The process can affect whole kidney and extend to surrounding tissue,
which then is known as diffuse form XP.
This type is the most common,
comprising up to 90% of cases,
and at the same time boasts the most characteristic imaging findings:
It is considered a gold-standard in imaging of XP.
At least 2-phase exam is performed with the use of intravenous contrast.
First visible change is kidney enlargement,
with the shape staying intact.
Often a hydronephrosis-like transformation is seen,
caused by hypodense masses (-10 to +30 HU) replacing renal parenchyma and by dilated calices.
The outline of these masses shows contrast enhancement – the specific shape created gave it a name of “bear paw sign”.
The biggest advantage of CT over other techniques lays in its ability to visualize renal calculi associated with XP,
as well as to assess the extrarenal involvement and potential complications (e.g.
fistulas,
abscesses).
- · Magnetic resonance imaging:
Although being reported as not able to provide additional information compared to other modalities,
MRI remains a viable diagnostic tool with similar findings.
Its use is limited mostly because of high cost,
low availability and the length of the procedure (which requires children to be sedated for the examination).
Signal intensity of lesions is high in T2 sequence due to the increased concentration of fat,
and intermediate in T1-weighted images.
If given gadolin-based contrast,
areas affected enhance well.
Renal calculi are not directly visible,
but may be indicated by a lack of signal in their location.
Extension to surrounding tissue is easily observed.
Being a low-risk and highly accessible modality without radiation exposure,
US is widely used in paediatrics.
Renal masses are hypoechoic,
while pelvis filled with calculi has high echogenicity and often creates an acoustic shadow.
As in other modalities,
extrarenal involvement can be found.
Still,
information obtained from US is insufficient for proper diagnosis making and treatment planning.
There are also certain atypical findings that may hinder the diagnostic process: unusual pelvic dilatation,
renal atrophy or absence of renal stones.
Diffused XP has to be differentiated with pyonephrosis,
hydronephrosis,
other types of chronic pyelonephritis and renal tuberculosis.
Rarely (10% of adult cases and reportedly more often in children) XP may be restricted solely to kidney in a so-called focal form.
The diagnostic procedure stays the same,
however there are no specific findings that would allow differentiation from other renal masses – patients are commonly treated for renal carcinoma,
and final diagnosis is made afterwards through a histopathological test of the removed kidney.
Focal XP appears as a single mass,
either hypoechoic or hyperechoic in US (given high concentration of lipid-filled macrophages),
enlarging the kidney and distorting its outline.
In CT,
a hypodense,
contrast-enhancing change is described.
Differential diagnosis is especially difficult in focal XP and must include:
- · renal abscess (more solid character),
- · lymphoma (bilateral changes without renal calculi),
- · Wilms tumour (appearance of pyuria may suggest XP).