Princess Margaret Hospital for Children

Transcription

From simple X-ray photographs to computer images produced by magnetic resonance imaging - there are a whole range of different techniques available to doctors for looking inside our bodies. How do they decide which one is most appropriate for a particular patient with particular symptoms? Well, that's just one of the questions we'll be addressing here, at the busy Princess Margaret Hospital in Swindon. Let's start by looking at the physical mechanisms involved. I'm joined by Alun Davies, who's head of medical physics here at the Princess Margaret Hospital. Alun, would you like to just tell us something about there range of techniques you have available here in the hospital? Well, the most common of all the medical imaging techniques is still the conventional X-ray. We have nine main X-ray rooms in this hospital. Additionally, we have a number of ultrasound sets, we have a gamma camera for nuclear medicine imaging, we have a CT scanner, that's a computer tomography scan, and an MRI unit, a magnetic resonance imager. Great, well shall we go have a look at some of this modern technology? Let's have a look in the MRI and CT unit. Right. What's the purpose of this - security barrier, is it? Yes, on the other side of this barrier, both the Ct and the MRI units, both of which have their attendant risks, The risk associated with CT can be well confined within the scanner room itself. On the other hand, the very strong magnetic field of the MRI scanner can potentially cause effects outside of the room, so, this is the first level of access control for this unit. Anyone who goes beyond this barrier will be closely supervised, and also required to fill in a fairly detailed questionnaire to ensure that they wont be affected by the strong magnetic field. But, as we've already been through that with you, we can go on. Thank you. So, this is where patients come for CT and MRI scans - shall we start with CT? Would you like to explain to me how these images here are formed? OK, we've got two video displays here. One is a conventional close circuit television display, which allows us to monitor the patient during the examination. - Nobody's in there at the moment. - No, that's right. The larger display has the cross section image that the CT scanner produces. This is one of a head. The CT scanner uses an X-ray tube, similar to those used in conventional X-ray sets, but this X-ray tube rotates around the patient, and detectors on the other side of the patient pick up the signal and from the whole data that's acquired during a single rotation,. a slice of the patient can be acquired, and displayed on the screen. So, CT scans are using X-rays, ionising radiation? That's right. Now, gamma rays are another form of ionising radiation, how do you use those to do medical imaging? In nuclear medicine we image the distribution of radio nuclides within patients. The radio nuclide will be injected generally into the patient, and depending on the compound to which it's attached, it will spread throughout one of the organ systems, or perhaps more than one organ system of the body. A gamma camera is capable of imaging that distribution. The important thing to, or one of the important differences to note between something like CT and nuclear medicine, is that in CT, we're imaging anatomy. In nuclear medicine we're looking at the function of the organs, rather than the anatomy. Let's move onto techniques that rely on non-ionising radiation. Now, this is the MRI machine. These images look very similar to the CT images we saw earlier on. Yes, both CT and MRI produce very high quality cross sectional images through the body, but that's really where the similarity between the two techniques ends. So, how are these images produced? well, the patient here is placed inside a very large static magnetic field, and as they're placed in that field, the protons which make up the nuclei of the hydrogen atoms will align along the magnetic field. As they align, they will also process around their own axis. The frequency of that procession is going to be determined by the magnitude of the static magnetic field. If we were to now switch on a radio frequency source, tuned in precisely to the rate of that procession, then a resonance effect will take place, and the protons will absorb the radio frequency energy. Switching off the source of the radio frequency energy, will then allow the protons to give up their energy, again as a radio frequency signal, and that can be detected by a sensitive coil or aerial, and that signal gives us a measure of the proton density, and information about the local chemical environment of the protons. So, the signal you get back, depends on the environment of the hydrogen atoms, is that right? That's right. As well as the density of the hydrogen atoms, the local chemical environment which they find themselves in, will also affect the size of the signal we get from the system. So, you get a different signal depending on whether it's in fat or water? That's exactly right. They are wonderful images. They are very nice, yes. So, we've looked at CT and talked about gamma cameras, and MRI, and all of those involve electromagnetic radiation, are there any other kind of waves that we can use to get images? Ultrasound is used for imaging. Shall we have a look at that? So, this is your ultrasound machine? This is one of the Doppler ultrasound machines we have in the hospital. Now, unlike the other techniques we've looked at, ultrasound employs no ionising radiation, and has a wide range of applications within medicine. the one that most people are familiar with, of course, is antenatal screening for pregnant mothers, and used for sizing the foetus and monitoring the progression of the foetus, but it has a wide range of applications beyond that - in the cardiac area, for looking at livers and in this case we have a Doppler ultrasound scan of a carotid artery. I could demonstrate how we do that using the transducer we've got here, which acts both as a transmitter and a receiver of ultrasound energy. It would simply a question of placing the transducer on your neck with a simple coupling gel to ensure that we don't lose any of the high frequency ultrasound energy, and with a bit of careful positioning, we could get an image similar to the one we've got on the screen. So, what's happening there? The ultrasound is going into my neck, and being reflected back out again? That's right. The ultrasound is reflected from any boundary within the body, and the time it takes for the reflected ultrasound energy to get back to the transducer is used to calculate the depth of that boundary within the patient. And therefore, using a large number of transducers which make up the array, an image of the structure can be formed. What about using the Doppler effect in ultrasound? I believe that's possible. That's right, yes. On the display here we have a demonstration of that. The colours represent. the flow of the blood If I start the tape going... ..you can see that the blue colour is showing blood flowing towards the transducer, and the orange, flowing away from the transducer, so we can see the transducer is just fixed in the centre here, and we're getting no Doppler signal directly underneath it.