Basics of How to Read Scans

8/14/2019 Basics of How to Read Scans

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Basics of Neuroradiological Imaging: Brain MRI or CT

Modern technology has given us the tools to view the living brain and skull in some detail. Inaddition, the arteries and veins of the brain and of the meninges can be visualized by mappingthe movement of blood through these vascular structures. The resultant images are powerful

tools to use in the diagnosis of the neurologically impaired patient.

Radiography (x-rays)

A plain radiograph of the head or spinal column provides hardly any information about thenormal anatomy of the brain or spinal cord - the so-called soft tissues. X-rays are best used for

hard tissue such as bone. Sometimes, displacement of parts of the brain can be visualized with an

x-ray if a radio-opaque substance such as calcium is present in the brain. For example, the pineal

gland often calcifies in adults and displacement of the calcified pineal gland can revealdisplacement of midline structures.

With contrast media, x-ray images can be more informative. In angiography, a radio-opaqueliquid injected into one of the carotid or vertebral arteries shows the branches of the arteries andveins. The chief value of this technique is for detecting arterial disease (occlusion, stenosis,

aneurysm) or displacements of blood vessels by lesions such as tumors. Computed tomography

(CT) has replaced the older techniques of pneumoencephalography and ventriculographywhere air was injected into the ventricular system or cranial cavity and then visualized by X-ray.

MRI and CT

The most routinely used methods to image the brain and skull are computed tomography (CT)and magnetic resonance imaging (MRI). CT is especially useful in visualizing the skull and the

brain in the early stages of subarachnoid or brain hemorrhage (such as a stroke.) On the other

hand, MRI shows brain anatomy in elegant detail, as well as cisternal relationships, cranialnerves, and a wide variety of clinical abnormalities. An MRI is the study of choice for tumor,

multiple sclerosis, and ischemic stroke. A contrast medium, gadolinium, is often added to

further evaluate tumor and abscess.


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Magnetic resonance angiography (MRA) is a MRI technique that visualizes arteries and veins by

measuring the velocity of flow in these structures.

Knowledge of cranial cross-sectional anatomy is very important for analyzing a head MRI or

CT. Once the normal structures are identified, abnormalities can be detected and a diagnosis may

be possible. Symmetry is an important concept in anatomy and is almost always present in anormal head MRI and CT unless the patient was incorrectly positioned with the head cocked at

an angle.

Comparison of brain MRI and brain CT images

Figure 1: LeftAxial MRI of brain; Right Axial CT of brain.

Obtaining and viewing images of the brain and skull

Patients lie on their back (supine) for imaging of the brain or spinal cord and the surrounding

bony structures of the skull and vertebral column. In this position, the dorsal surface of the

brainstem and spinal cord and the caudal aspect (occipital pole) of the cerebral hemispheres face

down. The ventral surface of the brainstem and spinal cord and the frontal pole are face up.

Images of the brain are commonly made in coronal, axial (horizontal), and sagittal planes.

Coronal scans are viewed as though you are looking the patient in the face, whereas axial scans

are viewed as though you are standing at the patients feet and looking toward his head while the

patient lies supine in the machine. Axial scans, in other words, show the cerebral hemispheres

from anterior (the more superior portion of the hemisphere), with the patients frontal area and

orbits at the top of the image and the occiput at the bottom. In both coronal and axial views,

the patients left side is tothe observers right.


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Figure 2. Left - patient in CT scanner; Right Patient in MRI scanner.

PLANES

MRI and CT images are commonly viewed in three planes: axial, coronal, and sagittal.

Figure 3: Planes of section


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Sagittal Coronal Axial

Figure 4. MRI of each plane of section

Sagittal - this slices the brain from the side, i.e., right to left.

Coronalthese are slices that look at the brain face first, similar to cutting a loaf of bread fromfront to back.

Axialthese are horizontal slices viewing the brain from the top down, like layers of a cake.

Sequence- A sequence sets the MRI scan to certain parameters, and collects information under

those parameters. Different sequences are best at showing different types of disease. Sequences

can include T1, T2 and contrast.

Figure 5. MRI sequences.


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Magnetic Resonance Imaging (MRI)

How MRI works (very, very briefly)

Protons (hydrogen) constitute a large proportion of body tissue. These atoms have electrons that

spin around the nucleus and as they do so, they induce an electrical current that creates amagnetic field. The protons are aligned randomly because of the changing magnetic effects on

each other. But when they are exposed to a powerful magnet, they stop pointing randomly and

align themselves in parallel with the external magnetic field but at different energy levels. Whenundergoing an MRI examination, the patient becomes a magnet, with all the protons aligning

along the external magnetic field and spinning at an angle with a certain frequency.

During the MRI procedure, short bursts of radio waves (electromagnetic waves) are sent into the

magnet containing the patient. These bursts are known as a radiofrequency (RF) pulse. The

energy of the RF pulse is absorbed when the RF pulse matches the frequency of the spinning

proton. This phenomenon is called resonance and is the resonance in magnetic resonance

imaging. But further magical manipulation, the computer generates two main types of MRIimage, T1-weighted and T2-weighted (more on that below).

Shades of Gray Matter

The routine MRI is not in color; instead, an MRI shows pixels from white to black, with many

shades of gray in between. The various shades of gray are described in terms of their signal

intensity.Low signal intensitymeans dark, and high signal intensitymeans bright. Lightercolors are said to have higher signal or to be more intense. Commonly used terms for intensity

are hyperintensityor hypointensity. These signify a region that is lighter or darker than

expected. This may represent a lesion, but is sometimes due to artifact.

Figure 6. Can you find the regions of hyperintensity? Which areas of this image show very low signal intensity?

