New search

Magnet resonance imaging

Synonyms:
MRI
Description:

MRI is a technique for visualizing internal structures of the body, particularly the soft tissues in form of sectional images.

The object to be examined is exposed to a strong static magnetic field, e.g. 3 T. The atomic cores (in general hydrogen cores, i.e. protons) take a certain orientation within the magnetic field. Through a radiofrequency pulse of a particular frequency the protons are excited and their previous orientation deranged. This frequency is dependent on both the strength of the static magnetic field and the type of atomic core; it is known as the Larmor frequency. Given a static magnetic field of 3 T, the according Larmor frequency is 128 MHz. During the nucleis' return into their initial state (relaxation), they emit an electromagnetic signal, which is used to calculate the picture.

While the static magnetic field is created by a superconducting coil, RF coils are responsible for the electromagnetic impulses. These RF coils each generate a gradient field in x-, y- and z-direction which is used to determine the position of a relaxing nuclei.

for localization. Only within a specific region within the gradient field in which the RF frequency corresponds with the Larmor frequecy, excitation is possible.

In daily clinical practice, open MRIs are normally operated from 0.2 - 1 T and closed MRIs from 1.5 - 3 T for the examination of patients. Only in research facilities, devices with higher magnetic fields from 7 T up to 11.7 T are used. For medical staff, the exposure is often provided in "mTh" in order to additionally account for a temporal component (hours) in a cumulated manner.

Frequency ranges:
  • 0–498.2 MHz (the Larmor frequency depends on the static magnetic field; for a current maximum of 11.7 T the corresponding Larmor frequency is 498.2 MHz)
Type of field:
magnetic

Measurements (acc. to literature)

