Tuesday, 16 January 2007

Have you ever asked yourself the question why we do CT scanning?

Well, we get very sharp and accurate pictures that allow us to determine, in some instances, the possible cause of a particular medical or surgical problem.

I am not a surgeon and therefore, it would be inappropriate for me to comment on imaging studies for surgical problems in any detail other than generally accepted concepts and as a result , I shall mainly concentrate on imaging modalities for medicine.


The CT head scan has revolutionised medicine in being able to see directly inside the cranium and to be able to identify a whole host of problems such as cerebral infarction/bleeding, tumours, raised intracranial pressure etc...

It is sometimes the only way to find out a problem.

I regularly hear of patients admitted with some type of confusion or reduced conscious level who, for example, may have a pneumonia or urinary tract infection, or an obvious acidosis / hypoxaemia or electrolyte disturbance. With the diagnosis being quite clear in most instances I have seen, on occasion, the patient is also put through the scanner to check their brain.

All patients with reduced consciousness should have a full physical examination first which includes a thorough neurological examination to see if the patient has any focal neurological impairment. If present, I would whole heartedly agree that a CT scan should be performed.

However, if there are no focal neurological signs and another causes of impaired conscious level can be identified, e.g. UTI causing an acute confusional state and with no signs of meningism, then performing a CT scan, in my experience, is usually normal and offers no additional value in diagnostic terms and the physician will have exposed the patient to radiation unnecessarily.

For example, hypercarbia as part of type 2 respiratory failure can result in cerebral oedema, but it also causes papilloedema. This is a physical sign that can be elucidated by fundoscopy which my provide the clue that there is cereberal oedema.

Moreover, I often hear that a patient needs a CT head scan prior to lumbar puncture. If there are no focal neurological signs and that includes NO PAPILLOEDEMA of the fundi (in the eye), then a CT head scan may not be required. This was confirmed in a paper in the New England Journal of Medicine several years ago (Hasbun et al, Computed tomography of the head before lumbar puncture in adults with suspected meningitis. NEJM 2001; 345: 1727-33)

However, as I understand it, fundoscopy is not routinely taught at medical school in Japan, which is quite different to the UK where it is taught. I also teach fundoscopy examination here in Japan, and when there is an emergency case in ER, on occasion, I am usually called to perform the fundoscopy examination. Hence, the only way to try and identify raised intracranial pressure (rather than ruling it out) is to perform a CT head at most institutions.

Fundoscopy is a dying art but it can potentially save a patient's life. A CT scan cannot always determine if there is raised intracranial pressure (false negative), and the same is true for an MRI scan. However, if fundoscopic papilloedema is present, then the patient has raised intracranial pressure until proven otherwise, and a lumbar puncture should not be performed as to do so might result in brain stem herniation( a good reference is Diederik et al, Community-Acquired Bacterial Meningitis in Adults. NEJM 2006. 354: 44-53).

The latter paper talks about repeated lumbar puncture or placement of a temporary lumbar drain to 'effectively reduce' intracranial pressure, although it does not allude to the mortality of such patients treated or the rate of cerebral herniation and as far as I am aware, this is not standard treatment.

I have personally seen a case of a patient with encephalitis whose only complaints were headache, fever and memory loss and papilloedema was identified by me, and when the CT was reviewed, there were indeed signs of raised intracranial pressure and cerebral oedema.

Hence, there must always be a good reason to do a CT head rather than just to screen the patient. If you suspect a brain tumour then a CT is reasonable, but if the cause of the confusion is due to a severe pneumonia, then performing a CT head scan with a normal neurological examination, might seem over intensive especially if there is no real clinical indication.

At the end of the day, the whole matter comes back to History and Physical Examination, as they are your guide to the cause of the problem (in most cases).

In order to consider doing any imaging, you have to consider what you are going to do with the result! For example, does every patient with a clinical catastrophic stroke with a GCS of 3/15 really require a CT head scan if the history and physical examination are consistent with the diagnosis? Most physicians would go ahead and scan such patients, if only for medical defence purposes, but a patient with a very low GCS usually portends a grave prognosis.

Some physicians would argue that to do a CT head scan in such circumstances could pick up an intracerebral problem that could be treated. However, in a terminal patient, it is sometimes far more humane to optimise their palliative therapy and make the patient as comfortable as possible in their last hours, days or weeks rather than putting them through perhaps unnecessary tests, treatments or surgery which the patient may not have wanted or in fact needed.

It is essentially, a case-by-case decision as no one patient is the same as the next, but the physician should always bear in mind 'am I going to gain anything from doing this test?'

So, when doing a scan, one has to take into account the social circumstances of a patient, their pre-morbid condition, their wishes, the severity of their present condition and whether by doing the scan, it will actually change your management of the patient.

CT Chest Scans

Pneumonia seems very common here in Japan. In the UK, most pneumonias are seen in the Winter/Spring months and very few are seen at other times. From my short experience here, the same is not true and pneumonia is common all year round with a higher number in Winter.

