Torsion testis- ultrasound/ Color Doppler video tutorial:

How does one diagnose torsion of the testis? The clinical examination of a torsed testis is characterized by pain and swelling of the scrotum on the side of torsion. In addition the testis is tender. This condition is seen mainly in the age group from 12 to 18 years.

The easiest and most inexpensive diagnostic method for torsion of testis is color Doppler examination of the scrotum. Whilst absence of vascularity or (poor or absent flow in the affected testicle) remains the hall mark of  torsion, there are additional signs of a torsed testis. These include (see the color Doppler ultrasound video clip above), a hypoechoic, swollen testis with a grossly thickened, edematous spermatic cord. The Color Doppler video above shows the mediastinum of the left testes is in an abnormal location, actually pointing laterally. This is also a diagnostic feature of torsion of the testis.

   This color Doppler video shows a transverse section through the left testes, as we pan from the upper pole to the lower pole. Along with the avascular left testis, note the mass formed along the lateral aspect of the testis by the enlarged and equally avascular left spermatic cord.  Any vessels seen in this imaging of torsion of the left testis are within the edematous left scrotal wall, an inflammatory response to the left testicular ischemia.                                                                                  

This video clip shows Power Doppler imaging of torsion of the left testes. Again, note the striking absence of blood flow within the left testes.
Here are a couple of still ultrasound images of the same case:

In the color Doppler image above note the stark contrast between the normally vascular right testis to the total absence of flow in the left testis.

In the lower image, observe the thickened cord, almost the same size as the adjacent left testis.
You can learn more on this topic at: http://www.ultrasound-images.com/acute-scrotum.htm