Bob Bury

CT and Nuclear Medicine

Last time, I explained a bit about how standard radiography using x-rays was used to form images of the insides of our bodies. Now I want to talk about all the other imaging methods that are available to the present generation of doctors. All I can do in the limited space available is to give you a flavour of the new 'scanning' technology, which has revolutionised medical practice. This time I'll deal with the two techniques which, like x-rays, use radiation (CT and nuclear medicine), next time, the two which don't (ultrasound and magnetic resonance imaging)..

Computed Tomography (CT scanning)

We

have seen that conventional x-ray techniques shine a wide beam of

x-rays at the patient, and capture the transmitted radiation with

its 'shadow' of the internal organs on a piece of fairly standard

photographic film. CT scanning uses x-rays as well, but in a completely

different way. Instead of the wide beam of rays, CT scanners produce

a very narrow 'pencil' beam. This passes through the patient and

hits not a film, but an electronic radiation detector which simply

records the amount of radiation passing through that bit of the

patient. Which is OK, but it just gives you a number, not a picture.

The clever part of CT scanning was to mount the x-ray tube and detector

on opposite sides of a circular, doughnut-shaped, gantry. If you

then rotate the tube and detector though a few degrees and repeat

the measurement, and keep on doing that until you have moved through

a full circle, a dedicated computer can reconstruct an image of

the internal organs in that thin slice of the patient (figure

1- click to enlarge). Advance the bed and patient a few centimetres

through the gantry and repeat the process, and you get another slice,

and so on until you have a set of images encompassing the part of

the patient you are interested in. The early scanners were only

big enough to get the head into, but it wasn't long before technology

allowed the construction of gantries which would take the whole

body.

We often talk of 'revolutionary' developments in medicine and other

sciences, but it would be difficult to overestimate the impact of

CT scanning on imaging, and on medicine in general. Let's put it

into context by thinking about brain imaging. Before CT, there were

two methods for demonstrating disease in the brain. One involved

the injection of contrast material (see previous article) into the

blood vessels supplying it (angiography), and the other depended

on the injection of air into the fluid-filled space around and within

the brain (pneumoencephalography). The first of these carries some

risk, and will only demonstrate the blood vessels, not the 'meat'

of the brain, the second only shows the internal and external surface

of the brain, and was exquisitely painful. So, dangerous and unpleasant

investigations, which still only delivered limited information concerning

the structure of the brain and any abnormality within it. Overnight

(almost!), CT produced images of the internal structure of the brain

which, even with the unsophisticated early machines, far surpassed

anything which had been possible previously. Suddenly, we could

'see' tumours and decide whether they were curable by surgery before

you operated. We could diagnose infarcts (damage to brain tissue

caused by bleeding or blood clots in the blood vessels supplying

the brain) in patients with strokes, and decide on the appropriate

treatment. With the advent of body scanners the same was true of

disease in organs such as the pancreas, which had previously been

completely inaccessible to imaging. I'll say a bit at the end of

next month's column about the impact all this had on those of us

lucky (and old!) enough to have been around at the time, but Sir

Godfrey Hounsfield, the inventor ofCT, thoroughly deserved his Nobel

prize.

<map name="Map"><area shape="circle" coords="174,102,22" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Left Kidney" title="Left Kidney" /><area shape="circle" coords="70,102,21" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Right Kidney" title="Right Kidney" /><area shape="circle" coords="124,111,21" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Vertebra (backbone)." title="Vertebra (backbone)." /><area shape="rect" coords="86,34,166,65" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Intestines" title="Intestines" /><area shape="circle" coords="92,146,14" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Muscle" title="Muscle" /><area shape="circle" coords="158,147,13" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Muscle" title="Muscle" /><area shape="rect" coords="82,3,161,13" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Skin" title="Skin" /><area shape="rect" coords="12,55,24,126" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Skin" title="Skin" /><area shape="rect" coords="69,15,175,25" href="http://www.thenakedscientists.com/HTML/Columnists/bobburycolumn6.htm#" alt="Abdominal wall muscles (6 pack !)" title="Abdominal wall muscles (6 pack !)" /></map>

The early scanners could take several minutes to scan one slice,

so imaging the whole chest or abdomen took a long time. Thanks to

developments in x-ray tube design, computing power and engineering,

scan times have come right down to a few seconds. Thanks to slip-ring

technology, the x-ray tube and detector can now revolve continuously,

and if the bed the patient is lying on moves slowly and continuously

into the gantry, the tube actually describes a spiral around the

patient, allowing information from a large volume of their body

to be acquired in a short time (quick enough, for example, for the

whole chest to be scanned during one breath hold, eliminating any

blurring due to respiratory movements). The dedicated imaging computer

can then reconstruct slices from that volume of material. The resulting

images are exquisitely detailed (figure 2), enabling individual

organs and their relationship to each other to be demonstrated.

