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Salters-Nuffield Advanced Biology Resources Extension 1.2 Student Sheet

In document NEW SPEC UNIT 1 (TOPIC 1) (Page 135-137)

Coronary angiography uses X-rays to detect obstructions in the coronary arteries of the heart. A radiologist or cardiologist inserts a catheter (thin flexible tube) through a small incision in a blood vessel in the arm, neck or groin (Figure 2). The catheter is then carefully threaded into the heart (Figure 3). The blood vessels of the heart are then studied by injection of a dye through the catheter. A rapid succession of X-rays is taken to view blood flow. Prior to the procedure the patient may be given a mild sedative; the site will be cleaned and numbed with a local anaesthetic. Angiography was once commonly used to check the condition of arteries, but now non-invasive techniques are used.

Figure 2 Coronary angiography – a catheter is inserted into an artery in the groin.

Figure 3 The catheter is carefully fed through the aorta into the coronary artery. Cardiac catheterisation can determine pressure and blood flow in the heart’s chambers, collect blood samples from the heart and examine the arteries of the heart. It is clearly a highly useful procedure. However, it is invasive and not without some risk.

Ultrasound is an imaging technique that works like radar – a pulse of sound energy is sent into the body and the reflections from internal tissue boundaries are detected. Ultrasound is used for viewing babies in the womb and is particularly well suited for this because it is very safe (compared to X-rays) and good for viewing tissue–tissue boundary features. The main drawback of ultrasonic imaging is the very high signal-to-noise ratio. This makes features hard to see against the background.

A CT, CAT or computerised tomography scan (Figure 4) is a method of taking pictures of the inside of the body using a very thin X-ray beam. As this X-ray beam passes through the body, it is absorbed in varying amounts by bones, tissues or fluid in the body, so that the beam that emerges from the body varies in intensity. This varying intensity is measured by a special device that converts this

information into a detailed picture. Sometimes dye is injected into a vein before a CT scan to improve the visibility of certain blood vessels and organs on the picture.

CT scanning was originally developed to help in diagnosing disorders of the brain. CT imaging of the head and brain can detect tumours, blood clots and blood vessel defects. However, the use of CT scanning has been expanded to include nearly every part of the body because it provides good soft tissue resolution (contrast). The short scan times (500 milliseconds to a few seconds) mean that CT can be used for all regions of the body, including moving parts. Thus, many internal organs can be seen with a CT scan, but not with regular X-rays.

The CT image can be processed after scanning in several ways. For instance, 3D display further enhances the value of CT imaging for surgeons. So-called multi-slice spiral CT scanning used with an ECG is a non-invasive method of imaging the heart and coronary arteries. It can show narrowing of the arteries and calcium deposits in the coronary arteries that form with plaque build-up. Thus, it can allow diagnosis that could prevent a heart attack.

Salters-Nuffield Advanced Biology Resources

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Extension 1.2 Student Sheet

Figure 4 A CT scan of a healthy human brain.

To find out exactly how a CT scan 3D image is produced, see the weblinks associated with this topic. EBCT is a special type of computed tomography that uses a sweeping electron beam to create the effect needed to make a CT image. EBCT is a very fast, non-invasive means of imaging the heart and coronary arteries. It eliminates the need for catheterisation and contrast injection which is required in conventional cardiac angiography. It is particularly useful in that it can show calcium deposits in the coronary arteries. These form along with the plaque build-up and can eventually lead to heart failure. Early imaging of calcium deposits in the coronary arteries allows aggressive preventive measures to be implemented, thus lowering the risk of heart attack.

MRI was formerly known as nuclear magnetic resonance (NMR) imaging – the name was dropped due to public fears about irradiation during scans, prompted by the word ‘nuclear’. The body to be examined is placed in a magnetic field and all the atoms are shaken up. The field is removed and the atoms emit their excess energy. The rate of emission is detected and is dependent on tissue type. It is this that determines the resulting image contrast. The main applications of MRI are in brain imaging and breast scanning. Others include the visualisation of torn ligaments and shoulder injuries, and the diagnosis of the early stages of stroke. MRI systems can also image flowing blood in virtually any part of the body. In many cases, the MRI system can do this without a contrast injection.

The MRI system builds up a 2D or 3D map of tissue types and then integrates all of this information to create 2D images or 3D models. MRI provides an unparalleled view deep inside the human body. The level of detail we can see is extraordinary compared with any other imaging technique. It is the method of choice for the diagnosis of many types of injuries and medical conditions.

If you had to undergo an MRI scan, you might be as confused as the author of this question, posted on a website:

Dear Alice,

What is the difference between CT and MRI? And what does with or without dye mean?

Write a reply from Alice and then compare your reply to the one given by Alice at the ‘Go Ask Alice’ website, which can be found in the weblinks for this topic.

Ambulatory or 24 hour monitoring: If no abnormality, disease or damage can be detected using the standard techniques but the patient still feels uneasy when performing stressful activities (such as climbing stairs) but feels okay under normal activity, then an ambulatory monitor may be used. An ambulatory monitor is a portable ECG system (often worn around the waist) that continuously monitors the heart’s electrical activity.

In document NEW SPEC UNIT 1 (TOPIC 1) (Page 135-137)