A transrectal ultrasound (TRUS) is an imaging procedure where a small probe is inserted into the rectum to create detailed pictures of the prostate gland and surrounding structures. Because the rectum sits directly behind the prostate, placing the probe there produces much sharper images than scanning through the abdomen. The procedure takes roughly 10 to 20 minutes and is most commonly used to evaluate prostate problems or guide a biopsy needle into suspicious tissue.
How the Procedure Works
The ultrasound probe is roughly the width of a finger. It emits high-frequency sound waves that bounce off tissues at different rates depending on their density. A computer translates those echoes into a real-time image on a monitor, letting the doctor see the size, shape, and internal structure of the prostate in detail. Abnormal areas often appear darker than the surrounding healthy tissue, particularly in the outer zone of the gland where most prostate cancers develop.
Beyond simple imaging, TRUS is frequently used as a live visual guide during procedures. When a biopsy is needed, the doctor watches the screen while directing a thin needle into specific areas of the prostate. The same guidance capability makes it useful for placing radiation seeds during cancer treatment, draining prostatic abscesses, and measuring the gland before surgery for an enlarged prostate.
Why Doctors Order a TRUS
The most common reason is a concern about prostate cancer. If a blood test shows a persistently elevated PSA level, or if a doctor feels a hard or unusual nodule during a digital rectal exam, a TRUS is typically the next step. It remains the most widely used imaging tool for evaluating the prostate and the primary method for guiding biopsies.
Cancer evaluation is far from the only use. Doctors also order TRUS to:
- Measure prostate size before treating benign prostatic hyperplasia (an enlarged prostate that causes urinary symptoms)
- Diagnose prostatic abscesses, which appear as dark cavities on the ultrasound and can be drained during the same session
- Investigate chronic prostatitis that hasn’t responded to standard antibiotics
- Evaluate male infertility, particularly when cysts or blockages in the reproductive tract are suspected
- Assess blood in semen (hematospermia) that doesn’t have an obvious explanation
- Examine pelvic organs in women when a transvaginal ultrasound isn’t possible, or to evaluate rectal and anal cancers in either sex
What the Experience Feels Like
If you’re having a TRUS for imaging only (no biopsy), the main sensation is pressure as the lubricated probe passes through the anal sphincter. Most people describe it as uncomfortable but brief. A topical anesthetic gel is applied to the rectal area beforehand to ease insertion.
When a biopsy is included, the experience is more intense. Studies show that 65% to 90% of patients report discomfort during biopsy, and about 30% describe significant pain. On a 1-to-10 pain scale, the average score falls around 3 to 4, though roughly 16% of patients rate it a 5 or higher. The most sensitive moment is the biopsy needle passing through the prostate’s apex, the lowest part of the gland. Pain during the initial probe insertion tends to predict how uncomfortable the biopsy itself will be, likely because both relate to anal sphincter tension.
To manage this, doctors typically inject a local anesthetic (usually lidocaine) around the nerve bundles near the prostate, a technique called a periprostatic nerve block. This is done under ultrasound guidance with a thin needle before the biopsy cores are taken. Combining the nerve block with the topical anesthetic gel provides the best pain control. General anesthesia is rarely needed.
Preparing for the Procedure
Preparation is straightforward. Most clinics ask you to use a rectal enema on the morning of the procedure to clear the lower bowel, which improves image quality and reduces infection risk during biopsy. Some centers prescribe a full bowel preparation solution the day before, though a simple enema on the day is the more common approach.
If you take blood thinners or anti-platelet medications, your doctor will likely ask you to stop them about a week before a biopsy. People who can’t safely pause anticoagulation are usually switched to a short-acting injectable alternative. Antibiotic prophylaxis is standard when a biopsy is planned, typically starting the day before the procedure and continuing for a few days after.
What the Images Reveal
A healthy prostate in a young man measures roughly 4 by 3 by 3.5 centimeters and has a volume of about 20 to 25 cubic centimeters, comparable in size and shape to a chestnut. The doctor evaluates whether the gland is enlarged, whether its internal zones look normal, and whether any dark spots suggest cancer or other abnormalities.
Prostate cancer typically shows up as a darker-than-normal area in the peripheral zone, the outer ring of the gland. An enlarged prostate from BPH looks different: it presents as asymmetric growth or multiple dark nodules in the transition zone, the inner portion surrounding the urethra. The doctor also checks the seminal vesicles, the small glands sitting above the prostate. Asymmetry, unusual enlargement, or a solid rather than fluid-filled appearance can signal that cancer has spread beyond the prostate itself.
TRUS Compared to MRI
TRUS has been the workhorse of prostate imaging for decades, but MRI, particularly multi-parametric MRI, has become an increasingly important complement. In a head-to-head comparison of first-time biopsy patients, MRI-targeted biopsy detected prostate cancer in 55.4% of cases compared to 41.4% for standard TRUS-guided biopsy. The gap was especially wide at lower PSA levels: among men with a PSA between 2.5 and 4, MRI-targeted biopsy found cancer nearly twice as often (56.6% vs. 29.5%).
This doesn’t mean TRUS is obsolete. Many centers now combine both technologies through a technique called MRI-TRUS fusion biopsy. Here, a patient first gets an MRI, and a radiologist marks any suspicious areas. During the subsequent biopsy, software overlays those MRI targets onto the live TRUS image, creating a three-dimensional model that lets the doctor aim the needle at specific lesions rather than sampling the gland randomly. The entire fusion procedure can be performed in an outpatient clinic under local anesthesia in a matter of minutes, using the same familiar ultrasound equipment. This approach is expected to reduce the total number of needle passes needed while catching more clinically significant cancers.
Risks and Complications
A TRUS used purely for imaging carries minimal risk. The probe doesn’t use radiation, and aside from temporary discomfort, side effects are rare.
When biopsy is involved, the risk profile changes. Infection rates after TRUS-guided biopsy range from 0.3% to 3.2% in most published data, though some estimates run as high as 5% to 7%. These infections can range from a urinary tract infection to, less commonly, bacteria entering the bloodstream. In one institutional review tracking outcomes since 2017, about 1% of procedures resulted in a urinary tract infection, 0.6% caused a bloodstream infection, and another 0.6% triggered a septic response without bacteria detected in blood cultures. Antibiotic prophylaxis before the procedure is specifically designed to minimize these risks.
Minor bleeding is common after biopsy and usually resolves on its own. Blood in the urine, stool, or semen can persist for days to weeks but is rarely dangerous.