How Is an Ultrasound of the Prostate Performed?

A prostate ultrasound is most commonly performed by inserting a thin, lubricated probe into the rectum, a technique known as transrectal ultrasound, or TRUS. The probe sits just millimeters from the prostate gland, which allows it to produce detailed images of the organ’s size, shape, and internal structure. First developed in the 1970s, TRUS remains the most widely used method for evaluating the prostate and guiding needle biopsies when cancer is suspected.¹ There are also transperineal and transabdominal approaches, each with different trade-offs in image quality, comfort, and infection risk.

Before the Procedure

Preparation varies by clinic, but a few steps are standard. Most urologists ask you to use a rectal enema beforehand to clear the lower bowel, which improves the ultrasound image and reduces infection risk if a biopsy is planned. A survey of urologists found that about four in five prescribe an enema, and a similar proportion prescribe an oral antibiotic (typically a fluoroquinolone) before biopsy.² Some centers have also started adding a povidone-iodine rectal cleansing step, which in one randomized trial eliminated positive urine cultures after biopsy compared with standard enema alone.³

If you take blood thinners, your doctor will usually have you stop them several days in advance to reduce bleeding. You may also be told to avoid aspirin. For a diagnostic ultrasound without biopsy, preparation is simpler: you may only need a full bladder or a brief enema. Either way, the visit is typically an outpatient affair, and you can usually drive yourself home afterward.

What Happens During a Transrectal Ultrasound

You will be asked to lie on your side, usually the left side, with your knees drawn toward your chest. The ultrasound probe, roughly the diameter of a finger, is covered with a protective sheath and coated with lubricating gel. The clinician gently inserts it a few inches into the rectum. Because the prostate sits directly in front of the rectal wall, this position gives the probe an unobstructed, close-range view.

Once the probe is in place, it sends out high-frequency sound waves that bounce off the prostate tissue and return to the probe, creating a real-time image on a monitor. The clinician systematically sweeps the probe to capture views of the entire gland, including the peripheral zone (where most cancers arise), the transition zone (where benign enlargement occurs), and the surrounding structures like the seminal vesicles. The imaging portion typically takes five to ten minutes on its own. If a biopsy is also being performed, the session runs longer.

The probe can also measure the prostate’s dimensions. Using the three measured axes, the volume is calculated with a mathematical formula. The most commonly used approach is called the ellipsoid formula, which multiplies the three dimensions by a standard coefficient.⁴ Research has shown that this formula tends to underestimate the actual prostate volume by roughly 18%, partly because ultrasound overestimates the gland’s height and length by about 10 to 11% while getting the width about right.⁵ For most clinical decisions, this level of accuracy is sufficient, but the measurement error is worth knowing about if your treatment plan depends on a precise size threshold.

What the Images Show

On a standard grayscale ultrasound, the prostate appears as a rounded structure with different shades of gray. Normal prostate tissue in the peripheral zone has a uniform, medium-brightness appearance. When cancer is present, it most often shows up as a darker area, described as hypoechoic, within that zone.¹ In one prospective study of 151 men, those with a hypoechoic lesion on TRUS had a cancer detection rate of about 69%, compared with a much lower rate in men without such a lesion. Hypoechoic lesions were also linked to higher-grade cancers.⁶

That said, not every dark spot is cancer, and not every cancer produces a dark spot. Many benign conditions, such as inflammation or atrophy, can also look hypoechoic. Conversely, some cancers match the surrounding tissue in brightness and are invisible on standard ultrasound. This overlap between benign and malignant appearances is the main limitation of conventional TRUS imaging, and it is why doctors rarely rely on ultrasound images alone to diagnose cancer. When color Doppler is added, the clinician can also evaluate blood flow patterns. Cancerous lesions tend to have thicker, more twisting blood vessels concentrated in the peripheral zone, while benign enlargement shows thinner, straighter vessels mainly in the transition zone.⁷

