basic neuroimaging (ct and mri)
TRANSCRIPT
Basic Neuroimaging (CT and MRI)
Dr Sean E Mc Sweeney
2007/2008
Indications for CT scan:
1) Significant Head Injury
2) Acute Stroke
3) Acute Headache (to look for SAH)
Indications for MRI:
4) Progressive focal neurological deficit
5) Epilepsy
6) Infections
7) Degenerative diseases
Multislice Helical CT scanner
Basic CT Physics
CT overcomes superimposition of structures
Measures and records small differences in tissue density
Highly collimated x ray beam passes through the head – with different tissues attenuating this beam differently
Detectors on the other side gather information about attenuation characteristics and with mathematical algorithms compute an image
Normal CT Brain ScanCT Scan above the level of the lateral ventricles
Frontal Lobe
Falx Cerebri
Centrum Semiovale
Parietal Lobe
Without Contrast
With Contrast
Normal CT Brain ScanCT Scan at the level of the lateral ventricles
Frontal Lobe
Frontal Horn of Lateral Ventricle
Occipital Lobe
Head of Caudate Nucleus
Thalamus
Posterior Horn of Lateral Ventricle
Normal CT Brain ScanCT Scan at the level of the 3rd Ventricle
Without Contrast With Contrast
Sylvian Vessels
Ant + Post Limbs of the Internal Capsule
Temporal Lobe
Head of Caudate Nucleaus
Frontal Horn of Lateral VentricleLentiform Nucleus
Tectum of Midbrain
Confluence of Venous Sinuses (Torcula)
Normal CT Brain ScanCT Scan at the level of the 4th Ventricle
ACA
Temporal Lobe
MCA
Basilar Artery
Pons4th Ventricle
Without Contrast With Contrast
Tissue Hounsfield Unit
Air -1000
Fat -40 to -100
Fluid 0 to 20
Soft Tissue 20 to 100
White Matter 20 to 35
Grey Matter 30 to 40
Acute Hemorrhage 55 to 75
Bone
1000
Black
White
Hounsfield Unit (CT Density)
Grey Matter
Example on CT
Air (-1000)Fluid CSF (0-20)
White MatterGrey Matter
Bone (1000)
Case Examples on CT
Lipoma (Fat) Calcification (in meningioma)
Acute hematoma
-80 HU 70 HU140 HU
Brain Windows vs. Bone Windows
CT of fibrous dysplasia of skull with brain and bone windows
Evolution of Hematoma on CT
Acute hematoma: 4 hrs
4 days
3 months after initial CT
Decreasing Density of Hematoma
• Acute : hyperdense (1 to 6 days)
• Subacute: isodense (6 to 12 days)
• Chronic : hypodense ( > 12 days)
Classic Infarct on CT
• Wedge shaped area of low density in right middle cerebral artery territory
ACAMCA
PCA
Acute Extradural hematoma
Extradural Hematoma - evolution
Acute extradural hematoma
Chronic epidural hematoma
Extradural Hematoma
• Extradural hematomas are located between the inner table of the skull and the dura.
• They are typically biconvex in shape because their outer border follows the inner table of the skull and their inner border is limited by locations at which the dura is firmly adherent to the skull.
• Epidural hematomas are usually caused by injury to an artery, although 10% of epidural hematomas may be venous in origin.
• The most common cause of an epidural hematoma is a linear skull fracture that passes through an arterial channel in the bone(middle meningeal artery).
• Epidural hematomas, especially those of arterial origin, tend to enlarge rapidly
Subarachnoid Haemorhage (SAH)
Acute Subarachnoid Haemorrhage in region of Circle of Willis (Yellow)
Subarachnoid Haemorhage
• CT without contrast– CT is the most sensitive imaging study in SAH .– Findings may be negative in 10-15% of patients with
SAH.– Maximum sensitivity is within 24 hours after the event;
sensitivity is 80% at 3 days, 50% at 1 week.– Look for evidence of hydrocephalus (trapped temporal
horns and "Mickey Mouse" appearance of ventricular system).
– Look for intraparenchymal clot, intraventricular hematoma, and interhemispheric hematoma
Hydrocephalus and SAH
• Haemorrhage can be seen in the ventricular spaces normally containing CSF, (dark grey replaced by white) which are dilated (Hydrocephalus) also the sylvian fissure are visable due to the presence of SAH.
CT Cerebral Angiography
• 3D CT Angiography of the Brain Allows Extremely Detailed and Precise Visualization of the Cerebral Vessels and the Circle of Willis that is Safe and Noninvasive
ACAMCA
CarotidPCA
Ct Angiography
A Subarachnoid hemorrhage in the right sylvian fissure (arrow), B, 3D volume-rendered image (lateral) shows inferiorly and posteriorly directed saccular aneurysm at the origin of the right posterior communicating artery (arrow). C,Preoperative right internal carotid digital subtraction angiogram shows corresponding saccular aneurysm. D, Angiogram shows successful clip placement in the posterior communicating artery aneurysm.
