Pseudoenhancement

Pseudoenhancement is an artifact encountered with contrast-enhanced CT, whereby the calculated density of a lesion is inaccurately increased. This phenomenon is most often problematic during evaluation of renal cysts by CT.

On CT, it can be challenging to distinguish cystic versus solid renal lesions. Homogeneous lesions measuring near-water density are generally assumed to represent simple cysts. On the other hand, lesion enhancement following contrast administration suggests the presence of a soft tissue mass and suspicious for malignancy (although not all solid masses enhance). Thus, it is important to identify true lesion enhancement for detection of renal malignancy.

Multiphase renal CT protocols are designed to assess for lesion enhancement between non-contrast and postcontrast acquisitions. Unfortunately, the calculated density of small (<1 cm) renal lesions have been shown to be spuriously increased on contrast-enhanced CT, particularly in the setting of substantial background renal parenchymal enhancement. This "pseudoenhancement" is thought to be due to a combination of partial volume averaging, beam hardening, and scatter radiation crosstalk on multidetector scanners . Indeed, pseudoenhancement artifact may be more pronounced on scanners with a higher number of detectors .

CT evaluation of renal lesions must factor in pseudoenhancement in the assessment for lesion enhancement. In general, renal lesions are considered definitively "enhancing" when there is ≥20 HU increase in attenuation from the precontrast series to the postcontrast series. An increase in attenuation of 10-20 HU is considered indeterminate, as this could actually be a benign cyst demonstrating pseudoenhancement.

Dual energy CT provides a novel approach in minimizing pseudoenhancement by the use of virtual monoenergetic images at a KeV range of 80 Kev to 90 KeV. At these KeV ranges beam hardening and partial voluming is minimized and there is no increase in radiation dose delivered to the patient .