The Transperineal Approach

Not every prostate ultrasound goes through the rectum. In a transperineal ultrasound, the probe or needle entry is through the perineum, the area of skin between the scrotum and the anus. This route avoids the rectum entirely, which has major implications for infection risk during biopsies. A large meta-analysis found that the transperineal route lowered the odds of hospitalization for infectious complications by 77% compared with the transrectal route.⁸ The PREVENT randomized trial reported zero infections in the transperineal group versus a 1.6% infection rate in the transrectal group.⁹

Transperineal scanning can use a standard abdominal probe placed against the perineum or a specialized biplane probe operating at 5 to 7 MHz, often followed by comparison with transrectal imaging at 7 MHz.¹⁰ Many centers are now shifting toward the transperineal route for biopsies, particularly for men at higher infection risk. The trade-off is that it may require local anesthesia to the skin of the perineum and, in some settings, sedation, though office-based procedures under local anesthetic have become increasingly common.

Can the Prostate Be Scanned Through the Abdomen?

Yes, and it often is. A transabdominal ultrasound places the probe on the lower belly, above the pubic bone, while your bladder is full. The full bladder acts as an acoustic window that lets the sound waves pass through to the prostate. This is the least invasive option and is commonly used in general urology clinics to get a quick estimate of prostate size.

The catch is accuracy. While transabdominal and transrectal volume measurements correlate well on average, the agreement between the two methods is loose enough to matter clinically. One study found that in about a quarter of patients, the two measurements differed by more than 30%.¹¹ Another study confirmed this lack of agreement using formal statistical comparison, finding that a transabdominal scan could read almost 10 mL higher or nearly 7 mL lower than the transrectal scan in the same patient.¹² For small prostates under about 30 cc, the two methods are close enough to be interchangeable. For larger glands, where size can change what surgery is recommended, transrectal or cross-sectional imaging is the safer choice.¹³

Pain Management and What the Procedure Feels Like

A diagnostic TRUS without biopsy is uncomfortable more than painful. Most men describe a feeling of pressure and fullness, similar to a digital rectal exam but lasting longer. The discomfort usually peaks at insertion and subsides once the probe is positioned.

When a biopsy is added, pain management becomes more important. The needle fires through the rectal wall or perineum into the prostate, and most men feel a sharp sting with each core. The standard pain-relief technique is a periprostatic nerve block, where the clinician injects a local anesthetic around the nerves near the prostate just before taking the tissue samples. A systematic review and network meta-analysis concluded that a periprostatic block, whether used alone or combined with intrarectal analgesia or sedation, meaningfully reduces pain compared with no block or rectal gel alone.¹⁴

Adding oral pain relievers on top of the nerve block can help further, particularly with the discomfort of probe insertion itself. A randomized trial found that men who received a combination of oral analgesics and a periprostatic block reported significantly lower pain scores at the moment of probe insertion and at the moment of the anesthetic injection itself, compared with men who received the nerve block alone.¹⁵ The combination of basal block plus periprostatic nerve block may also be especially helpful for younger patients, who tend to report more discomfort during the procedure.¹⁶

Anxiety also plays a real role in how much pain you perceive. One trial found that patients taught a simple diaphragmatic breathing exercise before the procedure had substantially lower anxiety scores and a greater drop in heart rate from pre- to post-procedure compared with those who received standard care.¹⁷ If your clinic offers any pre-procedure education or relaxation coaching, it is worth taking them up on it.

How Ultrasound Guides a Prostate Biopsy

The most common reason you will encounter a prostate ultrasound in practice is as the guidance system for a biopsy. The real-time images let the clinician see the needle on screen and direct it precisely into the prostate. In a standard systematic biopsy, the doctor takes samples from a grid of locations across the gland, typically eight or more cores per session.² Each core is a thin sliver of tissue shot from a spring-loaded needle, and the whole sampling process usually takes about 10 to 15 minutes.

