A Basic Overview of the Imaging Modalities Covered in BIO 335
Image grayscales and orientation
For healthcare students not familiar with imaging concepts
Radiographs (X-ray Images)
In the simplest terms, x-rays do one of two things when they encounter matter. They
either travel unimpeded through the vast empty spaces between the electrons and the
nuclei of atoms, or they interact. In other words, they either make it through or they do
not.
The more dense matter is, the less likely an x-ray photon will pass through it without
interacting at the atomic level. More x-rays will be absorbed in bone than will be
absorbed in soft tissue. As a result, the relatively few photons that make it through bone
will result in little exposure on a radiographic film, and the film will be light in that area.
More photons will make it through soft tissue, so the tissue around bone will be darker
due to the increased exposure to the film. Air, such in the lungs, will absorb very few
photons, so the air filled parts on a chest x-ray are very dark. The principle is the same for
digital radiographic equipment, although the gray scale may be reversed by the computer.
It is the difference in the number of photons that are absorbed from one tissue to another
that creates the contrast of densities on the image. As a result plain film or digital
radiographs display the following gray scale.
Dense bone = white
Concentrated contrast media in a vessel = white
Cancellous and compact bone = varying shades of light gray
Soft tissue = varying shades of dark gray
Fat = very dark to black
Air = black
Computed Tomography (CT)
CT uses x-ray photons to create sectional images the same as with radiographs up to the
point where x-rays exit the patient. Photons in a CT scanner are captured by detectors in
a ring around the patient, not x-ray film or a flat digital imaging plate. Nevertheless, after
the computer processes the data the gray scale is the same as for a radiograph or digital
image. In addition, CT is able to dramatically differentiate soft tissues densities, which
radiographs can not do. For example, on a radiograph of the skull brain tissue appears as
a homogeneous shade of light gray. On a CT scan many of the structures of the brain are
differentiated as varying shades of gray.
Since a CT scan is a digital image, it is possible to invert the gray scale of a CT, but this
rarely done.
Magnetic Resonance Imaging (MRI)
The physics of MRI is entirely different than that of x-ray interaction with matter. The
gray scale of an MRI image is resultant of the concentration of hydrogen in a part.
Because there are numerous methods to sample the MRI signal, the rules for the gray
scale are not as straight forward as they are x-ray absorption. However, the sensitivity of
MR is even greater than CT. Structures in the brain, for example, that are not visible on
CT show up nicely on MRI. MRI also allows imaging in multiple planes without
computer reconstructions. CT images are mostly limited to the transaxial plane because
of the orientation of the patient to the x-ray tube.
In addition, a contrast media is not necessary to demonstrate blood vessels (although
contrast agents are used in MRI). This is because that from the time the hydrogen in
blood absorbs energy, to when it dissipates that energy as the MRI signal, the blood has
moved. The demonstration of the vascular system is called Magnetic Resonance
Angiography (MRA).
The MRI images used in this course are T1 weighted. T1 images provide the best
identification of anatomy, and are frequently used for diagnostic examinations. For T1
images, the gray scale is as follows:
Fat = white
Cancellous bone = varying shades of gray, to white
Soft tissue = varying shades of gray
Dense bone = black
Air = black
CSF = black
Circulating blood = black
Other methods of sampling the MRI signal produce different gray scales. Looking at a T1
and T2 weighted brain scan side by side, it might appear as though the gray scale had
been inverted on the T2. The gray densities of the T1 brain tissue would be dark or black
on the T2, and the black CSF of the T1 would be white on the T2. This is not a result of
simply inverting the gray scale. On closer inspection it would be seen that some tissue
had not been inverted. T2 images are most often used to demonstrate a given pathology
that is known to best visualize using that sampling method.
Orientation of Cross Sectional Images
When viewing cross-sectional images there are two important conventions of image
display.
First, when viewing coronal images, the patient's right side is on your left as you look at
the screen. Regardless of what part you are looking at, imagine that the patient is in the
anatomical position, facing you.
Secondly, when viewing axial images, your orientation to the patient is as though you
were standing at the patient's feet, looking toward the head. Again, the patient's right side
is on your left as you view the screen.
Right and left must be marked on cross sectional images, but the images in this course
were cropped as close as possible to keep the file size small. As a result, the right and left
markers where clipped. Nevertheless, if you follow the above conventions, you will know
which side is right or left. In spite of these conventions, in real life there are times when
the orientation of the patient changes. Right or left markers are the most reliable
identifying factor.
For sagittal images, only unilateral parts are displayed on a single image. For example,
the cerebral hemisphere demonstrated on a sagittal section of the brain to the right of
midline, would be the right cerebral hemisphere. Unlike axial and coronal images, there
are no bilateral structures visible.