Difference Between MRI and Ultra-Sound

The main difference between MRI and ultra-sound is that MRI provides detailed anatomical information and is suitable for various applications but requires more time and resources, while ultra-sound is quick, portable, and relatively less expensive, making it a widely used imaging method in many clinical settings.

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 medical imaging techniques: the ways of making the interior pictures/images of the human body and illustrating them on paper or a computer screen to be observed by the clinician. After observing these images, clinicians 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.

Key Areas Covered

1. What is MRI 
      – Definition, Features, How It Works
2. What is Ultra-Sound
      – Definition, Features, How It Works
3. Similarities – MRI and Ultra-Sound
      – Outline of Common Features
4. Difference Between MRI and Ultra-Sound
      – Comparison of Key Differences

Key Terms

MRI, Scan, Ultra-sound

What is MRI? 

The word MRI stands for Magnetic Resonance Imaging. As its name suggests, it deals with the 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 a 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 briefly, it can be explained as follows.

How MRI Works

There are four major parts in an 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 in 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 spatially. This is the main cause of the sound of an 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 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.

What is Ultra-Sound?

Sound is mechanical energy, and ultra-sound is a type of sound which cannot be heard by the human ear. The 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 normal sound waves, and it is used to create images of the human body with its wave properties.

How Ultra-Sound Works

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.

Types of Ultra-Sound Scans

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 scans 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.

Similarities Between MRI and Ultra-Sound Scan

• Both of the methods do not involve radioactivity, and hence these two methods are named harmless scanning methods when compared 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 both be used for the same purpose on 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

Definition

MRI is a medical imaging technique that uses a strong magnetic field and radio waves to generate detailed images of the internal structures of the body, whereas ultra-sound is a medical imaging technique that utilizes high-frequency sound waves to create real-time images of the body’s internal structures.

Time Taken

Ultra-Sound generally requires less time to obtain an image compared to an MRI scan.

Image Resolution

MRI offers higher image resolution and detailed anatomical visualization compared to ultrasound.

Plane

MRI machines can produce images in any plane, allowing for multi-directional imaging. In contrast, ultrasound images are typically obtained in a single plane.

Reliability

An MRI is more reliable, and ultra-sound is more dependent on the operator.

Portability 

Ultra-Sound machines are portable and can be easily moved, whereas MRI machines are fixed and immobile.

Complexity 

MRI scans are more complex and expensive compared to ultrasound scans, which are generally more cost-effective.

Conclusion

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 are also some hazards and advantages to both methods. The main issue with the MRI is that when facing an MRI process, the patient has to follow many precautions to avoid damage 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 an MRI scan image. Patients with a pacemaker cannot ever face an MRI scan, and young children and infants usually require a general anaesthetic.

Image Courtesy:

1. “Siemens Magnetom Aera MRI scanner” By Ptrump16 – Own work (CC BY-SA 4.0)  via Commons Wikimedia 
2. “Carotid ultrasound” By National Heart Lung and Blood Insitute (NIH) – National Heart Lung and Blood Insitute (NIH), Public Domain) via Commons Wikimedia