0.2 T
Measurand Value Feature Remarks
magnetic flux density 0.3 mT (mean, measured) - arithmetic mean value of average exposure levels based on the total duration of the full shift of medical staff in a veterinary clinic; the static magnetic field B0 was 0.2 T for an open MRI system [1]
magnetic flux density 111 mT (maximum, measured) - maximum value in an academic veterinary clinic; the static magnetic field was 0.2 T for an open MRI system [1]
0.35 T
Measurand Value Feature Remarks
magnetic flux density 45 mT (maximum, measured) - maximum value of the measured magnetic flux density inside a 0.35 T MRT during the positioning of a patient; duration of positioning: approx. 7 min [2]
0.5 T
Measurand Value Feature Remarks
SAR 0.4 W/kg (maximum) - - [3]
cumulative exposure 5.78 mTh (measured) - occupational exposure for medical staff during the examination of a patient; manufacturer: Philips [2]
0.5 T - 7 T
Measurand Value Feature Remarks
magnetic flux density 5.3 mT (mean, measured) - arithmetic mean value of average exposure levels based on the total duration of the full shift of medical staff in nine different human MRI facilities; the static magnetic field B0 varied between 0.5 T - 7 T depending on the facility [1]
magnetic flux density 681 mT (maximum, measured) - arithmetic mean of the peak exposure levels of nine different human MRI facilities; the static B0 field varied between 0.5 T - 7 T depending on the facility [1]
0.65 T
Measurand Value Feature Remarks
cumulative exposure 0.65 mTh (measured) - occupational exposure for medical staff during the examination of a patient; manufacturer: Esaote [2]
1.03 T
Measurand Value Feature Remarks
cumulative exposure 1.03 mTh (measured) - occupational exposure for medical staff during the examination of a patient; manufacturer: Siemens [2]
1.5 T
Measurand Value Feature Remarks
electric field strength 4.0925 V/m (mean, measured) - at a distance of 100 cm (frequency range: up to 2 kHz) [4]
electric field strength 4.3733 V/m (mean, measured) - at a distance of 20 cm (frequency range: up to 2 kHz) [4]
electric field strength 5.8711 V/m (mean, measured) - at a distance of 50 cm (frequency range: up to 2 kHz) [4]
current density 0.71 mA/m² (simulated) - simulated expore for the cerebellum of a medical-laboratory assistant during the examination of a patient [5]
current density 0.88 mA/m² (simulated) - simulated expore for the retina of a medical-laboratory assistant during the examination of a patient [5]
current density 0.124 A/m² (simulated) - simulated expore for the bones of a medical-laboratory assistant during the examination of a patient [5]
current density 0.53 A/m² (simulated) - simulated expore for the heart of a medical-laboratory assistant during the examination of a patient [5]
current density 0.577 A/m² (simulated) - simulated expore for the kidneys of a medical-laboratory assistant during the examination of a patient [5]
current density 0.679 A/m² (simulated) - simulated expore for the stomach of a medical-laboratory assistant during the examination of a patient [5]
current density 0.679 A/m² (simulated) - simulated expore for the testis/prostate of a medical-laboratory assistant during the examination of a patient [5]
current density 0.71 A/m² (simulated) - simulated expore for the uterus of a medical-laboratory assistant during the examination of a patient [5]
current density 1.24 A/m² (simulated) - simulated expore for the small intestine of a medical-laboratory assistant during the care of a patient [5]
magnetic flux density 1.3525 µT (mean, measured) - at a distance of 100 cm (frequency range: up to 2 kHz) [4]
magnetic flux density 2.6023 µT (mean, measured) - at a distance of 50 cm (frequency range: up to 2 kHz) [4]
magnetic flux density 5.8709 µT (mean, measured) - at a distance of 20 cm (frequency range: up to 2 kHz) [4]
magnetic flux density 65.3 µT (maximum, measured) - magnitude of x-, y- and z-direction of the grdient field for an MRI of the type Achieva Nova 1.5 T manufactured by Philips [6]
magnetic flux density 118 µT (maximum, measured) - magnitude of x-, y- and z-direction of the grdient field for an MRI of the type Achieva 1.5 T manufactured by Philips [6]
magnetic flux density 0.005 T (mean, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during arrangements for the use of an automatic infusion pump; patient lying at the table close to the magnet [7]
magnetic flux density 0.006 T (mean, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during contrast administration into the arm of a patient; patient laying at the table was moved far from the magnet [7]
magnetic flux density 0.013 T (maximum, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during contrast administration into the arm of a patient; patient laying at the table was moved far from the magnet [7]
magnetic flux density 0.014 T (mean, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during contrast administration into the arm of a patient; patient laying at the table was moved from the magnet [7]
magnetic flux density 0.015 T (mean, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during contrast administration into the infusion tubing; patient stayed inside the MRI [7]
magnetic flux density 0.017 T (maximum, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during contrast administration into the infusion tubing; patient stayed inside the MRI [7]
magnetic flux density 0.08 T (mean, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during contrast administration into the permanent infusion cannula; patient lying partly in the MRI [7]
magnetic flux density 0.092 T (maximum, measured) - maximum value (normalized to 1.5 T) of the exposition of a medical-labour assitant during contrast administration into the arm of a patient; patient laying at the table was moved from the magnet [7]
magnetic flux density 0.093 T (maximum, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during contrast administration into the permanent infusion cannula; patient lying partly in the MRI [7]
magnetic flux density 0.12 T (maximum, measured) - median value (normalized to 1.5 T) of the exposition of a medical-labour assitant during arrangements for the use of an automatic infusion pump; patient lying at the table close to the magnet [7]
SAR 5.17 µW/kg (simulated) - simulated expore for the cerebellum of a medical-laboratory assistant during the examination of a patient [5]
SAR 6.4 µW/kg (measured) - simulated expore for the retina of a medical-laboratory assistant during the examination of a patient [5]
SAR 0.097 mW/kg (simulated) - simulated expore for the bones of a medical-laboratory assistant during the examination of a patient [5]
SAR 0.414 mW/kg (simulated) - simulated expore for the heart of a medical-laboratory assistant during the examination of a patient [5]
