Radiology 2

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.

• 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.

D) 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.

A) 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.

D. Relation of transducer to blood flow

C) 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.

A. 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

Faraday cage

Faraday cage shielding is also done in EEG cabins to prevent external electromagnetic disturbances. [AIIMS 04]

A. Gadolinium

D) Subarachnoid hemorrhage

• - 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

A. +22 to +46

B. Cerebral hemorrhage

In cerebral hemorrhage, blood follows the subdural space over the convexity of the brain and appears as a concave, hyper dense lesion.

- used in transducer of USG [AIIMS 04]

- PZT: Lead Zirconate Titanate) is polarized ceramic (though called crystal) used in USG transducer

B) Reflection

Frequency

A. More than 20,000/sec

Infrasound: <20/sec
Audible: 20- 20,000/sec (20Hz-20KHz)
Ultrasound: >20,000/sec

D. 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)

D. Ultrasound

- Mutation is produced only by ionizing radiations

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)

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.

B. 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.