The limitation of systematic biopsy is that it is essentially sampling blind. The needle follows a pattern rather than targeting a visible lesion, because many cancers do not show up on standard grayscale ultrasound. This is where MRI-ultrasound fusion has changed the game. In a fusion biopsy, a previously obtained MRI scan is digitally overlaid onto the live ultrasound image, letting the clinician steer the needle directly into suspicious areas identified on MRI. Fusion devices use different tracking technologies, from robotic mechanical arms to electromagnetic sensors to 3D ultrasound probes, to keep the two image sets aligned as the probe moves.¹⁸

The results of fusion biopsy have been consistently better than systematic biopsy for finding cancers that matter. A systematic review found that MRI-ultrasound fusion targeted biopsies detected more clinically significant cancers (a median detection rate of about 33% versus 24%) while using far fewer cores (a median of roughly 9 versus 37). About 9% of significant cancers were caught only by the targeted approach and would have been missed by systematic biopsy alone.¹⁹ Work with one fusion device in over 600 men showed that targeted biopsies were two to three times more sensitive for cancer detection than nontargeted systematic biopsies.²⁰

Risks and Common Side Effects

A diagnostic ultrasound without biopsy carries virtually no risk beyond temporary discomfort. When tissue sampling is involved, the picture changes. The most common side effects of a transrectal biopsy are blood in the urine and rectal bleeding. One study recorded hematuria in about 39 to 53% of men on the first day after biopsy, with rectal bleeding in 12 to 26%.²¹ These numbers may sound alarming, but the bleeding is usually minor and resolves within a few days. Blood in the semen can persist for several weeks and is harmless. Other complications such as fever, urinary retention, or significant pain occur in a small minority, roughly 3 to 8% for each individual complication.

Infection is the most serious risk of transrectal biopsy. Because the needle passes through the rectal wall, bacteria can enter the urinary tract or bloodstream. In one randomized trial, 10% of men in the standard-preparation group were hospitalized for febrile urinary tract infection after transrectal biopsy.³ Prophylactic antibiotics reduce this risk substantially, and most clinicians prescribe them as standard. The growing adoption of the transperineal route, discussed earlier, is driven largely by its dramatically lower infection rates.

Reassuringly, the number of cores taken does not appear to significantly increase complication rates overall.²¹ Whether the doctor takes 8 or 12 samples, the likelihood of any given complication is roughly the same.

Advanced Ultrasound Technologies

Standard grayscale ultrasound has well-known limits in distinguishing cancer from benign tissue. Several newer ultrasound-based technologies aim to close that gap without requiring a separate MRI.

Elastography measures tissue stiffness. Because prostate cancer is typically denser and stiffer than the surrounding gland, elastography can highlight suspicious areas in real time during a TRUS exam.²² Shear wave elastography, a more quantitative version, assigns a stiffness value measured in kilopascals to each region. In one study, using a cutoff of 37 kPa to flag suspicious tissue yielded a sensitivity of about 96% and a specificity of about 96%, and doubled the positive biopsy rate compared with conventional ultrasound-guided biopsy alone.²³

Contrast-enhanced ultrasound uses microbubble agents injected into a vein to light up the tiny blood vessels that tumors grow to feed themselves. Because cancers tend to develop their own abnormal blood supply, contrast enhancement can reveal hotspots of blood flow that are invisible on a standard scan.²⁴ Pilot work has shown that quantitative analysis of perfusion patterns obtained this way could help localize cancer-suspect areas and guide treatment decisions.²⁵

High-resolution micro-ultrasound is a more recent development that uses a much higher frequency transducer to produce images with resolution approaching that of MRI. When paired with MRI in fusion biopsy, one study found that micro-ultrasound detected clinically significant cancer at a similar rate to conventional ultrasound, suggesting it adds imaging detail but has not yet proven to catch more cancers in that setting.²⁶ The technology is still being evaluated, and its potential value may lie in reducing the need for a pre-biopsy MRI at all.

