Attenuation Correction (AC)

• The heart isn’t in the middle and there are lots of artifact from non-uniform attenuation area
• Cardiac PET imaging should only be performed with attenuation correction. Attenuation correction can be accomplished with a rotating line source or ring in a dedicated PET system or with CT or MRI in a hybrid system.¹
• The Hounsfield units (HU) generated by the CT scanner can usually be accurately converted into PET attenuation values.¹
• Typically adds less than 10 seconds to the cardiac scan time.¹
• Errors in attenuation correction from misregistration are typically much worse if the CT is acquired at full inspiration, and so the CT is often acquired at either end-expiration or during shallow breathing.¹
• Attenuation artifacts limit interpretation confidence and reduce specificity
• Ways to deal with attenuation artifacts
  • Multiple position imaging: Prone/Upright/Supine
  • Pre (Sorenson) and Post (Chang) reconstruction models.
  • Transmission scan
• Gadolinium-153 (15 Gd) External Line Source for AC
  • Commercially available for most SPECT systems
  • Annual expense to replace line source
  • No misregistration correction needed
  • Common artifact is right sided truncation in obese patients
• Image Registration
  • The reconstructed PET and CT images must accurately reflect the same 3D locations (i.e., the two images must be in registration). Such registration is often difficult because the PET and CT portions of all commercial combined PET/CT systems are not coincident (i.e., the PET and CT “slices” are not in the same plane) and the PET and CT gantries are contiguous. In practice, this means that the PET and CT acquisitions do not simultaneously image the same slice. In fact, because the bed must travel different distances into the gantry to image the same slice in the patient for PET versus CT, there is ample opportunity for misregistration via x, y, z misalignment of bed motion—or, of perhaps even greater concern, because of differential ‘‘bed sag’’ for the PET and CT portions, depending on the table design.¹
  • Attenuation Correction Accuracy
    • The use of the CT image for PET attenuation correction requires a transformation of the observed CT numbers in HU to attenuation coefficients at 511 keV.

Footnotes

1. Dilsizian V, Bacharach SL, Beanlands RS, et al. ASNC imaging guidelines/SNMMI procedure standard for positron emission tomography (PET) nuclear cardiology procedures. Journal of Nuclear Cardiology. 2016;23(5):1187-1226. doi:10.1007/s12350-016-0522-3 ↩ ↩² ↩³ ↩⁴ ↩⁵