Do I have to get a CT scan previous to removing my varicose veins? My Dr suggested yes.

In the majority of cases, ultrasound is the only imaging required prior to varicose vein treatment. However, it would be difficult to determine without doing a varicose vein consultation at your local vein doctor. 

It’s important to distinguish between varicose veins and spider veins when choosing between laser therapy, sclerotherapy, and vein surgery. With two board certified vein specialists, our center uses a combination of visual examination, functional flow measurements, ultrasound, transillumination, and infrared light to map out vein networks prior to therapy. Lasers can be used to treat deep varicose vein insufficiency, vein perforators, and varicose veins. Sclerotherapy is ideal for spider veins and reticular veins. Whereas surgery is reserved for severely incompetent varicose vein segments. 
Dr. Karamanoukian
Realself100 Surgeon

It is essential to communicate any concern to your provider. 
A venous ultrasound of the lower extremities can be done for diagnosis of venous insufficiency for varicose veins. 
Any other test like CT scan depends on the specific study indication. 

An ultrasound is typically all that is needed and is the standard of care for diagnosing venous problems.  If there are abnormal findings on ultrasound or physical exam to suggest venous pathology in the pelvis or abdomen then a CT venogram could help determine the anatomy and level of pathology.  An MRI would be an alternative that does not involve radiation and intravenous ultrasound can also be done but is more invasive and is typically done concurrently with intention to treat such as in placing venous stents for iliac vein compression.  

Most patients with varicose veins can be adequately evaluated using venous duplex ultrasound from the level of the groin crease and below. There are circumstances where a CT scan or MRV might be needed and valuable before proceeding with treatment. If your examination or history suggests a problem above the groin crease then these tests may be necessary. If you have a prior history of blood clots or varicose veins involving the labia, lower abdomen, or buttocks then a CT scan/MRV may be important to determine if there is a blockage or venous insufficiency in the veins of the pelvis or abdomen. An ultrasound is more than adequate in the case of routine varicose veins.

Vein problems between the groins and the feet are usually diagnosed by a venous duplex ultrasound.  If the vein problems are above the groin then a CTA  WITH VENOUS PHASE or MVI would be done.  Routine vein problems are diagnosed via duplex scan.

The standard of care is to perform an ultrasound. It should be done in the office by either your doctor or a technologist working with your doctor.