Ultrasound imaging

Technetium-99_m (^99mTc) is produced from the decay of molybdenum-99 (⁹⁹Mo). In hospitals,^99mTc is extracted from a molybdenum-technetium generator.

⁹⁹Mo is put into the generator at time t = 0 As⁹⁹Mo decays, the amount of^99mTc increases to a maximum at t = 24 hours. ^99mTc is extracted from the generator at regular intervals. At each extraction, the activity of^99mTc in the generator falls to zero and then increases again.

Figure 3 shows how the activities of⁹⁹Mo and^99mTc in the generator vary with t when extractions are made at t = 24 hours and t = 48 hours.

Show that about 6 × 10¹⁵ nuclei of⁹⁹Mo decay between t = 0 and t = 24 hours.

decay constant for⁹⁹Mo = 2.8 × 10⁻⁶ s⁻¹

90% of the⁹⁹Mo nuclei decay to form^99mTc nuclei. The other 10% decay to form different nuclei.

Estimate the number of^99mTc nuclei that are extracted from the generator at t = 24 hours.

Information about heart function can be obtained from a multi-gated acquisition (MUGA) scan or from a transthoracic echocardiogram (TTE) scan. Heart specialists use these scans to diagnose a range of heart conditions.

In a MUGA scan, a patient is injected with a compound containing^99mTc which binds to red blood cells. Using images of the red blood cells, a precise reading can be made of blood flow rate through the heart.

A TTE is an ultrasound scan of the heart. Images of the heart are seen on a monitor as a transducer is moved across the surface of the patient’s chest.

Compare the MUGA method of investigating heart function with the TTE method of investigating heart function.

In your answer you should:

• outline how an image of the red blood cells is obtained during a MUGA scan

• explain the advantages that a MUGA scan and a TTE scan have in common

• compare the strengths and weaknesses of a MUGA scan and a TTE scan.

Figure 2 shows a transducer used in a medical ultrasound scanner.

Explain why a backing material is used.

A beam of ultrasound is transmitted from muscle into bone.

Calculate the percentage of the incident intensity that is transmitted.

acoustic impedance of bone = 5.3 × 10⁶ kg m⁻² s⁻¹

Different ultrasound techniques are used to investigate the health of a patient’s kidneys.

Figure 3 shows the results of an ultrasound scan of a kidney using one technique.

Identify the type of ultrasound scan used to produce Figure 3.

Explain your answer.

Another ultrasound technique is used to measure the speed of blood flow in one of the kidney’s blood vessels.

Figure 4 shows an image formed using this technique. It shows how the speed of flow through the blood vessel varies with time. Point X shows this speed at one instant of time.

Determine the patient’s heart rate in beats per minute.

An ECG was made for the patient in Question 04.2.

Figure 5 shows one heartbeat from the ECG.

Annotate Figure 5 to show:

• units on both axes

• scales on both axes

• point Y that corresponds to point X on Figure 4.

Figure 5

A backing gel is used between an ECG pad and the skin of the patient. The gel is sticky. This property ensures that the pad is securely attached to the skin.

Explain:

• one other reason why the backing gel is needed

• one other property of the backing gel

• how the skin is prepared for the pad to be applied.

A patient has calcium kidney stones. Three types of scan are available to investigate the condition:

• a magnetic resonance (MR) scan

• a CT scan

• an ultrasound scan.

Calcium kidney stones contain no water and appear similar to bone in each of the scans.

Discuss the advantages and disadvantages of each option. In your answer you should

• refer to the relevant quality of the image obtained from each scan

• identify other factors that should be considered

• justify the type of scan you would recommend.

Figure 2 shows scanned images of three different human heads. Each image used one of the following scanning techniques:

• magnetic resonance (MR)

• ultrasound

• PET.

Figure 2

Identify the scanning technique used for each image. Go on to explain how the features of each image enabled you to identify the type of scan.

A: Scanning technique

B: Scanning technique

C: Scanning technique

Figure 1 shows an ultrasound transducer used to perform medical scans.

Explain how the transducer in Figure 1 operates in medical diagnosis.

In your answer you should explain how

• an ultrasound pulse is produced by the transducer

• the reflected ultrasound pulse is detected by the transducer

• the transducer can both transmit a pulse and receive the reflected pulse.

Ultrasound of frequency 1.0 MHz is used to scan a person’s liver.

Estimate the resolution of the scan.

speed of sound in liver tissue = 1600 m s⁻¹

Ultrasound travels from a transducer through the chest wall to an air pocket inside the lung. From the air pocket, the ultrasound is then incident on lung tissue.

Calculate the percentage of the incident ultrasound intensity that is transmitted into the lung tissue.

speed of sound in lung tissue = 1580 m s⁻¹

density of lung tissue = 1075 kg m⁻³

speed of sound in air = 330 m s⁻¹

Discuss whether an ultrasound scan would be suitable to investigate a tumour inside a lung.

Ultrasound is commonly used in medical procedures.

An ultrasound A-scan is used to find the length l of an eye as shown in Figure 6. Figure 7 shows the simplified A-scan for the eye. A short pulse of ultrasound is transmitted at time t = 0

The average speed of ultrasound in the eye = 1560 m s¹.

Calculate l.

Amniocentesis is a procedure where a tube is inserted into a uterus to remove some cells and fluid from around a foetus. For the procedure to be carried out safely the positions of the needle, foetus and placenta must be determined accurately.

Discuss whether an A-scan or a B-scan should be used for amniocentesis.

In your answer, you should:

• outline the differences between an A-scan and a B-scan

• describe the advantages and disadvantages of each type of scan

• explain why your chosen scan should be used for this procedure.

A patient with a suspected broken arm is going to have an X-ray image taken.

Explain the risk to the patient of exposure to X-rays.

Go on to discuss three ways by which the design and use of the X-ray equipment minimises this risk.

The blood vessel called the aorta passes through the abdomen. A second patient with a suspected fault in the wall of the aorta can be given an ultrasound scan or an X-ray of the abdomen.

Suggest, with reasons, which is the better procedure for investigating this suspected fault.

When ultrasound travels across a boundary from blood to the wall of the aorta there is a decrease in acoustic impedance across the boundary. This results in 0.0625% of the intensity of the incident ultrasound being reflected at the boundary.

Calculate the acoustic impedance of the aorta wall tissue.

acoustic impedance of blood = 1.64 × 10⁶ kg m⁻² s⁻¹