Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to investigate the anatomy and function of the body in both health and disease. MRI scanners use strong magnetic fields and radiowaves to form images of the body. The technique is widely used in hospitals for medical diagnosis, staging of disease and for follow-up without exposure to ionizing radiation.
What do TR and TE in MRI image mean? [IOM 11]
TE is Echo Time. It represents the time in milliseconds between the application of the 90o pulse and the peak of the echo signal in Spin echo and inversion recovery pulse sequences.
TR is Repetition Time. It is the amount of time that exists between the successive pulse sequences applied to the same slice.
In MRI, the field used is [AI 97]
C) 1.1 tesla
The tesla is the SI derived unit of magnetic flux density. One tesla is equal to one weber per square metre.
1 tesla is equivalent to 10,000G (gauss), used in the CGS system.
The field used in clinical practice range from 0.15 to 3 tesla (1500 to 30,000 Gauss) as compared to Earth's magnetic field of 0.5 Gauss.
All of the following statements about MRI are true, except [AI 12]
D) MRI is better for calcified lesions
MRI remains inferior to both plain x-ray and CT scan for delineating calcification, ossification, cortical destruction and periosteal reaction.
In color Doppler the color depends upon:
A. Relation of transducer to blood flow
NMR spectroscopy is based on the principle of [AI 91]
B) Quantum state changes of protons
The theory of nuclear magnetic resonance was elucidated by Edward Purcell.
The basic principle of MRI depend on the fact that nuclei of certain elements behave like small, spinning, bar magnets, and align with the magnetic force when placed in a strong magnetic field. Hydrogen nuclei (proton) in water molecules and lipids are currently used.
The strength of the MRI signals reflect the rate of Quantum state change of protons (Hydrogen ion). These transitions occur very slowly, on a time scale of 10th of seconds to seconds for most biological materials.
Contraindication for the use of NMR imaging is:
B. Intracranial metallic aneurysm clips
Contraindications:
- Intracranial metallic aneurysm clips
- Intra-orbital metallic foci
- Cardiac pacemakers
- Specific types of cardiac valves
Relative C/I:
- Claustrophobic pt
- Uncooperative pt not responding to conscious sedation protocols
MRI rooms are shielded completely by a continuous sheet or wire mesh of copper or aluminum to shield the image from external electromagnetic radiations, et, it is called: [AIIMS 03]
Faraday cage
Faraday cage shielding is also done in EEG cabins to prevent external electromagnetic disturbances. [AIIMS 04]
Which one of the following is the contrast used in MRI scan: [PGI 2000]
D. Gadolinium
CT is better over MRI [IOM 10]
A) Subarachnoid hemorrhage
CT scan:
- Invented by Godfrey Hounsfield [SGPGI 99]
- First CT scanner manufactured by Electro-Musical Instruments(EMI), England
- Unit of density of body tissues in CT scan: Hounsfield units
CT numbers (attenuation coefficient) expressed in Hounsfield units and compared according to Hounsfield scale:
Air: -1000
Water: 0 [AIIMS 05]
Soft tissue: +40 to +80
Bone: +1000 to +3000
On usual CT scale of densities (-1000 to +1000), brain tissues measures:
D. +22 to +46
On CT-scan, all are seen as hypodense area except: [JIPMER 01]
C. Cerebral hemorrhage
In cerebral hemorrhage, blood follows the subdural space over the convexity of the brain and appears as a concave, hyper dense lesion.
Pizoelectric crystals
- used in transducer of USG [AIIMS 04]
- PZT: Lead Zirconate Titanate) is polarized ceramic (though called crystal) used in USG transducer
Acoustic shadow in USG is due to [AI 94]
C) Reflection
Hertz is a unit of:
Frequency
Ultrasound refers to the sound waves with a frequency of:
C. More than 20,000/sec
Infrasound: <20/sec
Audible: 20- 20,000/sec (20Hz-20KHz)
Ultrasound: >20,000/sec
Real time sector scanning of neonates is preferred because of which of the following practical reason: [PGI 95]
C. Open fontanelles
- Real time sonographic sector scanner can use the anterior fontanelle as an acoustic window
- It has a wide field of view and requires only a small acoustic window (unlike other USG machines)
Which is not mutagenic: [PGI 93]
B. Ultrasound
- Mutation is produced only by ionizing radiations
Deleterious effect of ultrasound on small organism is:
B. Cavitation
- Some mechanisms for producing biological effects with ultrasound at therapy intensity levels include significant local heating, cavitation (bubble formation, collapse and oscillation) and micro-streaming (flow of bubbles)
Which of the following best estimates the amount of radiation delivered to an organ in the radiation field? [AI 10]
D. Absorbed dose
Absorbed dose - measures the total quantity of radiation delivered/deposited to an organ or tissue per unit volume.
Equivalent dose - measures the biological damage to an organ or tissue caused by the absorbed dose depending on the relative effectiveness of the type of radiation.(whether X-rays, gamma rays, electrons, protons, neutrons)
Effective dose - measures the biological damage by calculating an imaginary total body dose depending on equivalent dose for each organ and relative sensitivity of each organ to radiation.
Which of the following statements about "Stochastic effects" of radiation is true? [AI 10]
A. Probability of effect is a function of dose
Stochastic effects of radiation:
- Probability of effect is dependent upon dose (cumulative dose)
- These effects have no threshold, and these cannot be definitely be associated with radiation exposure; and occur both in general population and irradiated people
- E.g. Cancer, hereditary diseases/effects
Deterministic effects:
- "Severity" is dependent upon dose
- These effects have threshold, and threshold has to be exceeded before the effect becomes apparent
- These effects are definitely associated with radiation exposure and occur only in irradiated people
- E.g. Erythema, impairment of fertility, cataract, hair losses, myelosuppression, etc.
