Prostate Biopsy
Grayscale Trans-Rectal Ultrasound (TRUS)
- Probe frequency
- Increasing the frequency increases the resolution and decreases the depth of penetration
- Decreasing the frequency increases the depth of penetration and decreases the resolution
- The commonly used 7-MHz transducer produces a high-resolution image with a focal range from 1-4 cm from the transducer (best for peripheral zone where most cancers arise)
- Lower frequency transducers (e.g., older 4-MHz transducers) have a focal range from 2-8 cm but at lower resolution
- General urological uses of a TRUS (5):
- Assess prostate volume
- Locate focal prostate abnormalities
- Assess for obstructive cause of infertility (dilated seminal vesicles secondary to ejaculatory duct obstruction)
- Guide prostate biopsies
- Guide placement of brachytherapy seeds
TRUS Anatomy of the Prostate
- Traditionally described based on a pathologic zonal architecture:
- Anterior fibromuscular stroma (FS), which is devoid of glandular tissue
- Transition zone (TZ)
- Central zone (CZ)
- Periurethral zone
- Peripheral zone (PZ)
- CZ and PZ cannot be distinguished from each other on US, and are often collectively referred to as the PZ on TRUS
- The TZ is generally hyperechoic compared to the CZ and PZ
- Click here for image
- See video on GU Sonography of the Urinary Bladder, Scrotum & Prostate
- Calcifications along the surgical capsule, known as the corpora amylacea, highlight the plane between the PZ and TZ (multiple diffuse calcifications are a normal, often incidental finding and represents a result of age rather than a pathologic entity)
- Estimating Prostate Volume
- Most formulas assume that the gland conforms to an ideal geometric shape: either an ellipse (π/6 × transverse diameter × AP diameter × longitudinal diameter), sphere (π/6 × transverse diameter3), or a prolate (eggshaped) spheroid (π/6 × transverse diameter2 × AP diameter).
- All formulas reliably estimate gland volume and weight, with correlation coefficients > 0.90 with radical prostatectomy specimen weights, because 1 cm3 = approximately 1 g of prostate tissue.
- Planimetry is the most accurate means of volume measurement by US
- Planimetry allows for variation in shape as the area is calculated in consecutive ultrasonographic cross-sections. The area is multiplied with the distance between the cross-sections and the total volume is determined by summation of all contributions.§
- Most formulas assume that the gland conforms to an ideal geometric shape: either an ellipse (π/6 × transverse diameter × AP diameter × longitudinal diameter), sphere (π/6 × transverse diameter3), or a prolate (eggshaped) spheroid (π/6 × transverse diameter2 × AP diameter).