The prostate is a walnut-shaped gland surrounding the beginning of the urethra in front of the rectum and below the bladder. The prostate can become enlarged (particularly in men over age 50) and develop diseases like prostate cancer or inflammation (prostatitis). A large tumor can be felt by a rectal examination. The most effective way of detecting the early signs of prostate cancer is a combination of a prostate-specific antigen (PSA) blood test and a prostate ultrasound examination.
An abnormally high level of PSA can indicate prostate cancer or other prostate diseases such as benign prostatic hypertrophy or prostatitis. The transrectal sonography is an important diagnostic ultrasound procedure in determining whether there is any benign enlargement of the prostate or any abnormal nodules.
The imaging is performed with a rectal probe, yielding high resolution. High resolution 3D ultrasound provides reliable and accurate determination of the size and the location of cancer. Additionally, ultrasound elastography is a technique in development to improve the specificity and sensitivity of cancer detection. Ultrasound is also used to detect whether cancerous tissue is still only within the prostate or whether it has begun to spread out and to guide a diagnostic biopsy or ultrasound therapy.

See also Brachytherapy, and High Intensity Focused Ultrasound.

(PII) Pulse inversion imaging (also called phase inversion imaging) is a non-linear imaging method specifically made for enhanced detection of microbubble ultrasound contrast agents. In PII, two pulses are sent in rapid succession into the tissue; the second pulse is a mirror image of the first. The resulting echoes are added at reception. Linear scattering of the two pulses will give two echoes which are inverted copies of each other, and these echoes will therefore cancel out when added.
Linear scattering dominates in tissues. Echoes from linear scatterers such as tissue cancel, whereas those from gas microbubbles do not. Non-linear scattering of the two pulses will give two echoes which do not cancel out completely due to different bubble response to positive and negative pressures of equal magnitude. The harmonic components add, and the signal intensity difference between non-linear and linear scatterers is therefore increased. The resulting images show high sensitivity to bubbles at the resolution of a conventional image.
In harmonic imaging, the frequency range of the transmitted pulse and the received signal should not overlap, but this restriction is less in pulse inversion imaging since the transmit frequencies are not filtered out, but rather subtracted. Broader transmit and receive bandwidths are therefore allowed, giving shorter pulses and improved axial resolution, hence the alternative term wideband harmonic imaging. Many ultrasound machines offer some form of pulse inversion imaging.

See also Pulse Inversion Doppler, Narrow Bandwidth, Dead Zone, Ultrasound Phantom.

Pulsed ultrasounds are cycles of ultrasound separated in time with gaps of no signal. Pulsed sound waves are generated by short, strong pulses of sound from a phased array of piezoelectric crystals. The transducer, though emitting ultrasound in rapid pulses, acts as a receiver most of the time. In sonography, pulsed ultrasound is used to perform diagnostic or therapeutic procedures.

See also Pulse Average Intensity, Release Burst Imaging.

Rayleigh scattering is the backscattering of ultrasound from blood. The echoes detected from blood are created through interference between scattered wavelets from numerous point scatterers. Rayleigh Scatterers are objects whose dimensions are much less than the ultrasound wavelength. Rayleigh scattering increases with frequency raised to the 4th power and provides much of the diagnostic information from ultrasound. Doubling the ultrasonic frequency makes the echoes from blood 16 times as strong. The intensity of the backscattered echoes is proportional to the total number of scatterers, which means that the echo amplitude is proportional to the square root of the total number of scatterers.
At normal blood flow, the number of point scatterers in blood is proportional to the number of red blood cells. When blood flow is turbulent, or accelerating fast (e.g. in a stenosis), the number of inhomogeneities in the red blood cell concentration will increase.

See also Scattered Echo.

Diagnostic ultrasound imaging has no known risks or long-term side effects. Discomfort to the patient is very rare if the sonogram is accurately performed by using appropriate frequencies and intensity ranges. However, the application of the ALARA principle is always recommended.
There are reports of low birth weight of babies after applying more than the recommended ultrasound examinations during pregnancy. Women who think they might be pregnant should raise this issue with the doctor before undergoing an abdominal ultrasound, to avoid any harm to the fetus in the early stages of development.
Since ultrasound is energy, sensitive tissues like the reproductive organs could possibly sustain damage if vibrated to a high degree by too intense ultrasound waves. In diagnostic ultrasonic procedures, such damage would only result from improper use of the equipment.

Possible ultrasound bioeffects:
The thermal effect is controlled by the displayed thermal index and the mechanical index indicates the risk of cavitation.
An ultrasound gel is applied to obtain better contact between the transducer and the skin. This has the consistency of thick mineral oil and is not associated with skin irritation or allergy.
Specific conditions for which ultrasound may be selected as a treatment may be attached with higher risks.

See also Ultrasound Imaging Procedures, Fetal Ultrasound and Obstetric and Gynecologic Ultrasound.