Which of the following is the investigation of choice for assessment of depth of penetration and perirectal nodes in rectal cancer
Investigations for Rectal Carcinoma 
For Diagnosis -

Best diagnosed by sigmoidoscopy (rigid, not flexible) & Biopsy of tumor
Barium enema

For staging

TRUS (transrectal ultrasonography)
CT
MRI

TRUS — (transrectal ultrasonography)

The depth of tumor invasion can be accurately determined by TRUS
But it cannot detect cancer metastasis in lymph node (though it can detect enlarged lymph node, but it cannot predict if — cancer infiltration is present or not).

CT

CT scan is also helpful for evaluation of cancer extension.
But again, CT also cannot tell clearly about lymph node invasion by cancer cell.

MRI

It has become the most useful investigation tool (especially with the use of transrectal coils) for rectal carcinoma.
It can very well predict the invasion of tumor in the rectal wall as well as lymph node invasion with high accuracy.
Trans rectal ultrasound
CT Scan pelvis
MRI Scan
Double contrast Barium enema