What is the Difference Between Ultrasound and MRI?

Medical magnetic resonance equipment is a kind of medical imaging equipment that uses the characteristics of atomic spin motion to generate a signal after being excited by radio frequency pulses in an external magnetic field. It is detected by a detector and input to a computer, which is processed and converted to display an image on the screen.

Medical magnetic resonance equipment is a kind of medical imaging equipment that uses the characteristics of atomic spin motion to generate a signal after being excited by radio frequency pulses in an external magnetic field. It is detected by a detector and input to a computer, which is processed and converted to display an image on the screen.

Chinese name

Medical magnetic resonance equipment

Foreign name

MRI

Management category

Class III medical devices

Category Name

Medical magnetic resonance imaging equipment (MRI)

Development history of medical magnetic resonance equipment

In the 1930s, physicist Isidore Rabbi discovered that the nucleus in the magnetic field would be arranged in parallel in the forward or reverse order along the direction of the magnetic field. After the application of radio waves, the spin direction of the nucleus reversed. This is the earliest understanding of the interaction between the atomic nucleus and the magnetic field and the external radio frequency field.

In 1946, Purcell of Harvard University and Bloch of Stanford University discovered that when an atomic nucleus with an odd number of nucleus is placed in a magnetic field and a radio frequency field with a specific frequency is applied, the phenomenon that the atomic nucleus absorbs the radio frequency field energy will occur. This is the original understanding of the phenomenon of nuclear magnetic resonance.

In the 1970s, scientist Robert Lochl and his colleagues began the initial magnetic resonance research in the central laboratory in the Netherlands, and obtained the famous nuclear magnetic resonance image "Nottingham Orange"; in the 1980s, the laboratory received the first Human head magnetic resonance image and the world's first two-dimensional Fourier transform image. Therefore, the application of pulsed Fourier transform nuclear magnetic resonance is more extensive, especially in the diagnosis of many diseases with potential advantages.

Since the 21st century, with the improvement of science and technology, various key components of nuclear magnetic resonance have been developed worldwide, and various high-performance diagnostic and therapeutic equipment such as magnetic resonance imaging systems (MRI) have been developed. The rapid development of medical magnetic resonance equipment.

Working principle of medical magnetic resonance equipment

A radio frequency pulse of a specific frequency is used to excite hydrogen protons and absorb a certain amount of energy to cause resonance, that is, a magnetic resonance phenomenon occurs. When the radio frequency pulse is stopped, the excited hydrogen nucleus gradually releases the absorbed energy, and its phase and energy level are restored to the state before the excitation. This recovery process is called the relaxation process, and the time required to return to the original equilibrium state is called the relaxation time.

There are two kinds of relaxation time. One is the spin-lattice relaxation time, which reflects the time required for the spin nucleus to transfer the absorbed energy to the surrounding lattice. It is also a 90 ° RF pulsed proton from longitudinal magnetization to lateral magnetization The time required to return to the state before excitation of longitudinal magnetization is called T1. The other is the spin-spin relaxation time, which reflects the process of transverse magnetization decay and loss, that is, the time that transverse magnetization is maintained, called T2. The T1 of normal tissues and pathological tissues of different human organs is relatively fixed, and there are certain differences between them, as is T2. This difference in relaxation time between tissues is the basis of MRI imaging.

Magnetic resonance imaging mainly includes three aspects: 1) excitation of radio frequency (RF) pulse sequences that generate magnetic resonance phenomena and measuring magnetic resonance signals; 2) spatial coding for determining signal positions; 3) measuring the measured magnetic resonance signals and their positions The information is reconstructed into a magnetic resonance image.

Indications for medical magnetic resonance equipment examination

1. Diagnosis of early ischemic lesions in the brain, intracranial hemorrhage and head fractures, and other lesions such as tumors, inflammation, vascular lesions, infections, etc.

2. Examination of lesions in the intracranial transitional area without artifacts, avoiding CT examination of skull base lesions due to the influence of bones.

3. Neck lesions can clearly show the throat, throat, thyroid, lymph nodes, blood vessels and muscles.

4. Due to the "empty effect" of the blood vessels in the mediastinum and the high signal of fat, the chest has special diagnostic value for hilar lymph nodes and space occupying lesions.

5. Applying gated technology can make accurate diagnosis of myocardial, pericardial and congenital lesions, and quantitative analysis of cardiac function.

6. Examine liver lesions and identify liver cysts, cavernous hemangioma, and liver cancer by T1-weighted and T2-weighted.

7. Examine the kidney and ureter. The fat around the kidney makes a good contrast of the magnetic resonance image, the kidney parenchyma and the urine form a good contrast, and it shows a good effect on ureteral stenosis and obstruction.

