The Basics of Neuroimaging

G. Hathout, M.D.

Professor, Division of Neuroradiology UCLA Dept. of Radiology

In the old days (e.g. 1970), neuroradiologic diagnosis consisted of “X-ray radiography”:

Plain films


Angiography, where masses in the brain were imputed by their effect on the cerebral vessels (i.e. “round-shift” or “square-shift”).

Problems with the Old Days

X-rays take in a large field of view.

They collapse 3D structures into a 2D projection.

There is a superimposition of structures (bone on soft tissue).

There is insufficient tissue contrast between different brain tissues on plain film.

These factors made it impossible to visualize the brain in any real sense.

CT: The beginning of the Modern Era

Developed by Sir Godfrey Hounsfield in 1972 (Nobel Prize!) Represents a major advance over plain radiographic methods Still an X-ray technology, but …

Uses a highly collimated x-ray beam to image the patient slice by slice

A narrow X-ray beam is passed through the slice with non-attenuated x-rays registered by a detector. As this apparatus rotates, X-ray attenuation is
measured at multiple different angles.

These different projections are mathematically analyzed to break up the slice into small rectangular portions called pixels (picture elements), with an
X-ray attenuation coefficient assigned to each pixel.

Therefore, we can actually see the different components of the brain, not collapsed on each other.

. CT overcomes the two main limitations of plain film:

Images patient in slices

Solves problem of superimposition of structures.

Remember: CT measures linear X-ray attenuation of each pixel, which depends mostly on electron density within that pixel.

Higher electron density = higher X-ray attenuation = Bright on CT

Ct is able to visually distinguish tissues based on different X-ray attenuations

The X-ray attenuation coefficients can be quantified into units called Hounesfield units (HU):

Human eye can detect tissue differences of about 4-6 HU.

CT can distinguish soft tissue (brain), fluid (ventricles and CSF), blood, fat and calcification (pineal gland, cysticercosis, skull).

Typical spatial resolution: 0.5 mm in 256 x 256 pixel image.

CT has come a long way from first generation scanners (grainy with 3-4 mm resolution and 80 x 80 pixels).

Examples of what CT can do: refer to slides.

The development of CT contrast: iodine-based contrast detects a leaky blood-brain barrier. Iodine has a high atomic number (53), therefore is bright on CT.
Normal brain does not enhance. Vessels, tumors and infections enhance.

Magnetic Resonance Imaging: The star of the Modern Era.

A completely different imaging technique developed in the late 70’s to early 80’s. Images hydrogen protons based on their magnetic properties (hydrogen
protons have spin, and therefore a “magnetic moment,” making them behave like tiny compass needles). The physics and math behind MRI is quite complex, but
we can think of MR imaging as three “easy” steps.

Bring hydrogen protons into equilibrium with a big magnetic filed (that is the magnet in the MRI scanner). Typical imaging is at 1.5 Tesla. 1 Tesla is
10,0000 Gauss. Earth’s magnetic field: about 0.5 Guass!

Disturb the equilibrium (done with radiofrequency pulses).

Allow the system to “relax” back to equilibrium

As the system “relaxes” back to equilibrium, it does so along two pathways: spin- lattice relaxation and spin-spin relaxation (we won’t worry about what
those are now). The speed of relaxation varies for hydrogen protons based on their chemical environment (hydrogen in water, fat, protein, etc). Relaxation
is governed by exponential time constants.

For spin-lattice relaxation, the time constant is called T1, and for spin-spin, it is called T2.

Protons always spin, or precess, around the main magnetic field at a characteristic speed called the Larmor frequency. At equilibrium, protons are spinning
“out of phase.” There is a net vertical magnetization, but no net horizontal magnetization.

Disturbing the equilibrium:

destroys the vertical magnetization, and

puts all the protons in phase, creating a transverse (horizontal) magnetization.

. Return to equilibrium means a gradual:

Recovery of vertical magnetization (T1 relaxation, governed by T1 time constant)

Dephasing of spins, leading to loss of transverse magnetization (T2 relaxation governed by T2 time constant).

Different tissues are characterized by different T1 and T2 times.

The MR signal and appearance on MRI images thus depends on 3 parameters:

Proton density per voxel (volume element) – how many hydrogen protons are there to generate signal?

The T1 time of those protons

The T2 time of those protons

. Comparison of MR and CT:

Proton density (PD) on MR is the analog of electron density on CT.

T1 and T2 relaxation have no CT analog. The ability of MR to use these additional tissue parameters allows it to have a much greater tissue contrast than
CT. That is the magic of MR. Despite a slightly lower spatial resolution (on the order of 1mm), it has a much higher contrast resolution for tissues of the
brain, making it the gold standard imaging technique.

