Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 109

1 If the magnetic susceptibility χ of a diamagnetic tissue is ‐10−3 the magnetic flux density within that tissue in a 1.5 T scanner will be:1.499 T1.5 T1.501 T1.51 T15 mT

2 The magnetic field strength H at the centre of a long solenoid of length 150 cm, with 1800 turns carrying 1000 A is1000 A m−1200 000 A m−1400 000 A m−1600 000 A m−11 200 000 A m−1

3 The magnetic flux density produced in question 2 is500 G0.5 T1.0 T1.5 T5000 G

4 Which of the following does not affect the magnetic force on a soft ferromagnetic material with a high magnetic susceptibility?The distance to the bore openingThe fringe field spatial gradient dB/dzThe exact value of susceptibility χThe size of the objectIts electrical conductivity.

5 A metal object saturates magnetically at 1 T. The maximum force as it approaches a 3T scanner with a spatial gradient of 5 T m−1 is proportional to1 T2 m−13 T2 m−15 T2 m−19 T2 m−115 T2 m−1

6 When a previously un‐magnetized ferromagnetic object is introduced into an external B field, the maximum torque occurs when the angle between the long axis of the object and the field lines is:0°30°45°60°90°

7 The induced electric field in a circular cross section of tissue from the z‐gradient is proportional to:The tissue conductivityThe area of the cross sectionThe rate of change of magnetic fieldThe radius of the cross sectionTissue density.

8 If the diameter of the heart is 8 cm, what current density would a uniform dB/dt of 100 T s−1 induce in it, assuming tissue conductivity of 0.2 Sm−1?1.6 mA m−2400 mA0.8 A m−20.4 A m−24.0 A m−2

9 If we change field strengths from 1.5T to 3T then (keeping our sequence the same)SAR will stay the same but B1 will halveSAR and B1 will both doubleSAR will double while B1 remains the sameBoth SAR and B1 will increase by four timesSAR will increase by a factor of 4 while B1 remains the same.

10 In a pulse sequence, if we halve the duration of the RF pulse whilst keeping the flip angle and TR the sameSAR will increaseSAR will not changeSAR will decreaseThe duty cycle is doubledWe cannot predict what will happen.