Acute Subdural
Acute subdural (White) on chronic (Yellow) and mass effect :midline shift (Red arrow)
Acute Subdural
• Subdural hematomas are located between the dura and the brain.
• Their outer edge is convex, while their inner border is usually irregularly concave (cresenteric).
• Subdural hematomas cross the intracranial suture lines; this is an important feature that aids in their differentiation from epidural hematomas.
• Subdural hematomas are usually venous in origin, although some subdural hematomas are caused by arterial injuries.
• The classic cause of a posttraumatic subdural hematoma is an injury to one of the bridging veins that travel from the cerebral cortex to the dura.
The MRI machine
1.5 Tesla magnet
Magnetic resonance imaging
• Magnetic resonance imaging is based on imaging of protons within the human body
• Images produced are based on a computer calculation of how protons within different soft tissues react to the application of a strong magnetic field and radiofrequency pulse
MRI Is Similar to an Orchestra
Different music can be produced by changing the parameters used
MRI Sequences - NORMALS
T1
T2
FLAIR
T1 vs. T2 Appearances
MRI Indications
• Neurology- acute cerebral infarction
- brain tumour
- white matter disease- spinal cord problems- posterior fossa-better than CT
- paediatric-avoids radiation
- head trauma
Acute Infarct: CT vs. MRI
CT very useful to r/o bleed
MRI can show acute infarcts: useful if thrombolytic therapy
being considered
DWI
Brain tumor-GBM
Multiple sclerosis
Multiple sclerosis
• MRIs typically demonstrate more than 1 hyperintense white matter lesion on T2.
• Lesions may be observed anywhere in the CNS white matter (supratentorium, infratentorium, and spinal cord).
• Typical locations for MS lesions include the periventricular white matter, brainstem, cerebellum, and spinal cord.
• Ovoid lesions perpendicular to the ventricles are common in MS and occasionally are called Dawsons fingers.
• Perhaps the most specific lesions in MS are noted in the corpus callosum at the interface with the septum pellucidum
• Proton density (PD) MRI has an advantage over standard T2 imaging because, on PD series, MS lesions remain hyperintense while CSF signal is suppressed
HSV Encephalitis
Spinal Cord Lesion on CT
Ewing’s sarcoma arising from the cervical spine
Spinal Cord Lesion on MRI
Ewing’s sarcoma arising from the cervical spine with cord compression
T1 sagittal T1 sagittal C + T2 sagittal
Cord astrocytoma
T1 T1, with contrast
T2
Acoustic Neuroma
Blood Products
Acute hematoma best seen on CT
Subacute and chronic hematoma better evaluated on MRI
Primary (hypertensive) bleeds occur in the basal ganglia; for bleeds at other locations, hunt for a cause
Subacute Hematoma on MRIRight paraasagittal T1WI Left paraasagittal T1WI
(normal side)
Axial FLAIR image Axial T2 image
Chronic Resolved Hematoma
Now shows “slit” like cavity with hemosiderin stain
Advantages of MRI v CT
• No ionising radiation• Multiplanar imaging capability• Non-invasive vascular imaging• Cardiac imaging• Excellent contrast resolution-much better
than CT• No streak artifacts from bone• Safer contrast agent (Gadolinium)• Molecular imaging
Disadvantages of MRI v CT
• More expensive
• More motion sensitive
• Limited availability
• Contraindicated in certain patients
• Acute haemorrhage difficult to see (SAH)
Contraindications for magnetic resonance imaging
• Cranial metal i.e. head, brain, orbit
• Pacemaker
• History of metalwork, shrapnel
• Claustrophobic
Positron Emission Tomography
What is Positron Emission Tomography (PET)
• Positron emission tomography (PET scan) is a diagnostic examination that involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons.
• Positrons are emitted from a radioactive substance administered to the patient. The subsequent images of the human body developed with this technique are used to evaluate a variety of diseases
PET Scanning
• F-18 2 fluoro-2 deoxy-D-glucose (FDG) is a glucose analog labeled with positron-emitting fluorine-18.
• Most malignant tumors are metabolically active and take up increased FDG relative to normal tissue.
• FDG is highly sensitive in identification of malignant tumors.
PET-CT
• PET-CT is the fusion of functional and anatomic information acquired almost simultaneously.
• By combining the structural anatomic information with functional data, we are able to visualize form and function
PET-CT
Lung cancer with metastatic mediastinal lymph nodes
Lung cancer with metastatic Adrenal mass