ModalityUS
Ultrasound (sonography)
Real-time imaging with high-frequency sound waves and no radiation. A study mixes still images with video clips (cine loops), which are stored as multi-frame DICOM files.
On this page
Ultrasound imaging, also called sonography, uses high-frequency sound waves to show the inside of the body in real time. It can show moving structures, such as the beating heart and flowing blood, and it uses no ionizing radiation. Ultrasound of the heart is called echocardiography. A study usually includes still images and short video clips called cine loops.
Key facts
| Fact | Details |
|---|---|
| DICOM modality code | US |
| Typical number of images | From about 10 to more than 100 still images and loops |
| Typical study size | A few MB for still images; several hundred MB or more for studies with many loops |
| Exam duration | Usually 15 to 60 minutes |
| Radiation | None |
| Contrast agent | Usually none; microbubble contrast agents are used in some exams |
How it works
A sonographer or doctor moves a handheld probe, called a transducer, over the skin. A gel between the probe and the skin removes air, which blocks sound. The probe sends short pulses of sound at frequencies far above human hearing, in the megahertz range. The sound reflects from boundaries between tissues, and the probe receives the echoes. The machine calculates the depth of each boundary from the time each echo takes to return, and it updates the image many times per second.
The basic grayscale image is called B-mode. Doppler ultrasound measures blood flow: color Doppler shows its direction and speed as colors over the image, and spectral Doppler shows the speed as a graph. Some probes are used inside the body, for example in the vagina, the rectum or the esophagus, to get closer to an organ. Bone and air stop sound, so ultrasound cannot look inside bones or through air-filled lungs and bowel.
Uses
Common uses include:
- pregnancy, to check the growth and position of the fetus
- the liver, gallbladder, kidneys, bladder and other abdominal organs
- the pelvic organs, thyroid, breasts and testicles
- the heart (echocardiography) and blood vessels, for example to look for blood clots in leg veins
- muscles, tendons and joints
- guiding needles for biopsies and injections
Ultrasound machines can be moved to the bedside and are often used in emergencies. The quality of an exam depends strongly on the skill of the person who performs it.
Preparation and what to expect
Preparation depends on the exam. For the gallbladder and upper abdomen, you may be asked not to eat for several hours. For some pelvic exams, you need a full bladder, so you drink water beforehand. Many exams need no preparation.
You lie on a table, and the sonographer spreads gel on the skin and presses the probe against it. The gel can feel cold, and the pressure can be uncomfortable over a sore area. You may be asked to hold your breath or to turn. An internal exam uses a covered probe. After the exam you wipe off the gel and can return to normal activities. In some clinics the sonographer does not discuss the results, and a doctor reports them later.
Safety
Ultrasound uses no ionizing radiation, and standard diagnostic ultrasound has no known harmful effects on humans. However, sound waves carry energy. They can heat tissue slightly and in some cases create tiny gas bubbles (cavitation). The machine usually shows two safety numbers on the screen: the thermal index (TI) and the mechanical index (MI). Operators follow the ALARA principle and use the lowest output and the shortest time needed, especially in pregnancy. For this reason, the FDA discourages ultrasound done only to make keepsake images or videos of a fetus. Microbubble contrast agents rarely cause allergic reactions.
What an ultrasound study looks like in DICOM
Ultrasound images have the modality US. A still image uses the SOP class Ultrasound Image Storage (1.2.840.10008.5.1.4.1.1.6.1). A cine loop is one Ultrasound Multi-frame Image Storage file (1.2.840.10008.5.1.4.1.1.3.1). In it, Number of Frames (0028,0008) gives the number of frames, and Frame Time (0018,1063) gives the time between frames in milliseconds. 3D data can use Enhanced US Volume Storage (1.2.840.10008.5.1.4.1.1.6.2), but many machines save 3D data in vendor-specific forms or only as rendered pictures.
An ultrasound image is usually a copy of what the machine shows on its screen, often 640x480 to 1024x768 pixels or more, with 8 bits per sample. Even grayscale images are often saved in color. Color is stored as RGB, or as YBR_FULL_422 when the frames are JPEG-compressed. Lossy JPEG compression is common, and long loops can be stored as compressed video, such as H.264, inside the DICOM file.
The patient's name, the date, the hospital name, measurements and labels are often burned into the pixels. The attribute Burned In Annotation (0028,0301) should say so, but it is not always reliable. Removing tags does not remove this text, so de-identification of ultrasound images also needs masking or cropping.
Measurements depend on Sequence of Ultrasound Regions (0018,6011). It describes each calibrated area of the screen, such as the image and a Doppler graph, and gives the physical size of a pixel in Physical Delta X (0018,602C) and Physical Delta Y (0018,602E). Some machines also record Mechanical Index (0018,5022) and Soft Tissue Thermal Index (0018,5027). Measurements, such as fetal size, are often sent as a separate structured report with the modality SR.
How to view the images
Use a viewer that can play multi-frame files, such as Weasis (free, open source), Horos (free, open source, macOS), OHIF Viewer (free, open source, web browser) or MicroDicom (free for non-commercial use, Windows). Most viewers let you change the playback speed and step through single frames. Distance measurements are correct only inside calibrated regions.
A loop can be exported to a video file, but the export loses the DICOM data and may lose quality (see converting DICOM). See also how to open DICOM files.