CT advances • Recent advances in CT are quickly incorporated into PET/CT • Manufacturers are now able to install PET/CT with 128 slices • The lower cost of SPECT means that “high-end” CT is rarely installed in SPECT-CT
In practice Cables Aperture / bore X-Ray Tube Fan beam Detectors
Fast gantry rotation ~ 0. 33 second rotation Courtesy Im. PACT
Helical scanning • • Also known as spiral scanning Introduced in late 1980 s Continuous rotation Continuous table feed
Helical MSCT
Multi-slice CT • Dual slice – Elscint (1991) • Four slice – GE, Philips, Siemens, Toshiba (1998) • Sixteen slice – GE, Philips, Siemens, Toshiba (2002) • 32, 40, 64 slice … – and beyond. . .
Larger detector arrays • 128/256 row scanners – Allow whole organ coverage in single rotation 256 row detector (256 x 0. 5) = 128 mm Z - axis Aquilion 64 detector Detector mock-ups courtesy of Toshiba
Multi slice scanning • Multiple parallel rows of detectors Multi slice • For single irradiation width, four simultaneous data sets acquired – 4 axial images – 4 helical sets of data – (or 8, or 16, or …) Banks of detectors
QC for SPECT/CT or PET/CT • Routine checks (noise, CT number accuracy) • Geometrical alignment of SPECT & CT/PET images • Check of accuracy of correction – – head and body sized water phantoms – Is the CT attenuation correct? Homogeneous phantom – AC Emission values constant across the phantom?
Image noise Scanner’s water phantom • • • CT 01 Measured daily – weekly 40 % of phantom diameter Standard deviation Central slice Noise image
Radiation Dose Low dose CT may be used if the CT scan is not for diagnosis
Radiation dose • • • CT chest ~ 7 m. Sv CT abdomen ~ 8 m. Sv CT calcium cardiac scoring ~ 3 m. Sv CT coronary angiogram CCTA ~ 12 m. Sv Rapid improvements in technology (gating, iterative reconstruction) so CCTA dose may fall to under 5 m. Sv 99 m. Tc MIBI / tetrafosmin ~ 8 – 12 m. Sv
Radiation dose from CT • Absorbed dose (m. Gy) • Effective dose (m. Sv) – Energy absorbed per unit mass – Not related to scan length – Proportional to intensity of x -rays – Radiation risk for stochastic effects – Related to scan length – Related to anatomical position
Scan parameters affecting absorbed dose • Scan parameter settings: – Tube voltage (k. V) – Tube current (m. A) – Rotation time (s) – Helical pitch • …and to a lesser extent: – Focal spot size – Irradiated slice width
Dose versus pitch Pitch = 1 Pitch = 2 Pitch = 0. 5 Higher pitch reduces dose
Diagnostic Reference Levels in CT DRLs: European, national, local Doses collected for standard patient DRL set at the third quartile value of distribution (i. e. 75% of reported doses lie below the DRL) 35 DRL 30 25 Frequency • • • 20 15 75% 10 25% 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 CTDIvol (m. Gy)
CT Patient Dosimetry Calculator