Ch 28 Imaging of neuro Flashcards
Which form of imaging has the highest spatial resolution?
Radiography
highest spatial resolution (i.e., the ability to resolve fine detail
CT - pros
CT is generally thought to be a superior bone imaging modality, the opposite may be true with an infiltrative type of pathology.
tomographic or slice oriented, thereby eliminating the summation effects and depth perception losses associated with radiography.
CT readily identifies acute hemorrhage and fracture
Which imaging modality has the highest contrast resolution?
mRI
contrast resolution (i.e., the ability to discriminate tissues of differing composition)
MRI- pros/cons
pros
- discriminate between soft tissues, including gray and white matter
- no radiation
- invaluable for elucidating the precise localization or extent (long scan time)
- ideal fro head trauma
- MRI can provide physiologic information through the use of specialized pulse sequences
cons
- longer scan
MRI study more time-consuming, the added perspective gained by direct multiplanar MR images
- metals can render all or parts of an MR image nondiagnostic
What is the difference in MPR image acquisition in CT and MRI?
With CT, sagittal and dorsal planes are reformatted/reconstructed after acquisition of transverse images
With MRI, images for each anatomical plane are obtained using seperate acquisition
CT
images are constructed as an x-ray tube within the gantry rotates around the patient while emitting x-ray photons
attenuation of photons occurs as they pass through the patient and is largely related to electron density.
Directly opposite the x-ray tube, electronic detectors (as opposed to film) absorb the remaining x-rays and convert them into a digital signal
data can be acquired continuously as the patient table is advanced, resulting in a spiral acquisition
What are the terms (-suffix) used to describe the level of brightness in radiography, CT and MRI?
Radiographs = opacity
CT = attenuation or denstiy
MRI = intensity
What are the Houndsfield units of air, fat, water, brain, acute to subacute clotted blood, mineral and bone, metal?
List some causes of hypoattenuation on CT scan
Cystic or fluid-filled
Necrosis
Oedema
Fattu infiltration
Gas
List some causes of hyperattenuation in CT
Haemorrhage
Mineral
Metal
Densely cellular/fibrotic
CT - vertebral canal
use of CT for intracranial structures has limitations compared to MRI
contrast is largely provided by epidural fat.
When pathology reduces epidural fat, identifying the underlying lesion or its relationship with the meninges and spinal cord may be difficult
Loss of contrast can be compensated for by the use of myelography with CT; added contrast between the subarachnoid space and the spinal cord helps to delineate intradural and extradural structures
in nonchondrodystrophic dogs or in chondrodystrophic dogs in which the most likely diagnosis is not intervertebral disc disease, CT may not resolve the lesion.
when pathologic soft tissues infiltrate and replace the normal signal void of bone, bony abnormalities become apparent
What produces the signals in MRI?
Mobile hydrogen atoms within the tissue
Bone: tissues devoid of hydrogen protons will have no signal ( therefore hypoinense)
MRI
1) H+ ions in body (water and lipid) experience magnetic field and align
2) Protons begin to “wobble” or precess around axis bc of angular momentum. Some protons are spin-up and some spin-down, but net magnetization is spin up (Mo)
3) Rate of precession is proportional to strength of magnetic force. Affected by subtracting magnets in MRI, nearby protons, magnetic substances within tissues
4) Spins perturbed by radiofrequency pulses through excitation. Mo now experiences second magnetic field (B1) and precesses around both magnetic fields. Both have same frequency and pulse in resonance. The simultananeous precessing results in downward spiral or “nutation” of Mo towards xy plane
5) RF frequency is removed and electrical voltage is transmitted to receiving coil. The signal decays as protons relax and i) return to equilibrium allowing longitudinal magnetization to recover (T1) ii) stop precessing in union (T2 decay)
6) T1 and T2 relaxation are different depending on substance. Can weight images by adjusting parameters of pulse sequence
Define pulse sequencing
A series of timed events by which a radiofrequency pulse is used to creaste a signal
Which are the only pulse sequences upon which all others are built?
Spin echo (considered the work-horse of clinical MRI and is used to produce T1W, T2W and proton density-weighted images
Gradient echo
What is FLAIR?
Why is it useful?
Fluid-attenuated inversion recovery - suppresses the signal from fluid
Give the ability to distinguish pure fluid structures (nulled signal) from solid, but hihgh-water content lesions such as oedema within tissue (high signal)
help distinguish high protein fluid (edema) from pure fluid (CSF)
WHat are STIR sequences?
Why is it useful?