I have seen many chest x-rays (CXR) with old tuberculosis (TB) usually in the elderly age group. The vast majority have completely inactive disease. Without the usual symptoms of cough, fever, sweats, weight loss etc is unconvincing to me to perform a CT chest scan, unless there is CXR evidence of active disease.

Pneumonia can usually be diagnosed on CXR WITHOUT the need for a CT chest scan. However, again I have seen at various hospitals the acquisition of CT scans for uncomplicated and quite obvious pneumonias and I see this as unnecessary.

History and Physical should at the very least provide enough information to make a diagnosis of pneumonia (except perhaps atypical pneumonia, where chest sounds may appear normal) and CXR can usually suffice to confirm the diagnosis. If the suspicion is strong for an atypical pneumonia, with a normal CXR, then I would be the first to say 'get a CT'. However, the 'reflex' to always obtain a CT chest for pneumonias is not necessary as it usually does not change the management in the vast majority of cases I have experienced.

The patient is going to usually receive a Broad-spectrum antibiotic unless there is definite identification of the organism and sensitivities are known, in which case, a narrow spectrum antibiotic would be used. Basically, antibiotics are going to be given and hence, doing a CT in an uncomplicated pneumonia does not help treat the patient.

Blood cultures, sputum examination, urinary antigens may be far more helpful than a CT.

Repeat CXR after clinical resolution, normally at 4 weeks, is the usual way to detect any unusual features that come to light such as the hiding malignancy, and it is at that time that a CT should be done, if of course, there were no sinister signs of an underlying disorder on admission.

If on the other hand, the patient's hypoxaemia is out of keeping with a small area of consolidation on the CXR, then one must always consider a complicating Pulmonary Embolism, and the modality of imaging would be a SPIRAL CT or the new technique of MR pulmonary angiography, rather than a Plain CT. Hence, when thinking of imaging, one must always consider what one is looking to diagnose or exclude. Doing a Spiral CT will aide in the diagnosis of PE and hence, it would alter the patient's treatment, for example, the patient would need heparin added or thrombolysis or thromboembolectomy.


The abdomen was once described by my best friend as a 'box of magic tricks', as was told to him by his General Practitioner. Clearly, the abdomen being only partially radiolucent to Xrays is difficult to image.

I think that formal imaging of the abdomen requires an even higher level of suspicion of a serious problem, because doing a CT exposes the patient to a higher level of radiation.

History and Physical Examination may be helpful, but an ultrasound performed by an experienced doctor or ultrasonographer specialist may provide the answer, thereby negating the need for a CT scan.

For example, I have been informed that at another hospital, a patient was admitted with a history of fresh rectal bleeding which was consistent with a clinical diagnosis of inflammatory bowel disease. Whereas the patient should have been prepared for a flexible sigmoidoscopy or colonoscopy, the patient was put in the CT scanner, and the report confirmed that it looked like inflammatory bowel disease. The patient still needed a colonoscopy in any case, which is the Gold Standard Test, so was the CT necessary? No.

The usual way of imaging is an abdominal X-ray to rule out Toxic Megacolon or perforation. Only if perforation is suspected, but not detected on Xray, would a CT be required, although a traditional decubitus abdominal film might also provide the answer. Thus, CT should not be routinely performed in such cases. In cases where there is straight forward peritonitis, a CT can be a very helpful tool.

If on the other hand, the physician was considering a bleeding divertiulum or an abscess, for the former problem, a colonoscopy may still provide an answer (bearing in mind not to perforate an diverticulum) or ultrasound scanning the area and then draining percutaneously. If colonoscopy was considered too dangerous, then a barium enema can also provide the answer and although several pictures of the abdomen are taken, it provides less radiation than with a CT.

Patients with an appendicitis normally have a good history and in advanced cases, a physical examination consistent with the diagnosis. Performing an ultrasound may reveal the swollen appendix or ' free fluid'. Watching the patient, the temperature, pulse, blood pressure should be the reason for surgical intervention or conservative management, and doing an abdominal CT, although producing very fine pictures, is exposing the patient to radiation, which in a young female patient is of great concern in respect of possible future reproductive problems.

Finally, you must be worn out reading today's blog, but fertile female patients requiring any abdominal imaging involving X-rays MUST be pregnancy tested. Although the patient may say that are not pregnant or there is no possibility of being pregnant, there are some cases when the patient IS pregnant, and proceeding with a scan in the absence of testing the patient is negligent. Pregnant women can shed some of their uterine lining in early pregnancy which may look like a period and so, it is always best to test anyway, with the patient's consent of course.

You should always explain to the patient the risk of exposure to, for example, a full body CT scan.

David Brenner and Carl Elliston of Columbia University for Radiologic Research have been able to calculate the life time risk of developing cancer due to radiation from a single body scan which is 0.08%. That is equal to one in every 1,250 people who has a full body CT will develop cancer as a result. That is quite a worrying figure!

Extrapolating this further, a 45 year old person having a full body CT every year for 30 years would cause one in 50 to develop a radiation-induced cancer.

If you are interested you can read the article for yourself: Brenner, J. J. and Elliston, C.D. Estimated radiation risks potentially associated with full body CT screening. Radiology. Vol 232 (September) P 735-738. 2004.

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