SAR 0.45 mW/kg (simulated) - simulated expore for the kidneys of a medical-laboratory assistant during the examination of a patient [5]
SAR 0.529 mW/kg (simulated) - simulated expore for the stomach of a medical-laboratory assistant during the examination of a patient [5]
SAR 0.536 mW/kg (simulated) - simulated expore for the testis/prostate of a medical-laboratory assistant during the examination of a patient [5]
SAR 0.554 mW/kg (simulated) - simulated expore for the uterus of a medical-laboratory assistant during the examination of a patient [5]
SAR 0.968 mW/kg (simulated) - simulated expore for the small intestine of a medical-laboratory assistant during the examination of a patient [5]
SAR 0.07 W/kg (maximum, simulated) - maximum value of a simulation model (age: 5 years, sex: female, height: 1.09 m, weight: 17.9 kg) in the abdomen [8]
SAR 0.11 W/kg (maximum, measured) - maximum value of a simulation model (age: 11 years, sex: female, height: 1.47 m, weight: 35.4 kg) between abdomen and pelvis [8]
SAR 0.13 W/kg (maximum, simulated) - maximum value of a simulation model (age: 26 years, sex: female, height: 1.63 m, weight: 59.1 kg) in the abdomen [7]
SAR 0.13 W/kg (maximum) - maximum value of a simulation modle (age: 14 years, sex: male, height: 1.69 m, weight: 59.1 kg) in the abdomen [8]
SAR 0.16 W/kg (maximum, simulated) - maximum value of a simulation model (age: 34 years, sex: male, height: 1.77 m, weight: 76.2 kg) between sternum and abdomen [9]
SAR 0.18 W/kg (maximum, simulated) - maximum value of a simulation model (age: 37 years, sex: male, height: 1.82 m; weight: 120.2 kg) between sternum and abdomen [9]
cumulative exposure 2.9 mTh (measured) - occupational exposure for medical staff during the examination of a patient; manufacturer: Siemens [2]
2.5 T
Measurand Value Feature Remarks
SAR 0.4 W/kg (maximum) - maximum value [3]
3 T
Measurand Value Feature Remarks
electric field strength 0.4 V/m (maximum, simulated) - maximum value of the electric field inside the central nervous system of a patient during an examination [10]
electric field strength 0.6 V/m (maximum, measured) - maximum value of the electric field inside the whole head of a patient during an examination [10]
magnetic flux density 3.31 µT (maximum, measured) - magnitude of x-, y- and z-direction of the grdient field for an MRI of the type Magnetom Allegra 3 T manufactured by Siemens [6]
magnetic flux density 276–679 mT (mean, measured) - exposure of a patient during an MR examination of the body or extremities (exposure duration: 207 - 231 s); according to personal dosimeters [11]
magnetic flux density 316–647 mT (mean, measured) - exposure of the medical staff during escort or assist of a patient during an MR examination (exposure duration: 130 - 176 s) [11]
magnetic flux density 399–845 mT (mean, measured) - exposure of a patient during an MR examination of the head or neck (exposure duration: 197 - 319 s); according to personal dosimeters [11]
magnetic flux density 515–1,250 mT (maximum, measured) - exposure of a patient during an MR examination of the head or neck (exposure duration: 319 - 509 s); according to personal dosimeters [11]
magnetic flux density 545–920 mT (maximum, measured) - exposure of the medical staff during escort or assist of a patient during an MR examination according to personal dosimeters (exposure duration: 340 - 513 s); according to personal dosimeters [11]
magnetic flux density 660–1,250 mT (maximum, measured) - exposure of a patient during an MR examination of the body or extremities (exposure duration: 318 - 372 s); according to personal dosimeters [11]
magnetic flux density 1,400 mT (measured) - maximum value of the measured magnetic flux density inside a 3 T MRT during the positioning of a patient; duration of positioning: approx. 5 min [2]
SAR 0.09–0.31 W/kg (maximum) - in the knee during a 3D gradient echo sequence [12]
SAR 0.94–1.4 W/kg (maximum) - in the head during a 3D gradient echo sequence [12]
SAR 1.46–1.74 W/kg (maximum) - in the pelvis during a 3D gradient-echo sequence [12]
SAR 2.45–2.61 W/kg (maximum) - in the head during a 2D fast spin echo sequence [12]
SAR 2.56–3.14 W/kg (maximum) - in the pelvis during a 2D fast spin echo sequence [12]
SAR 2.59–6.97 W/kg (maximum) - in the head during a 2D fast spin echo sequence [12]
cumulative exposure 26.83 mTh (measured) - during the examination of a patient; manufacturer: Philips [2]
4 T
Measurand Value Feature Remarks
current density 29 mA/m² (maximum, simulated) - maximum value of the current density inside a patient for an x-z image at chest height; the distance from the foot of the bed to the center of the magnet was 1.55 m [13]
current density 70 mA/m² (maximum, calculated) - maximum value of the current density inside a patient for an x-y image over the whole body; the distance from the foot of the bed to the center of the magnet was 2 m [13]
current density 86 mA/m² (maximum, simulated) - maximum value of the current density inside a patient for an x-z image at chest height; the distance from the foot of the bed to the center of the magnet was 2 m [13]
current density 100 mA/m² (maximum, simulated) - maximum value of the current density inside a patient for an x-y image over the whole body; the distance from the foot of the bed to the center of the magnet was 1.55 m [13]
current density 120 mA/m² (maximum, simulated) - maximum value of the current density inside a patient for an x-y image over the whole body; the distance from the foot of the bed to the center of the magnet was 0.8 m [13]
current density 47 A/m² (maximum, simulated) - maximum value of the current density inside a patient for an x-z image at chest height; the distance from the foot of the bed to the center of the magnet was 0.8 m [13]
4.7 T - 11.7 T
Measurand Value Feature Remarks
magnetic flux density 3 mT (mean, measured) - arithmetic mean value of average exposure levels based on the total duration of the full shift of medical staff in five different experimental animal research facilitites; the static magnetic field B0 varied between 4.7 T - 11.7 T [1]
magnetic flux density 221 mT (maximum, measured) - arithmetic mean of the peak exposure levels of five different experimental animal research MRI facilities; the static B0 field varied between 4.7 T - 11.7 T depending on the facility [1]
7 T
Measurand Value Feature Remarks
SAR 2 W/kg (maximum, simulated) - in the head of a 14-year-old Caucasian boy [14]
SAR 2.6 W/kg (maximum, simulated) - in the head of a 34-year-old Caucasian man [14]
SAR 2.9 W/kg (maximum, simulated) - in the head of a 26-year-old Caucasian woman and a Japanese man [14]
SAR 4 W/kg (maximum, simulated) - in the head of an 11-year-old Caucasian girl [14]
SAR 3.4 kW/g (maximum, simulated) - in the head of a Japanese woman [14]

