I have had kidney cancer ct scan now show another small lesion in the same kidney but ultrasound does not?

It's possible.: Ultrasound and ct scanning are two imaging techniques to evaluate the kidney. They are two different technologies, so they have different performance characteristics and are best in certain situations. For cysts, ultrasound is an excellent modality. For small renal masses or lesions, a multiphase contrast ct scan is the best, and may be able to detect lesions not seen on ultrasound.

Mri: Mri can be another excellent imaging modality in these scenarios. If the renal mass is so small it's difficult to determine on ct then it is likely safe to follow with another ct in 6 - 12 months. But asking about MRI is one thought in the short term.