How Ultrasound Compares with MRI

You might wonder why ultrasound is still used when MRI seems to dominate the conversation around prostate imaging. The answer is practical. MRI offers superior soft-tissue contrast and is now recommended before biopsy in many guidelines, but it is expensive, takes 30 to 45 minutes inside a scanner, and is not always available quickly. Even with its advantages, MRI still misses a meaningful number of prostate cancers.²⁷

In terms of raw lesion detection, an early multi-institutional comparison found that MRI identified about 60% of tumors larger than 5 mm, while ultrasound detected about 59%, with no statistically significant difference between them.²⁸ Modern multiparametric MRI has since improved considerably, but so has ultrasound with the addition of elastography, contrast enhancement, and micro-ultrasound. In current practice, the two modalities are often used together rather than as competitors: MRI identifies the targets, and ultrasound guides the needle to them in real time.

Managing Anxiety Before and During the Exam

The idea of a rectal probe understandably makes many men nervous. Research into patient experience around prostate biopsy suggests that pre-procedure education meaningfully reduces stress. One study found that a 20-minute one-on-one simulation session, in which a clinician walked the patient through the preparation, the procedure itself, and what to expect afterward, helped patients become familiar with the biopsy environment and process.²⁹ The diaphragmatic breathing technique mentioned earlier produced a large effect on anxiety, roughly four times the reduction in anxiety scores compared with standard care.¹⁷

If you have a procedure scheduled, ask your clinic whether they offer any preparation materials or relaxation guidance. Even without a formal program, practicing slow, deep belly breathing in the waiting room and during the exam can make a noticeable difference. Knowing the timeline also helps: most men are surprised by how quickly it is over. A diagnostic scan takes minutes. Even a full biopsy session, from the time the probe goes in to the time it comes out, typically wraps up within 15 to 20 minutes.

1
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3
PubMed Central. The effect of povidone-iodine-enhanced rectal enema on procedure-related infectious complications before transrectal ultrasound-guided prostate biopsy
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PubMed. The correlation between prostate volume, transition zone volume, transition zone index and clinical and urodynamic investigations in patients with lower urinary tract symptoms
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PubMed Central. Hypoechoic lesions on Transrectal Ultrasound and its correlation to Gleason grade in the diagnosis of Clinically Significant Prostate Cancer
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Eurasian Journal of Medicine and Oncology. Comparison of transrectal ultrasonographic and color Doppler features of benign prostatic hyperplasia and prostate cancer in older men
8
PubMed. Infectious Complications After Transrectal Versus Transperineal Prostate Biopsy: A Systematic Review and Meta-analysis
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PubMed Central. Application of Bland-Altman Method in Comparing Transrectal and Transabdominal Ultrasonography for Estimating Prostate Volume
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PubMed. Comparison of Transabdominal and Transrectal Ultrasound for Sizing of the Prostate
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PubMed Central. Effectiveness of periprostatic block to prevent pain in transrectal prostate biopsy
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PubMed Central. Efficacy and safety of three different analgesic methods for patients undergoing transrectal ultrasound-guided prostate biopsy
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PubMed. Effect of Diaphragmatic Breathing on Procedural Anxiety During Transrectal Prostate Biopsy
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PubMed Central. Target detection: magnetic resonance imaging-ultrasound fusion-guided prostate biopsy
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PubMed. Detection of Clinically Significant Prostate Cancer Using Magnetic Resonance Imaging-Ultrasound Fusion Targeted Biopsy
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PubMed Central. MRI-ultrasound fusion for guidance of targeted prostate biopsy
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PubMed Central. Complication rates after prostate biopsy according to the number of sampled cores
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PubMed Central. Real-time elastography of the prostate
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PubMed Central. Contrast-enhanced ultrasound imaging of prostate cancer
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PubMed. First results of endocavity evaluation of the microvascularization of malignant prostate tumors using contrast enhanced ultrasound (CEUS) including perfusion analysis
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PubMed. Diagnostic performance of high-resolution micro-ultrasound and conventional ultrasound in fusion biopsy for clinically significant prostate cancer detection
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PubMed. Advanced ultrasound in the diagnosis of prostate cancer
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PubMed Central. Effectiveness of stress management in patients undergoing transrectal ultrasound-guided biopsy of the prostate