For example tumours can be seen, and the exact degree to which they

have spread to surrounding structures can be documented. This may,

sadly, show that the tumour is inoperable, but at least that will

spare the patient the distress of an unnecessary operation; on the

other hand, the scan pictures may help the surgeon to plan the operation

in some detail before the patient ever gets near to the operating

table, increasing the chances of success. If radiotherapy is to

be used, the images can be used directly in planning the radiation

treatment, guiding the beam to ensure that it hits the tumour without

damaging the surrounding normal tissue.

Nuclear Medicine

Also known as isotope imaging, scintigraphy or radionuclide imaging,

nuclear medicine (NM) has actually been around for a while. This

is a completely different way of seeing inside patients. All the

techniques we have mentioned so far shine a beam of radiation through

the patient, and use the transmitted radiation to form an image.

In NM, the source of radiation is introduced into the patient's

body, and the emitted rays are used to paint the picture. The radiation

is produced by the decay of relatively short-lived radioactive isotopes

with the production of gamma rays. These are essentially the same

as x-rays, but with shorter wavelengths and higher energy. The trick

with NM is to choose a chemical which will be taken up in the organ

we are interested in, then 'label' it with a suitable radioactive

isotope. The isotope we use most is technetium, because it is readily

available, has suitable physical characteristics for imaging, and

also has the ability to link readily with other molecules to provide

a whole range of radiopharmaceuticals suitable for investigating

all the major body systems. The radiopharmaceutical is injected

(usually) into a vein.

The

radiation that emerges from the patient is detected by a gamma camera.

The business end, or head, of the gamma camera contains a large

single crystal of sodium iodide about a centimetre thick and up

to 40cm across. When gamma rays are absorbed by the crystal, a tiny

flash of visible light is emitted. These flashes are detected and

amplified by an array of photomultiplier tubes, and the resulting

image is stored in a computer. Cameras can have two heads, and most

modern scanners have the ability to rotate around the patient, rather

like a CT scanner, and produce images of a slice through the patient,

as well as producing static images (figure 3). The beauty of NM

is that it provides functional imaging. In other words, it produces

images that are related to the function of the organ concerned,

rather than simply telling us about its appearance. In fact, the

anatomical information in NM scans is fairly low resolution in comparison

with other imaging techniques, but often we don't care too much

about the size and shape of an organ, we're much more interested

in how well it's working. It all depends on the clinical situation.

Take the kidney, for example. In a patient with a malignant tumour

of the kidney, we want to know exactly how big the tumour is, how

far it has spread and whether it will be possible to remove it surgically.

For that, we need a CT scan. For a patient with a kidney stone that

is causing obstruction to the passage of urine down the ureter to

the bladder, on the other hand, we want to know how bad the obstruction

is, and what effect it is having on the function of the kidney.

In this case, we don't care what it looks like, we need to know

what it is doing; we need a NM scan. After injecting a radiopharmaceutical

which is rapidly cleared from the blood by the kidneys and excreted

in the urine, we can position the gamma camera over the kidneys

and capture rapid-sequence images over the course of half an hour

or so, following the changes in levels of radioactivity as the pharmaceutical

is excreted. This will give us a measurement of the relative function

of the kidneys and also tell us just how severe the obstruction

is.

Often,

of course we want to know about structure and function, and so NM

techniques are complementary to the other imaging methods we have

been looking at. Almost any organ or tissue and many of the physiological

processes in the body can be imaged in this way. Cardiac efficiency

can be measured by tagging the patient's blood cells with an isotope

and observing the passage of blood through the pumping chambers

of the heart, and by using a different radiopharmaceutical, blood

flow to the muscle of the heart itself can be mapped, allowing us

to detect and quantify coronary heart disease (figure 4). Perhaps

the simplest application of nuclear medicine is the bone scan. By

injecting a labelled phosphate compound which is taken up by actively

metabolising bone cells, we get an image which doesn't show the

detailed bone structure like ordinary x-rays, but instead maps the

activity of the skeleton. Why is that helpful? Well, any damage

to a bone, whether caused by injury, infection or tumour, will result

in increased activity by the bone cells in the area, as part of

the repair process. This will be seen as a 'hot spot' on the bone

scan, often long before x-rays become abnormal. For example, the

x-rays of the athlete's leg seen in figure 5 (right) were normal,

but the bone scan clearly shows the hot area due to a stress fracture.

PET imaging

PET imaging (positron emission tomography) is a new technique in

nuclear medicine which is rapidly gaining clinical acceptance. In

PET, isotopes which emit positrons are used. A positron is a positively

charged electron: it is anti-matter. As soon as a positron meets

an electron, which it does almost immediately, there's a lot of

them about, they annihilate each other with the production of two

gamma rays moving in directly opposite directions. These hit a ring

of detectors surrounding the patient and form an image. Some of

these isotopes are very short-lived, and can be used to label molecules

which play a central role in cellular metabolism; glucose, for example.

There is no space to go into PET in detail here, but it is having

a huge impact in oncology (cancer medicine) due to its ability to

detect small areas of active tumour, allowing early treatment (figure

6 - below), and in cardiology. It is also producing exciting developments

in the study of the brain, as it is possible to image changes in

brain activity in real time, as patients perform different tasks,so

that we can 'see' patients thinking.

 

- May 2005

About the Author

Consultant Radiologist