CT Scan vs Ultrasound

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Ultrasound and CT Scan (Computed Tomography) are two of the most widely used medical imaging techniques. The techniques use different principles to generate an image for diagnostic purposes.

Comparison chart

CT Scan versus Ultrasound comparison chart
	CT Scan	Ultrasound
Radiation exposure	The effective radiation dose from CT ranges from 2 to 10 mSv, which is about the same as the average person receives from background radiation in 3 to 5 years. Usually, CT is not recommended for pregnant women or children unless absolutely necessary.	No radiation
Cost	CT Scan costs range from $1,200 to $3,200; they usually cost less than MRIs (about half the price of MRI).	Ultrasound procedures cost $100-$1,000.
Time taken for complete scan	Usually completed within 5 minutes. Actual scan time usually less than 30 seconds. Therefore, CT is less sensitive to patient movement than MRI.	Ultrasound usually takes about 10-15 minutes.
Ability to change the imaging plane without moving the patient	With capability of MDCT, isotropic imaging is possible. After helical scan with Multiplanar Reformation function, an operator can construct any plane.	Present
Details of bony structures	Provides good details about bony structures	Ultrasounds are usually not used for bony structures. Instead they are used for internal organs of the body.
Principle used for imaging	Uses X-rays for imaging	High Frequency sound waves (ultrasound) are used for imaging
Details of soft tissues	A major advantage of CT is that it is able to image bone, soft tissue and blood vessels all at the same time.	Detailed with advanced technology

Sonogram of a fetus in its mother's womb

How Ultrasounds and CT Scans Work

Different medical imaging principles are applied for ultrasounds and CT scans. As a result, the process of scanning is also different.

The CT scan of a person's torso.

Principles

A CT scan creates a 3D image of an organ or of the interior body structure. It does this by compiling multiple X-ray images created by low-powered rays passing several times over the same body area, from different angles. A computer merges all the images into a final result that enhances clarity and definition.

In an ultrasound, a sound wave is produced in short pulses at the desired frequency and focused at the region of interest in the body. These sound waves are partially reflected back from the body, received by a transducer and sent to the ultrasonic scanner, where they are processed and transformed into a digital image. The formation of the image depends on the time and the strength of the echo, and is displayed on the computer screen for analysis.

CT Scan Procedure

During CT scans, patients are moved through the scanning system. A source of X-rays and an X-ray detector also rotate in synchronicity so that the X-rays that are passed through the region of interest can produce different image slices in axial or helical mode. The multiple images are then computed to create a view of the organ. The prepared CT image can be immediately viewed on a television monitor or recorded for storage and analysis later.

Ultrasound Procedure

During a medical ultrasound, a probe is passed over the region of interest to send sound waves into the area. To minimize air bubbles between the probe and the skin, a jelly is applied to the region first. The patient is sometimes asked to change positions to get a better view of the target area. The images obtained measurements can be viewed or stored for use later.

Cost Comparison

CT Scan costs range from $1200 to $3200 - they are usually more expensive than ultrasonography. The cost of ultrasound depends on the area examined and usually ranges form $100 to $1000. The cost might vary in different countries.

Applications of Ultrasound vs CT Scan

The advantages of CT scans are most evident in screenings for cancer (tumors), injuries, or abnormalities inside the body. The scans can also be combined with other techniques, such as ultrasound or Magnetic Resonance Imaging (MRI), for greater definition and precision.

Ultrasound is used for diagnostic applications, such as visualizing muscles, tendons, internal organs, to determine its size, structures, any lesions or other abnormalities. Obstetric sonography is used to visualize fetuses during pregnancy. Other applications of ultrasound include removing kidney and gall stones, lipectomy and other applications.

Advantages and Risks

What makes CT scans more useful than X-rays is that the CT result offers a higher-contrast image between tissue types. In addition, a CT scan can home in on a specific structure or area within the body, eliminating possible obstructions such as other organs, bones, or tissue. From a single scan, a doctor can look at different angles and planes, increasing diagnostic ability. This level of accuracy can help patients avoid further procedures, such as colonoscopies or exploratory surgery.

However, the use of X-rays in CT scans is often associated with higher risks for cancer, especially lung and colon cancer, and leukemia. Newer CT units use lower doses of X-rays for shorter scan periods to help combat this problem. Another concern is that the contrasting agents used to enhance visibility may have potentially serious long-term effects.

Ultrasound is less expensive and comparatively safer technique than CT scans. It has a variety of applications in the biomedical, industrial and other areas.

The advantages of ultrasonography far outweigh any risks associated with this technique. However, several studies have highlighted the harmful effects of ultrasound on pregnant mammals such as mice though this effect has not yet been shown in humans. Also, increased exposure to ultrasound waves leads to heating up of tissues, changes in pressure and other mechanical disturbances.

Preventative value

A study financed by the U.S. government showed that current and former heavy smokers can, with annual CT scans, reduce their odds of dying from lung cancer by as much as 20%, as early detection of diseases greatly increases chances of survival.^[1]