8. Examination of pelvic lesions is the best imaging method for vascular and lymphatic, tumor, inflammation, and metastatic tumors in the pelvic cavity.

9. Examination of the joints of the extremities, joint soft tissue, osteomyelitis, tumors in the soft tissue and vascular malformations have a good display effect.

Composition of magnetic resonance imaging equipment for medical magnetic resonance equipment

Magnetic resonance imaging equipment consists of magnets and magnet power sources that generate magnetic fields, gradient field coils and gradient field power sources, radio frequency transmitters / receivers, system control and data processing computers, imaging operation and image analysis workbenches, and movable examination beds.

Comparison of Medical Magnetic Resonance Equipment

The common examination equipment in hospitals include magnetic resonance imaging equipment, CT machines, X-ray examinations of CR and DR, and ultrasound examination equipment. The main differences are shown in Table 1.

Table 1 Comparison of commonly used equipment in hospitals

Common equipment	Technical principle	Instrument use characteristics
MRI	Nuclear magnetic resonance	The images of cross section, sagittal plane and coronal plane can be obtained with good spatial resolution.
CT	X-ray	Cross-section images can be obtained with high density resolution.
CR and DR	X-ray	CR is an IP board and DR is completed by PACS. The image acquisition is higher than that of a CT machine, but the diagnostic range is not significantly different.
DSA	X-ray	Used during interventional surgery, angiography, surgical silhouettes, and images show the direction of blood vessels to facilitate interventional surgery.
B-mode ultrasound	Ultrasound	Widely used in the diagnosis of cardiology, digestive medicine, urology and obstetrics and gynecology.

Medical magnetic resonance equipment classification

The magnet that generates the magnetic field is the core of the MR imaging system. According to the nature of the magnet, medical magnetic resonance imaging equipment is divided into three types: permanent magnetic resonance imaging system, normally guided magnetic resonance imaging system, and superconducting magnetic resonance imaging system.

The following compares the manufacturing process, magnetic field characteristics, energy and other consumption, and price, and introduces the advantages and disadvantages of the three types of magnets, as shown in Table 2.

Table 2 Comparison of three magnets

Magnet type	advantage	Disadvantage
Normally conducting magnet	Easy to manufacture, low price, magnetic field can be turned off	Limited magnetic field uniformity, limited field strength, large power consumption, high stability power supply required, restricted import and export of magnetic field
Superconducting magnet	Field strength up to 4T, high magnetic field uniformity, good transient stability	High price, low temperature environment required, large fringe field range, possibility of overshoot, and restricted import and export
Permanent magnet	Do not use magnetic field power supply, do not consume electric power, limited fringe field, transverse magnetic field, low price, convenient import and export	Limited magnetic field uniformity, limited field strength, sensitive to ambient temperature, heavy weight

Advantages and disadvantages of medical magnetic resonance equipment

Advantages: non-ray imaging, non-invasive, harmless; can create axial, sagittal, coronal or any angle image without patient movement; no need to inject contrast during cardiovascular and cerebrospinal imaging , Safe, painless, and can be used for functional analysis.

Disadvantages: expensive, time-consuming, and difficult to meet a wide range of applications; spatial resolution is not as good as CT; not suitable for some critically ill patients; patients with cardiac pacemakers can not be applied, so as not to cause the pacemaker to malfunction, causing life threatening.

Precautions for use of medical magnetic resonance equipment

1. The patient must remove all metal objects and preferably change clothes.

2. During the scanning process, the patient's body should not directly touch the inner wall of the magnet and various wires to prevent the patient from being burned.

3. Tattoos, cosmetics, hair dyes, etc. should be removed beforehand as they may cause burns.

4. Patients should wear earplugs to prevent hearing damage.

5. It is strictly forbidden to scan with a cardiac pacemaker and nerve stimulator in the body.

6. If metal foreign bodies (dentures, contraceptive rings, metal implants, post-operative metal clips, etc.) in the body are within the scanning range, scan carefully to prevent metal objects from moving or generating heat, which may cause patient damage. Artifacts can interfere with diagnosis.

7. There is an aneurysm clip in the body, and those with metal foreign bodies in the eyeballs should not be scanned.

8. Pregnant women and babies should get a doctor's consent for another scan.

Further reading of medical magnetic resonance equipment

[1] Wu Jie, Yuan Hangying, Yan Jun, et al. Risk Factor Analysis and Management of Medical MRI Imaging Equipment [J]. Chinese Journal of Medical Physics, 2014, 31 (03): 4918-4919 + 4928.

[2] Shao Dongning. Problems in the operation of low magnetic field medical diagnostic magnetic resonance equipment [J]. Chinese Journal of Radiological Medicine and Protection, 2003 (06): 63.