This patient has multiple sclerosis (MS).


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EnhancingSometimes MRI scans are done using contrast.This involves intravenous injection of a contrastagent, usuallygadolinium. The contrast will increase, or enhance, the signal of certain types of

lesions. For example, an active multiple sclerosis lesion will enhance under contrast, whereas

inactive multiple sclerosis lesions will not enhance.

Figure 7. MRI of patient with left-sided stroke. Image on the left has no contrast. Iimage on the right with

gadolinium enhancement.

MRI Weighting: T1 and T2

The most commonly used techniques for MRI imaging are called T1-weightedand T2-

weighted(if you are interested, also look up Flair and Diffusion-weighted). These different

techniques emphasize the normal brain or potential lesions in different ways. The different types

of MRI represent computer manipulation of the data with each technique serving to visualize or

emphasize certain aspects of anatomy or pathology.

T1 weightedimages are useful for brain anatomy (brain tissue or parenchyma.) The brain

appears medium gray and CSF is dark gray, and air is nearly black. In certain MRIs, it is possible

to distinguish gray matter from white matter. Most tumors appear dark, with low signal intensity.

Fat has high signal intensity (very bright) on T1. Gadolinium contrast added to the T1 may light

up a tumor or abscess. If a lesion has dark signal on T1, and bright signal with gadolinium, it is


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a contrast enhancing lesion. The enhancement may be the same throughout the lesion, making

it homogeneously enhancing.

gray matter

fluid-filled space

white matter

air-filled space

fat

Figure 8 T1-weighted sagittal MRI, no contrast enhancemment. Look for areas of high-fat-content where tissues

appear as bright areas of high signal intensity (hyperintense). Look for areas of high-water-content tissues which

appear as dark areas of low signal intensity (hypointense).

T2-weighted images are great to evaluate CSF spaces, which appear bright white. Brain appearslight gray. The T2 sequence is used to check the size and symmetry of the ventricles, and to look

for bright signal of edema surrounding an aggressive tumor or a subacute stroke or hemorrhage.

Figure 9. Left - T1-weighted MRI. Note that the ventricles are black. Right T2-weighted MRI. Note that the

ventricles are bright white.

Also the T2 images are used for multiple sclerosis, with its characteristic periventricular whitematter changesbright splotches around the lateral ventricles. The bright signal is the scarring ofthe myelin sheath in the white matterthe sclerosis shows up on T2 images.


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Figure 10. Left - T1 image - no contrast. A tumor is seen on the onpatients left side. RightT2-weighted image,

no contrast. The tumor is emphasized. Note on T2 images, high-fat-content tissues appear as dark areas of low signal

intensity (hypointense while high-water-content tissues appear as bright areas of high signal intensity (hyper-

intense)


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Computed Tomography

CT is an x-ray imaging technique that measures the effects that tissue density and the varioustypes of atoms in the tissue have on x-rays passing through the tissue. This technique is

valuable in clinical diagnosis because the density of many cerebral lesions is greater or less

than the density of normal brain tissue.The higher the atomic number, the greater the abilityof the atom to attenuate, or stop, x-rays. The computer transforms the attenuation intensities intonumbers that represent values found at all points in the volume of the tissue slice. The values are

expressed in Hounsfield units (HUs). HU values are used in an arbitrary scale where bone is

specified as +1000 (very white), water as zero, and air as -1000 (very black). Using this scale,HU or CT numbers represent specific shades of gray for each of the various points located in the

slice.

Figure 11. CT gray scale [gradient bar]

Some approximate numbers:

bone +1000calcification +150 to +200

blood +100

soft tissue +20 to +60brain +30

fluid 0 to +15

CSF +5

water 0fat -20 to -200

air -1000

Skull (bone)

Ventricle (fluid-filled space)

brain

Figure 12. Axial CT scan.


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CT is a fast and accurate method of detecting hemorrhages such as recent subarachnoid

hemorrhage. An acute subarachnoid hemorrhage in a noncontrast CT scan appears hyperdense(white) in contrast to the subarachnoid spaces and cisterns, which normally are hypodense

(dark).

Figure 13. LeftCT scan showing subacute subdural hematoma (arrowheads). Note the compression of gray and

white matter in the left hemisphere due to the mass effect. Right CT scan showinghigh density blood

(arrowheads) that fills the sulci over the right cerebral convexity in this subarachnoid hemorrhage.

A CT scan can be enhanced by using iodinated contrast material injected intravenously

followed by the CT examination. Iodine has a large atomic number and attenuates x-rays. As a

result, vasculature is visualized as hyperdense (white) structures. The contrast material may alsoenhance neoplasms or areas of inflammation, because the contrast agent leaks from the vesselsinto the cellular spaces owing to a breakdown of the blood-brain barrier.


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Self-study summary:

1. What are the 3 commonly used planes for brain imaging?

2. Describe the most common (advantageous) uses of each neuroradiological technique below:

Xray:

MRI:

CT:

3. Which of the 3 techniques shows the most detailed brain anatomy?

4. Which technique is most closely related to the plain xray, MRI or CT?

5. T1 and T2 weighting are related to which imaging technique?


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6. On the scale between darkest and brightest, make check marks the charts below to indicate

the relative intensity of each substance listed:

T1weighted MRI:

DARKEST------------------------------------ ----------------BRIGHTEST

Fat

CSF

Brain

Tumor

How would the tumor appear if gadolinium was given intravenously?

T2weighted MRI:

DARKEST------------------------------------ ----------------BRIGHTEST

Fat

CSF

Brain

Tumor

Another disease state that T2 is commonly used to diagnose is _________________________.

Basics of How to Read Scans

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