References

  1. Schaap K et al. (2014): Exposure to static and time-varying magnetic fields from working in the static magnetic stray fields of MRI scanners: a comprehensive survey in the Netherlands.
  2. Acri G et al. (2014): Evaluation of occupational exposure in magnetic resonance sites.
  3. Wilkening GM et al. (1990): Health effects of nonionizing radiation.
  4. Israel M et al. (2013): Electromagnetic Field Occupational Exposure: Non-Thermal vs. Thermal Effects.
  5. Farrag SI (2014): Numerical computation of specific absorption rate and induced current for workers exposed to static magnetic fields of MRI scanners.
  6. Andreuccetti D et al. (2013): Weighted-peak assessment of occupational exposure due to MRI gradient fields and movements in a nonhomogeneous static magnetic field.
  7. Karpowicz J et al. (2013): The pattern of exposure to static magnetic field of nurses involved in activities related to contrast administration into patients diagnosed in 1.5 T MRI scanners.
  8. Murbach M et al. (2014): Whole-body and local RF absorption in human models as a function of anatomy and position within 1.5T MR body coil.
  9. Kondur AK et al. (2008): Implantable cardioverter defibrillators save lives from lightning-related electrocution too!
  10. Laakso I et al. (2013): Computational dosimetry of induced electric fields during realistic movements in the vicinity of a 3 T MRI scanner.
  11. Yamaguchi-Sekino S et al. (2014): Occupational exposure levels of static magnetic field during routine MRI examination in 3T MR system.
  12. Boss A et al. (2007): Tissue warming and regulatory responses induced by radio frequency energy deposition on a whole-body 3-Tesla magnetic resonance imager.
  13. Liu F et al. (2003): Calculation of electric fields induced by body and head motion in high-field MRI.
  14. de Greef M et al. (2013): Specific absorption rate intersubject variability in 7T parallel transmit MRI of the head.