Imaging

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Info bulb.pngBy the 23rd Century imaging has become simple, Dr McCoy uses a single scanning device to show every part of the body in every way. Life as we know it though remains more complicated. How do we pick the technology or imaging to request?

Contents

Introduction

Imaging technology and its application is essential in medicine. Still photography, video and motion photography, ultrasound, endoscopic, nuclear medicine and radiology techniques are extremely useful in aid diagnosis and medical interventions that would be done with difficulty if at all without them.

Requesting

Most hospitals require cards to be filled out to request that the radiology department perform an imaging investigation. Some trusts now have computer based sytems. Who can do this is governed by a combination of IRMER and local written procedures. The main point is that an x-ray exposure must be justified and authorised. Practioners (e.g. radiologists) may give this authorisation or the trained operator who will perform the procedure (e.g. radiographers). They must be sure that the procedure is justified based on the request that you send. This means that requests must be filled with care and phrases like "Because my reg wants it" just don't wash (see Tips for requesting imaging).

X-rays

Computed Tomography (CT)

CT scanning uses ionising radiation to produce cross sectional images through the body.

Magnetic Resonance Imaging (MRI)

MRI uses similar mathematical and processing principles to CT to produce images from very different and complex radio frequency data. The data arises from relaxation of atoms with unpaired spin aligned by a magnetic field and subjected to radiofrequency radiation at different angles and polarisation. Understanding the physics is challenging. It generally requires more acquisition time than CT, and can not be used in patients with contraindications to exposure to high magnetic fields such as permanent pacemakers. The most common form of MRI is proton resonance imaging, which is assumed to be the case unless qualified by terms such as 31P MRI. The protons imaged are effectively those in water molecules, and as a result MRI can give more detailed structural information than CT. This is illustrated by its ability to show the focal demyelination of multiple sclerosis. It can delineate inflammatory processes and most tumours better than CT. Techniques such as diffusion weighted imaging (DWI, which images the Brownian motion of water) can quickly demonstrate tissue ischaemia or tumour associated oedema and echo planar imaging (EPI) can freeze body motion to a 100 millisecond time frame. Magnetic resonance angiography is advancing rapidly with many competing techniques and potential contrast media of which gadolinium chelates are presently the most common used. Functional MR can look at biochemical processes.

Ultrasonography

Ultrasonography uses sound waves (above the frequency heard by humans) to generate an image, and therefore does not involve the use of ionising radiation. It is particularly useful for looking at solid organs within the body.

The frequency of the probe used will depend on the area being scanned. Frequency being inversely proportional to wavelength the higher the frequency used in the probe the greater the spatial resolution but the lesser the depth of tissues that can be imaged. As a rule therefore, higher frequency probes tend to be used for areas which are superficial and require greater detail eg: musculoskeletal work, whereas areas which require greater depth of ultrasound penetration such as the abdomen will require a lower frequency probe. On this basis it becomes apparent why abdominal ultrasound in particular is more useful for imaging slim patients than obese ones, where it can often be near impossible to obtain a diagnostic scan! It also explains why ultrasound is such a useful modality for imaging children.

Echocardiogram

The use of ultrasound probes to image the heart. Comes in two main forms: trans-thoracic (Probe is placed on the chest wall) and trans-oesophageal (probe is placed within the oesophagus).

Obstetric Imaging

Ultrasound is believed to not be hazardous to the foetus, but common sense suggests minimising exposure and power. Imaging may be transabdominal or transvaginal.

Vein Location

Detection of venous thromboembolism

Nuclear Scans

Positron emission tomography (PET) scan

see Positron emission tomography

Single Photon Emission Computed Tomography (SPECT)

see SPECT.

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