Short Tau Inverstion Recovery - supresses fat signalling
Allows assessment of high water contect fluid or soft tissues against a background of suppressed fat
Useful for vertebral and paravertebral soft tissue pathology
Whar are T2* sequences used for?
Useful for identifying haemrrhage or blood clots
What sequence is particularly useful for radiation planning?
spoiled gradient echo
MRI
Transverse (A) and sagittal (B) postcontrast T1W images of the brain and sagittal (C) T2W image of the cervical spine of a dog with a cystic meningioma in the cerebellum. Note the dural tail in A (arrow). The “cystic” component of the lesion is hypointense. In B, ventral displacement of the cerebellum causes compression of the medulla and effacement of the sub- arachnoid space. In C, syringohydromyelia and intramedullary edema are noted in the cervical spinal cord (arrows). T1W spin echo or T2W fluid-attenuated inversion recovery (T2 FLAIR) images may help differentiate spinal cord edema from syringohydromyelia
MRI
Transverse postcontrast T1W image of a dog with a presumed glial neoplasm. A hypointense mass in the left temporal/piriform lobe is exhibiting a mass effect and only mild ill-defined contrast enhancement. The presumption of glial tumor is based on the imaging characteristics and the anatomic location of the lesion, as glial tumors have a propensity to arise in the piriform lobe.
MRI
Dorsal postcontrast T1W image of a dog with a suspected histiocytic sarcoma. (Abnormal histiocytic cells were present in the cerebrospinal fluid.) A large, intensely contrast- enhancing mass can be seen in the left temporal lobe. Although adjacent meningeal enhancement is evident (arrowhead), this is believed to indicate meningeal spread from an intra-axial, not an extra-axial, neoplasm. The mass is otherwise round and is not broad-based; ependymal (arrow) and more widespread meningeal enhancement is typical of round cell neoplasia
MRI
Transverse postcontrast T1W image of a cat with bilateral otitis media/interna with intracranial extension. Note the enhancing material in the tympanic cavities and the diffuse pachymeningeal hyperenhancement and focal fusiform thicken- ing of the dura adjacent to the left tympanic cavity (arrow) compatible with a small abscess.
MRI
Sagittal (A) and transverse (B) T2W images in a dog with cervical spondylomy- elopathy (cervical vertebral malformation/instability). Marked stenosis of the vertebral canal and compression of the spinal cord from enlargement of the dorsal laminae at the C6-C7 intervertebral space can be seen. The gray matter of the spinal cord is hyperintense, consistent with edema, necrosis, gliosis, microcyst formation, and/or demyelination (arrow). Despite severe compression, the dog was ambulatory with moderate ataxia and tetraparesis
MRI
Sagittal (A) and transverse (B) T2W images of the cervical spine of a dog with an acute onset of left hemiparesis. A focal left intramedullary T2 hyperintensity can be seen at the level of the C3-C4 intervertebral disc extending caudally to the level of C4. Volume and signal intensity of the nucleus pulposus are reduced, and narrowing of the C3-C4 intervertebral disc space is evident. Mild extraneous extradural material is shown dorsal to the affected disc, along with minimal cord compression. No contrast enhancement is noted. Findings are compatible with presumptive acute noncompressive nucleus pulposus extrusion
MRI
Sagittal T2W image of an English Bulldog with congenital anomalies of the thoracolumbar vertebrae and chronic stenosis of the vertebral canal at the level of the T11-T12 inter- vertebral space. On transverse images (not illustrated), stenosis was due to the combination of a bulging intervertebral disc, degeneration of the articular processes, and scar tissue formation following hemilaminectomy. Chronic cord compression has likely resulted in dilation of the dorsal subarachnoid space (white arrow) and atrophy of the spinal cord at the level of the intervertebral disc. Intramedullary T2 prolongation is noted cranially at the level of T11 (black arrow) and may be compatible with edema, necrosis, gliosis, microcyst formation, and/or demyelination.
MRI
A, Sagittal T2W images of a Dachshund with an acute onset of neurologic signs and surgical diagnosis of extrusion (herniation) of mineralized intervertebral disc material at T11- T12, and (B) a Dalmatian with long-standing neurologic signs and presumed chronic annulus bulges involving the T12-T13 to L2-L3 intervertebral discs. Magnetic resonance imaging (MRI) signal intensity is similar in both types of disc abnormalities, making correlation of imaging findings with signalment and clinical signs critical.
Imaging features of discospondylitis
in cats
Photodynamic detection of a canine glioblastoma using 5-aminolevulinic acid
M. Yamashita
that photodynamic detection
using 5-aminolevulinic acid might be useful for intraoperative fluorescence-guided resection of malignant gliomas in dogs.
