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The Basics


Left and Right
The first step in reviewing radiology images is knowing which side is left and which side is right. The images are displayed in a standard fashion.  When looking at a frontal image, the image is oriented as if you are looking at the patient.  In this example of a Chest X-Ray, notice how the right side is labelled with and R.  This same convention is applied when reviewing an axial or transverse image from a CT or MRI, imagine that the patient's feet are directed towards you.   

Imaging Planes
There are three primary imaging planes that are utilized in neuroimaging: 
- Axial plane: Transverse images represent "slices" of the body  
- Sagittal plane: Images taken perpendicular to the axial plane which separate the left and right sides (lateral view)
- Coronal plane: Images taken perpendicular to the sagittal plane which separate the front from the back. (frontal view)

CT vs MRI: How can you tell the difference?
It is important to recognize the type of imaging study you are reviewing.  Is the study a CT or an MRI? 
CT: Computed Tomography 
Imaging Plane: CT images are acquired only in the axial plane.  The axial data set can then be used to reconstruct images in other planes, sagittal and coronal are the most common. 
Windows: Images can be "windowed" to bring out different structures, which is a post processing step.  For neuroimaging we primarily review images using a "brain window" to look at the parenchyma and ventricular system.  We use a bone window to evaluate the osseous structures and air filled cavities including the paranasal sinuses and temporal bone.   
White and Black: You will notice that air within the sinuses is black, the brain parenchyma has a gray appearance and the skull is bright white
MRI: Magnetic Resonance Imaging 
Imaging Plane: MRI can be acquired in any plane, not just axial
Sequences: In MR, each different type of image is referred to a sequence.  The primary MR sequences include T2, T1, T1 with contrast, Diffusion and FLAIR.  Each sequence has to be acquired separately, which means that an MRI will take a lot longer to perform than a CT.  
White and Black:You will notice that the same structure may be bright or dark depending on the type of sequence; CSF for example is bright on T2, but dark on T1. The tissue and imaging characteristics are a lot more complicated than CT. 
One might confuse the head CT and the T1 MR sequence.  Both images display a structure that is white in the periphery and has CSF that is dark.  On MR this white structure represents the subcutaneous fat and on CT the bright structure is the skull.  The way to tell a head CT from a brain MR is in evaluating the brain parenchyma.  
1) MRI has superior soft tissue anatomic resolution than CT
2) The distinction between the gray matter and white matter is much greater with T1 and T2 MR than with CT
3) The color of the white matter on T1 MR is lighter than gray matter.  This relationship is opposite on CT: notice how the color of the white matter is darker than the gray matter.  This difference is related to the presence of fat containing myelin in the white matter.