Difference Between MRI and Ultra-Sound

Did you ever come across a medical scan in your life? If so or not, you may have heard the words MRI scan and Ultra-Sound scan. What are these methods? What is the difference between MRI and Ultra Sound? Though you have heard about the words you might be seeking the answers to these questions. Simply, we can identify them as the medical imaging techniques: the ways of making the interior pictures/images of the human body and illustrate them on paper or a computer screen to be observed by the clinician. After observing these images, clinician can make important decisions on diagnosis, which stand for the well-being of the patient. Let’s see what MRI and Ultra Sound are and how they engage in medical imaging.

What is MRI? How MRI Works?

The word MRI stands for Magnetic Resonance Imaging. As its name, it deals with magnetization procedure in order to create an image. Three types of MRI machines can be identified according to the position of the patient when taking the image. Those are Closed MRI, Open MRI and Standing or Sitting MRI. The most popular, widely used one is closed one and the newest one is a standing one. The machine is used to generate images of the soft tissues, bones and joints. From the first MRI machine in 1977 to the present, the functionalities have been changed slightly but the operating principle of the MRI remains the same and, in briefly, it can be explained as follows.

There are four major parts in a MRI machine: Primary Magnet, Gradient Magnet, RF (Radio Frequency) coil and the computer. The human body contains 70% of water (H2O); that means it consists of a large number of hydrogen atoms. These atoms are spinning randomly in various planes and various directions. Under the influence of the magnetic field (around 3 Tesla) of the primary magnet, these hydrogen atoms arrange themselves in parallel (majority of the atoms) and anti-parallel (minority) directions and spin around the axis of the magnetic field. After the primary magnet, the gradient magnets are activated, and they produce three magnetic fields that are perpendicular to each other in spatially. This is the main cause of the sound of a MRI machine. By changing the magnetic field generated by the gradient magnets we can change the precession of H atoms rather than along the axis of the primary magnetic field. When the gradient magnetic field is removed, these atoms return back to the original position along the primary magnetic field and thus release energy. This is called as Relaxation. The energy released is in the form of an electrical signal and it is detected by the RF coil. The computer that is directly connected to the RF coil identifies these signals and these analog signals are converted into digital signals by an A/D (Analog to Digital) converter. Next the signals are stored for a short time period for data acquisition. Once that is completed, these signals (data) are transformed by a method called Fourier Transformation and the final image is created.

Difference Between MRI and Ultra-Sound | Pediaa.com

What is Ultra-Sound? How Ultra-Sound Works?

Sound is a mechanical energy and ultra-sound is a type of sound which cannot be heard by the human ear. Human ear cannot hear above 20 KHz , but the industrially used ultra-sound is at a range of MHz to GHz. Ultra-sound has the properties of the normal sound waves and it is used to create images of the human body with its wave properties. The operating theory of ultra-sound is not complex as that of MRI. It can be simply explained as follows. 

The transducer / probe emits ultra-sound wave. When this sound wave hits a target, many changes happen to the wave such as reflection, refraction and scattering. These mutations are sensed by the same transducer. For example, the depth of an organ can be identified by the time difference between the outgoing and incoming pulses of the wave. Acoustic shadowing is used to identify the rigid parts and acoustic enhancement is used to illustrate the liquids inside the bodies. By increasing the amplitude of the ultra-sound wave, we can observe the deeper structures of the human body as reflected wave also has significant amplitude. The clinician can also observe the interior body movements as a video as reflected waves come in sequentially.

In medical imaging, we can identify three main types of ultra-sound scans. They are external, internal, and endoscopic scans. In the external scan method, a lubricant gel is applied on the skin to move the transducer smoothly on the skin and to ensure the touch between the transducer and the skin. External ultra-sound scan can be used to examine the heart, liver, kidneys, and thyroid. The internal ultra-sound scan is done by placing the transducer on the vagina or the rectum. It is mainly used to check the pregnancy status and the womb. The endoscopic scan is done through a tube entered into the body through the mouth where the ultra sound transducer is placed on the tip of the tube. This method can be used to look into the abdomen.

Comparison of MRI and Ultra-Sound Scan

The main similarity of these two methods is that both of the methods do not involve with radioactive and hence these two methods are named as harmless scanning methods when comparing to x-rays and CT scanning. Either of the method can be used to produce both still and moving pictures of the inside body. The ultra-sound and MRI can be both used for the same purpose in many occasions, but the ultra-sound scan is widely used as it is not expensive as the MRI scan.

Difference between MRI and Ultra-Sound

  • It takes minimum time to take an image using an ultra-sound than an MRI.
  • The image resolution is higher in MRI than that of ultra-sound.
  • The MRI machine, unlike the ultra sound, can produce images in any plane.
  • The MRI is more reliable, and ultra-sound is more dependent of the operator.
  • The ultra-sound machine is pretty much portable in these days, but MRI is a fixed one.
  • MRI is more cumbersome and expensive than the ultra-sound scan.


In the innovative modern world, there is a huge range of medical imaging methods from x-rays to PETS. Among them MRI and Ultra-Sound methods are still widely used even though they are not the newest methods. We saw that there are similarities as well as differences between MRI and Ultra-Sound. There also some hazards and advantages in both methods. The main issue with the MRI is that when facing a MRI process the patient has to follow many precautions to avoid damages to himself/herself as well as the machine. A patient must have to remove all of his/her metallic parts from the body before the scan. Even a tattoo might be a considerable thing to get a MRI scan image. The patients with a pacemaker cannot ever face a MRI scan and young children and infants usually require a general anesthetic.

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