the market: an injectable form of radioactive gallium that collects in lymphomas and some other tumours (and in infections), imaged one to five days later. It has mostly been replaced by FDG-PET but is still FDA-approved and occasionally used.

ApprovedSPECT tracer

Indium-111 pentetreotide · OctreoScan

The original nuclear medicine scan for finding neuroendocrine tumours, used before PET scans like Ga-68 DOTATATE became available. It is still FDA-approved but has largely been superseded because PET gives sharper pictures with less radiation.

ApprovedSPECT tracer

Technetium Tc-99m medronate (MDP)

The standard, decades-old bone scan: a radioactive tracer that sticks to areas of bone rebuilding, so a gamma-camera picture afterwards shows where cancer may have spread to bone, or where a fracture or infection is healing.

EmergingSPECT tracer

Technetium Tc-99m PSMA agents (PSMA-I&S)

A version of the PSMA prostate cancer scan that uses the same cheap, widely available radioactive tag as a standard bone scan instead of the PET tracers. Surgeons can also carry a small radiation detector into the operating room to help find and remove cancerous lymph nodes. It has not been through formal drug approval; it is used as an academic, in-house preparation.