MRI Flashcards
How to evaluate hydrocephalous… measurement
callosal height - range of 3.7mm to 7.3 mm though abnormal range overlaps
What radiographic diagnosis is happening in this images where the white arrows are pointing?
Transtentorial herniation and foramen magnum herniation
What type substances will strengthen the magnetic field of an MR machine?
What type of substances will weaken it?
Strengthen: paramagnetic and ferromagnetic
Weaknen: Diamagnetic
Magnetic susceptibility artifact is when?
There is local alteration of the magnetic field reulting in spatial misregistration and image distortion
What common surgical material can cause suspetibility artifact?
Suture and can remain for years
Susceptibility artifact is proportional to what?
The strength of the magnet… the bigger the magnet the more susceptibility artifcat you see.
What sequences exhibit more susceptibility artifacts?
Long echo time (TE) or T2W images
Besides ferro and paramagnetic objects, where can you see susceptibility artifact?
air-tissue or air-bone interface
Ex. Frontal sinus, nasal cavity, mouth and neck
What type of sequences improves visibility of areas effected by susceptibility artifact
Spin-echo rather than gradient echo.
What other actions can be taken while planning an MRI to reduce suspectibility artifact?
Decrease voxel size (most useful: done by decreasing FOV and/or slice thickness)
Increase reciver bandwidth
Changing the frequency encoding direction.
CSF signal loss occurs due to?
High velocity or turbulent CSF flow
Where does CSF signal void usually occur?
In areas of narrowing
Ex: Mesencephalic aqueduct, lateraly ventrical, fourth ventrical and syrinxes
Explain why CSF Signal void happens?
A spin-echo is performed by admitting an intial 90 degree RF pulse and then followed by a 180 degree refocusing RF pulse.
In CSF signal void, the protons that have received the 90 degree RF pulse have moved on before being exposed to the 180 degree refocusing FR pulse.
Therefore the protons that enter that area have received no excitation pulse and therefore do not admit a signal
What affects the CSF signal?
Turbulence
obstruction
Cardiac motion
How does cardiac motion affect CSF signal?
During systole the blood enters the choroid plexus and parenchymal vasculature causing CSF to flow faster in an antegrade direction (systolic pseudogating) - leads to decrease in signal
Opposite during diastole (diastolic pseudogating) - leads to increase in signal
What are some ddx for CSF signal void?
COMS
Intra and extraaxial masses
Intraventricular tumors
Hydrocephalous
CSF signal void occurs in what weighting?
T2W
Ghosting is what in MR?
Macroscopic motion causes structures to be in different positions duringt the phase-coding process
Aliasing is AKA?
Wraparound artifact
Aliasing occurs when?
When parts of the anatomic structures being imaged are outside the FOV.
The structures are then superimposed on the opposite side of the image/FOV
You can correct aliasing in MR by?
Increasing FOV
Using oversampling technique
Apply a saturation pulse
FLAIR images null out what type of fluid?
Low-protein fluid (CSF or water)
Incomplete suppression of FLAIR can indicate?
Artifact (propofol, increased O2, motion, inhomogeneity of the magnetic field)
Hemorrhage
meningitis
neoplasia (leptomeningeal mets)
Post-contrast T1W MRI pseudolesions?
Trigeminal Nerve enahncement (CNIII)
Choroid Plexus enhancement
Branches of the trigeminal nerve
ophthalmic
maxillary
mandibular
Why does CN3 and choroid plexus contrast enhance in face of no pathology?
Incomplete blood nerve barrier
Where is the choroid plexus locacted?
Ventral portion of the lateral ventricles
dorsal aspect of the 3rd ventricle
Caudal dorsal aspect of the 4th ventricle
Hyperintensity of the neurohypohysis is commonly seen in what species?
Dogs—-NOT cats
Reasoning behind T1W hyperintensity of the neurohypophysis as an artifact?
Storage of arginine vasopressin (in humans but is correlated to dogs as well)
Decrease is neurohypophysis signal on T1W could indicate?
Diabetes mellitus
Diabetes insipidus
Hypersecreation of Arginine vasopressin
What is external hydrocephalus?
CSF in the subarachnoid space around the brain
Bilaterally symetrical brain lesions think what type of ddx?
Metabolic
Toxin
Storage
T1 hyperintensity pre-contrast material?
My Very Best Friend is Cool
Melnan, Magnesium, Mucin
Vasopressin
Blood
Ferrous objects, fat
Cu, Ca+2
Hepatic encephalopathy is hyperintensities
Thamine definency is seen where?
Thalamus and Brainstem
Size of pituitary gland in a Cat? Dog?
Cat: ~5mm in height and 3.5mm in width
Dog: 4.5mm in height and 6mm in width
Big findings for a AHNCNPE
Acute hydrated non-compressive nucleus pulposus extrusion
Focal T2 hyperintensity
Non-compressed cord
Changes in the cord or epidural fat
Suggested cut off for length of L2 single shot fluid signal attenuation with developing or not developing myelomalacia?
>7.4 of L2 is high risk for developing
What percentage of BCS dogs do not have a septum lucidum?
80% - Free flowing lateral ventricles
What are the clinical questions that need answered when looking at meningitis?
- Distribution: Cerebellum vs cerebrum vs brainstem… this will help the clinician know the clinical signs to watch out for
- Necrotizing areas - prognosis
- Amount of edema or area - prognosis
- Mass effect/herniation - manitol treatment
An annular tear is when what happens?
The annulus tears with no extrusion of disc material. This can be painful as the outer 1/3 of the disc annular ring is highly innervated with pain fibers
