How specific is ct scan to confirm avn in a bone in compare to mri and bone scan? ?

MRI is best for...: New onset/recent avn. Bone scan is next, especially with a technique called spect/ct, and ct is third. Once avn is being repaired by the body, it can be seen on ct as new bone formation.

CT is good but: Diagnosis on ct takes specific anatomical changes which can be identified at earlier stages by bone scan and mri. Mri is probably best as you can see marrow changes ( edema, etc. ) and bone changes ( fracture/destruction, if present , at the same time).

MRI or bone scan: It may depend on particular early avascular necrosis in hip is best detected by either MRI or bone scan by showing the decreased blood supply. It is quite specific in majority of cases. Ct can detect later changes of avn such as sclerosis of dead bone or fracture of femoral epiphysis. Avn in small bones of hands and feet may be easier detected by mr examination.

Good question: You really want to detect avn in early stage to enable early intervention and avoid prosthetic hip replacement! more important question is how sensitive imaging modality in detecting early avn. Mri reported sensitivity of 97% & specificity of 98% in the diagnosis of avn of the hip. This is more sensitive & specific than ct or bone scan. Ct is not specific unless asterisk sign is appreciated.