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GCSE Questions: Medical Physics

- scanning and therapy

Q1.

Doctors sometimes need to know how much blood a patient has.

They can find out by using a radioactive solution.

Radiation from radioactive substances can harm your body cells.

The doctors use a radioactive substance called Technicium 99-m which loses half of its radioactivity every six hours.

After measuring how radioactive a small syringe-full of the solution is they inject it into the patient's blood.

They then wait for 30 minutes so that the solution has time to become completely mixed into the blood.

Finally, they take a syringe-full of blood and measure how radioactive it is.          

Example: If the doctor injects 10 cm3 of the radioactive solution into the patient and this is found to have been diluted 500 times by the blood there must be 10 × 500 = 5000 cm3 of blood

(a)     After allowing for background radiation:

10 cm3 of the radioactive solution gives a reading of 7350 counts per minute;

a 10 cm3 sample of blood gives a reading of 15 counts per minute.

Calculate the volume of the patient's blood. (Show your working.)

The examiners expect you to do the same as they did in the example.

7350/15 = 490

so the radioactive solution has been diluted 490 times.

therefore there must be 490 x 10 = 4900 cm3of blood

(4 marks)

(b)     The doctor's method of estimating blood volume will not be completely accurate. Write down three reasons for this.

You are expected to use the information given to you in the diagram! Always look carefully at any diagrams you are given - they are not put there to make the examination paper 'look pretty'!

some of radioactive solution gets into cells/body organs

some of radioactive solution gets into urine (in the kidney)

the radioactive solution becomes less radioactive during the test

 

(3 marks)

(c)    Explain why Technicium 99-m is a suitable radioactive substance to use.

The half life is just about right, long enough to carry out the procedure but not to interfere with the patient's lifestyle too much or put him/her in danger from excessive exposure. There is a lot of information on this radionuclide on this page if you are interested.

It won't lose (too) much radioactivity during the test

It won't stay radioactive/harm cells for too long after test is over

(2 marks)

(Total 9 marks)