Three main flavors of MR images:

proton density (PD) weighted

T1 weighted images (reflecting differences in PD and T1 times)

T2 weighted images (reflecting differences in PD and T2 times).

Another look at T1 and T2:

On T1 weighted images, tissues with a short T1 time are bright (such as fat). Tissues with a long T1 time (water) are dark.

On T2 weighted images, tissues with a long T2 time are bright (such as water). Tissues with a short T1 time (fat, or fatty white matter) are relatively

A few of many additional twists and refinements:

FLAIR images (Fluid Attenuation Inversion Recovery) – T2 weighted image which nulls signal from “free” water.

Fat-saturation images (T1 or T2 weighted images which null signal from fat).

GRE T2 images: T2 images that are more sensitive to blood products. 4.

Development of MRI contrast.

Gadolinium, a rare earth metal, is used as a chelate. CT “sees” iodine. MRI cannot “see” gadolinium (MR “sees” only hydrogen protons). Therefore, contrast
works by a completely different mechanism: it shortens the T1 time of adjacent hydrogen protons, making them brighter.

. Comparison of MRI and CT:

CT is fast, and less motion sensitive. Used for fast screening.

CT is better at detecting calcium (bone lesions, fractures) as well as acute blood, because oxyhemoglobin is diamagnetic and does not affect T1 or T2 time
of hydrogen protons.

-Therefore, CT is used in trauma to find bleeds and fractures, and in such settings as ruling out subarachnoid hemorrhage. It is also easier, faster and
cheaper in such settings as detecting subdural hematomas and hydrocephalus.

For all “finesse” work requiring high contrast resolution, MRI is the test of choice.

PET imaging: Positron Emission Tomography Moving from structural to functional imaging.

Lower spatial resolution (about 6mm), but images something that MRI and CT cannot: tissue metabolism.

Use a radioactive tracer that emits positrons. These annihilate with electrons when they are released, and create two photons given off at a 180 degree
angle. A detector ring detects photons, and can roughly localize their origin.

The most common tracer is Fluorine 18, which is used in a glucose molecule to replace a hydroxyl group, to create 18-FDG (fluorodeoxyglucose). FDG is taken
up by cells like glucose, but gets trapped inside in the Krebs cycle.

The more metabolically active a group of cells are, the more FDG they take up, and the more positrons are emitted, making their location “hot” on PET.
Thus, we have a metabolic map.

Uses of PET:

Originally to study brain activity, but has found its greatest use in oncologic imaging, to detect tumors, since they are so metabolically active.

In the brain, it is used to study dementias, seizures, neurodegenerative disease, and tumors.

Many very creative ligands, such as dopamine analogues, or opiate receptor ligands are created to study how theses chemicals function in the brain. This is
the world of molecular imaging.

XIX. Now, what’s all this fuss about MRI.

Let’s see what MRI can do in seeing intricate pathology

MRI often lets us see structures that no other modality can, like individual cranial nerves.

When it can’t, it gives us enough regional localization that we can tell what neuroanatomic structures are likely to be affected by a given lesion.

Test questions:

Which of these is not an advantage of CT over plain films:

Images patient in slices in stead of a full volume.

Solves problem of superimposition of structures.

Delivers a lower radiation dose.

Has better tissue contrast resolution.

Answer: c. CT has a much higher radiation dose.

Which of these is not an advantage of MRI over CT:

Better tissue contrast resolution.

Absence of ionizing radiation.

Better spatial resolution.

Better visualization of the posterior fossa.

Answer: c. CT has a spatial resolution on the order of 0. 5mm, while MR is at about 1mm).

Which of the following is true about spin-echo MRI:

Tissues with shorter T1 times are brighter on T1 weighted images.

Tissues with shorter T2 times are brighter on T2 weighted images.

Gadolinium contrast is not detected directly by MRI, but rather because it lengthens the T2 time of adjacent hydrogen protons.

On T1 weighted MRI images, white matter is slightly darker than gray matter; i.e. white matter is “gray” and gray matter is “white.”

Answer: a. Tissues with shorter T1 times are brighter on T1 weighted images.

Which of the following is not a function of the 7th cranial nerve?

Motor innervation to the ipsilateral facial muscles.

Parasympathetic innervation to the parotid gland for ipsilateral salivation.

A visceral sensory component, subtending taste to the anterior two-thirds of the ipsilateral tongue.

Secretomotor functions to the lacrimal gland for ipsilateral lacrimation.

Answer: b. The parotid glands are innervated by the 9th cranial nerve. The submandibular and sublingual glands are innervated by the 7th nerve.

Recommended Readings:

Nolte, The Human Brain, 5th edition. Chapter 11: Organization of the Brainstem , and Chapter 12: Cranial Nerves and Their Nuclei.