X - Questions markscheme

1.       (a)     (to a maximum of 7 marks) e.g.

•    X-ray source + detectors round patient …

•    … rotated around patient …/ the signal / X-ray passes through the
same section of the body from different directions.

•    … producing a (thin) slice / cross-section.

•    Idea of absorption / less gets through / more is absorbed …

•    by dense material / bone / material of high Z / High Z related to
materials such as bone / Low Z to materials such as soft tissue

•    attenuation is by the photo-electric effect

•    the possibility of using a contrast medium.

•    better than a simple X-ray at differentiating other organs.

•    patient is moved a small distance and the process is repeated /
process continues in a spiral.

•    a computer (analyses the data) / identifies the position of organ/bone …

•    … and forms a 3-D image.                                                                          7

  (b)   •    Patients are exposed to ionising radiation. (1)

•    (Ionising radiation) could cause cancer / damage cells (1)

          Plus a maximum of ONE from:-e.g. (1)

•    It’s expensive.

•    Time consuming / uses valuable resources, etc..                                          3

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  2.     (a)     Low energy X-rays are absorbed by the skin / undesirable as can cause
damage / greater ionising (1)                                                                             1

  (b)   I = I0e–µx (1)                              ln I = ln Io – µ x
Io =  (1)                     ln Io = ln 347 + 250 × 0.025
Io = 1.79 × 105 Wm–2 (1)                                                                                3

  (c)   P = I × A (0)
P = 347 × π × (0.10 × 10–2)2 (1)
P = 1.09 × 10–3 W (1)                                                                                       2

  (d)   (i)      P = 18 × 100 / 0.15 (1)
P = 12000 W (1)                                                                                      2

  (ii)   12000 / 7.5 × 1017 (= 1.6 × 10–14 J = energy of each electron) (1)
0.5 m v2 = 1.6 × 10–14 (0)
v = 1.9 × 108 ms–1 (1)                                                                             2

(iii)    tube current = 7.5 × 1017 × 1.6 × 10–19 = 0.12 A (1)
V × I = 12000 (1)
V =12000 / 0.12 = 100,000 V or 100 kV (1)                                           3

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  3.     Formation of image to a max 3 e.g.

          X-rays are detected by a film / scintillation counter etc., (1)
High ‘Z’ means high attenuation / low transmission
[Allow atomic mass / nucleon number] (1)
shadow on the film / reference to exposure after attenuation (1)
Reference to photoelectric effect / energy range around 1–100keV /
absorption ∞ Z3 (1)

          Explanation of the use of a contrast medium to a max.4 e.g.

          X-rays do not differentiate / show up soft tissues well …(1)
… as similar absorption / ‘Z’ is similar / ‘Z’ is low for these tissues. (1)
Contrast medium has high ‘Z’ / absorbs X-rays strongly.(1)
It is usually taken orally / as an enema / can be injected.(1)

          Example of type of structure that can be imaged to a max.1 e.g.

          digestive tract / throat / stomach.(1)                                                            to a max. 8

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