Unlike regular sound, ultrasound can be directed into a single direction. The echoes received by a stationary probe will result in a single dimensional signal showing peaks for every major material change.
To generate a 2D picture, the probe is swiveled, either mechanically or through a phased array of ultrasound transducers. The data is analyzed by computer and used to construct the image. In a similar way, 3D pictures can be generated by computer using a specialized probe. In this way, a photo of an unborn baby may be made.
Some ultrasonography machines can produce color pictures, of sorts. Doppler ultrasonography is color coded onto a gray scale picture. From the amount of energy in each echo, the difference in acoustic impedance can be calculated and a color is then assigned accordingly.

See also Densitometry and 3D Ultrasound.

Ultrasound therapy uses high energy sound waves to treat different diseases. Historically, the use of ultrasonic waves in therapy began before the wide use as a diagnostic medical imaging tool. Dependend on the intensity, ultrasound therapy reach from the thermal effect used in physical therapy to the destruction of tissue with lithotripsy.

Types of ultrasound treatment:
See also Thermal Index, History of Ultrasound, Interventional Ultrasound, and B-Mode Acquisition and Targeting.

Handheld ultrasound systems are portable devices for smartphone or tablet and are increasingly common in emergency, intensive care and veterinary medicine, but also in the pocket of the stationary doctor. This type of ultrasound machine enables immediate diagnoses directly on site (handheld point-of-care-ultrasound / HPOCUS) and quickly provide information regarding the patient's further care. Handheld ultrasound machines fit into a single-use plastic cover and can be easily disinfected, making them particularly useful in infectious environments.
The most striking advantage of handheld POCUS devices is the small footprint. The design is very compact, lightweight (approx. 200 g/0,44 lbs. - 500 g/1,1 lbs) and flexible. Due to this compactness and the necessary technical compression, the quality of the imaging is still limited compared to 'high-end devices', but sufficient to the extent that handheld ultrasound devices are already successfully used in many medical disciplines.
Depending on the model, handheld ultrasound systems run on Android, iOS, Windows or proprietary operating systems. They are connected to the end device via USB cable or wirelessly via Bluetooth or WiFi. The respective end device is used as an ultrasound monitor to display the ultrasound images. The associated app is operated via touchscreen, although some devices have a few buttons, e.g. for recording ultrasound images or freeze images. The images can be stored and managed on the end device itself, the inserted memory card or in the cloud. Theoretically, also a private smartphone can be connected, but this can lead to complications with reimbursement.
See also Portable Ultrasound Machine, Ultrasound Technology, Environmental Protection, Ultrasound Accessories and Supplies and Sonographer.

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.

An ultrasound couplant is a material that propagates acoustical waves. The couplant eliminates air from the interface and adapts the contours of the transducer to the skin. The used material is an ultrasound gel that mimics the acoustic characteristics of biological tissue.