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Introduction

Basics of CT Imaging

The first CT scanner was developed in 1972 by Sir Godfrey Hounsfield. It wasn't until the mid to late 80's that CT was available initially in teaching hospitals for assessing stroke patients. Prior to this there was no way to discern ischaemic and haemorrhagic strokes ante-mortem. The CT scanner consists of an X-ray tube and on the opposite side of the cylinder where the patient lies is a set of x-ray detectors. Through a process of data acquisition the X-ray tube and detectors rotate around the patient and acquires a vast amount of imaging information which then undergoes image reconstruction. Between the x-ray tube and the detectors there is a loss of attenuation as it travels through structures and this is known as the attenuation coefficient which reflects the degree to which the x-ray intensity is reduced by the material. These values are scaled to give values known as Hounsfield units seen in the table below. The overall appearance can be altered by varying Window level and window width. Different windows can be used to look at different structures as shown below. Helical scans mean that there is continuous movement of the patient through the gantry whilst imaging.

Different Hounsfield units

MediumHounsfield UnitsAppearance
Air-1000Black
Fat-80 to -100Black
Water0Black
CSF +5Black
White matter +30Dark Grey
Grey matter +40Light Grey
Acute haemorrhage +70White
Bone+400 to +3000Bright White

CT is really 4000 shades of grey from -1000 for air to +3000 for bone

Different Windows W=Width L=Length

IndicationParameters
Brain window to best show white/grey matter differentiationW=80 L=40
Bone windows for bone pathologyW=3500 L=700
Subdural windows for small or isodense subdurals W=250 L=70

Clinically there are no real contraindications for CT if clinically indicated. Scans are incredibly quick nowadays. The only real difficulty is with agitated or confused patients in whom scan quality will be degraded with movement artefact. It may be occasionally necessary to intubated and ventilate a patient prior to scanning. The main downside is radiation exposure to about 2 mSV of exposure which is about 8 months of background radiation exposure.

Non contrast CT scan in Ischaemic stroke

Non contrast CT scan is the standard imaging modality for hyperacute stroke care. It is fast, cheap, accessible, and very sensitive for haemorrhage and there is no problem with pacemakers or monitoring equipment. Claustrophobic or monitored patients can be scanned relatively easy. Done early many scans will be normal despite significant clinical findings. Stroke is primarily a clinical diagnosis and not a radiological diagnosis.

Clinical Indications for urgent CT i.e. within 1 hour of arrival at hospital
  • Anticoagulant treatment, a known bleeding tendency
  • Depressed level of consciousness
  • Unexplained progressive or fluctuating Symptoms
  • Papilloedema
  • Neck stiffness or fever
  • Severe headache at onset

Caution : What may be missed on a CT scan
  • Isodense Subdural haematoma
  • Brainstem or Posterior fossa pathology though more recent scanners are much improved
  • Low attenuation lesions near skull missed 'beam hardening'
  • Suspected Haemorrhage over 2 weeks ago (get GRE MRI)
  • Multiple sclerosis plaques

Look at as much brain imaging as possible, the ones that you request and more. The key is to see a wide variety of normality and to build up some pattern matching skills and experience in identifying important structures and lesions. You will have an advantage over the radiologist who has only the clinical details on the card whereas you have the patient.

Imaging Anatomical Landmarks

Make sure you understand the terms axial, sagittal and coronal. Axial is simple horizontal slices from above down. Sagittal is in the same plane as cutting the brain in the midline or parallel to this (parasagittal). The falx cerebri that separates the brain hemispheres lies in the sagittal plane. Coronal is the plane of my daughter's hair band. A bit like a tiara or 'corona'.

It is vital to have a good inner representation of the cerebral vessels as the enter the skull and join the circle of Willis and the circle of Willis as well and the position of the vessels. A few simple points will help greatly.

The circle of Willis (COW) is at the level of the midbrain. Find the midbrain and then look for the vessels. This is a good place to start looking for the hyperdense MCA as the MCA leaves the COW laterally within the sylvian fissure.

Next: >> Introduction to Stroke Imaging Part 2


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