Neuroimaging in Neurologic Disorders¶
Chapter 434 | Harrison's 22e · Part 13 – Neurologic Disorders · Chapter 434
Key Clinical Points¶
- MRI is generally more sensitive than CT for detecting lesions in the central and peripheral nervous systems.
- Diffusion-weighted imaging (DWI) is the most sensitive technique for detecting acute ischemic stroke, encephalitis, abscess, Creutzfeldt-Jacob disease, and acute demyelinating lesions.
- CT is preferred for rapid assessment of patients with suspected acute stroke, hemorrhage, or trauma, as well as for visualizing fine osseous detail.
- Photon-counting CT (PCCT) provides superior resolution (0.2 mm) compared to conventional 64-slice CT (0.625 mm).
- Specific imaging modalities are selected based on clinical suspicion (e.g., SWI for chronic subarachnoid blood, CTA/MRA for vascular anatomy).
- Premedication protocols for contrast allergy vary by the time remaining before administration.
- MRI has specific contraindications including various metallic and electronic devices.
- DTI tractography is essential for mapping white matter pathways to plan surgical interventions in tumors.
- Vessel wall imaging (e.g., 3D T1-weighted cube) helps identify inflammatory changes in the arterial wall.
- Acute stroke management utilizes multimodal imaging to identify the 'mismatch' between core infarction and hypoperfusion.
DEFINITION & CLASSIFICATION¶
• Imaging Modalities: Numerous noninvasive options for evaluating patients with neurologic disorders. ◦ Computed Tomography (CT) ◦ Magnetic Resonance Imaging (MRI) and its variations: → Diffusion-weighted MRI (DWI) → Diffusion tensor MRI (DTI) → Susceptibility-weighted imaging (SWI) → Arterial spin label imaging (ASL) → Perfusion MRI (pMRI) ◦ Advanced CT techniques: → CT angiography (CTA) → Perfusion CT (pCT) → Dual-energy CT → Photon counting CT ◦ Specialized Imaging: → MR neurography (MRN) → MR vessel wall imaging → MR spectroscopy (MRS)
ETIOLOGY & PATHOPHYSIOLOGY¶
• Imaging Principles: ◦ MRI Sensitivity: Generally more sensitive than CT for detecting lesions in the peripheral and central nervous systems (CNS). ◦ DWI Mechanism: Sensitive to the microscopic motion of water; used to detect acute ischemic stroke, encephalitis, abscess, Creutzfeldt-Jacob disease, tumors, and acute demyelinating lesions. ◦ CT Utility: Faster acquisition; preferred for uncooperative patients or those with suspected acute stroke, hemorrhage, and acute intracranial or spinal trauma. Superior for visualizing fine osseous detail.
DIAGNOSTIC APPROACH¶
- Initial Assessment Identify the urgency of referral to a specialist. → Goal: Rapidly identify patients with nervous system infections, acute strokes, spinal cord compression, or treatable mass lesions.
- Selection of Technique (Table 434-1) • Hemorrhage: ◦ Acute parenchymal → CT, MR ◦ Subacute/chronic → MRI ◦ Subarachnoid hemorrhage → CT, CTA, lumbar puncture → angiography ◦ Chronic subarachnoid blood → MR with SWI • Ischemic Infarction: ◦ Hemorrhagic infarction → CT or MRI ◦ Bland infarction → MRI with diffusion > CT, CTA, angiography • Vascular Pathology: ◦ Carotid or vertebral dissection → MRI/MRA > CTA ◦ Vertebral basilar insufficiency → CTA, MRI with DWI MRA ◦ Carotid stenosis → CTA, MRA > US • Mass Lesions & Infections: ◦ Neoplasm (primary or metastatic) → MRI ± contrast ◦ Infection/abscess → MRI ± contrast ◦ Immunosuppressed with focal findings → MRI ± contrast ◦ Vascular malformation → MRI ± angiography • Other Neurologic Conditions: ◦ White matter disorders → MRI ◦ Acute demyelinating disease → MRI ± contrast ◦ Dementia → MRI > CT, contrast if mass ◦ Trauma (Acute) → CT ◦ Shear injury/chronic hemorrhage → MRI + SWI ◦ Headache/migraine → MRI > CT ◦ Seizure (First time, no focal neurologic deficits) → MRI > CT ◦ Seizure (With neurologic deficit, immunocompromised, or cancer) → MRI ± contrast > CT ◦ Partial complex/refractory → MRI ◦ Cranial neuropathy → MRI ± contrast ◦ Meningeal disease → MRI ± contrast • Spine: ◦ Low-back pain (No neurologic deficits after > 6 weeks) → MRI or CT ◦ Low-back pain (With focal deficits) → MRI > CT ◦ Spinal stenosis → MRI or CT ◦ Cervical spondylosis → MRI, CT, CT myelography ◦ Infection → MRI ± contrast, CT ◦ Myelopathy → MRI ± contrast ◦ Arteriovenous malformation → MRI ± contrast angiography
MANAGEMENT & TREATMENT¶
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Premedication for Contrast Allergy (Table 434-2) • 13 hours prior to examination: Prednisone, 50 mg PO or methylprednisolone, 32 mg PO • 7 hours prior to examination: Prednisone, 50 mg PO or methylprednisolone, 32 mg PO • 1 hour prior to examination: Prednisone, 50 mg PO; Diphenhydramine, 50 mg (IV, IM, or oral) [optional] • Immediately prior to examination: Benadryl, 50 mg IV (alternatively, can be given PO 2 hours prior) • Alternative for non-oral patients: Hydrocortisone, 200 mg IV for each dose of oral prednisone. • Accelerated IV Premedication (for duration < 4–5 hours): ◦ Option 1: Methylprednisolone sodium succinate (e.g., Solu-Medrol) 40 mg IV OR Hydrocortisone sodium succinate (e.g., Solu-Cortef) 200 mg IV immediately, and then every 4 h until contrast; plus Diphenhydramine 50 mg IV 1 h before. ◦ Option 2: Dexamethasone sodium sulfate (e.g., Decadron) 7.5 mg IV immediately, and then every 4 h until contrast; plus Diphenhydramine 50 mg IV 1 h before. ◦ Option 3: Methylprednisolone sodium succinate 40 mg IV OR Hydrocortisone sodium succinate 200 mg IV plus Diphenhydramine 50 mg IV, each 1 h before administration. Note: Options with duration < 4–5 hours have no evidence of efficacy and are for emergent situations.
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Contraindications to MRI (Table 434-4) • Cardiac/Circulatory: Cardiac pacemaker or leads, internal defibrillatory device, Swan-Ganz catheter. • Prostheses & Implants: Cochlear prostheses, bone growth stimulators, spinal cord stimulators, electronic infusion devices, intracranial aneurysm clips (some), ocular implants or metallic foreign bodies, McGee stapedectomy piston prosthesis, DuraPhase penile implant, magnetic stoma plugs, magnetic dental implants, magnetic sphincters. • Vascular/Other: Ferromagnetic inferior vena cava filters, coils, stents (safe 6 weeks after implantation), tattooed eyeliner (contains ferromagnetic material).
KEY PEARLS & HIGH-YIELD POINTS¶
• Imaging Technology Highlights: ◦ Photon-counting CT (PCCT): Provides resolution of 0.2 mm (compared to 0.625 mm for standard 64-slice CT). ◦ DWI Utility: Essential for identifying restricted diffusion in abscesses and acute infarcts. ◦ Vessel Wall Imaging: Used to detect inflammation/thickening in the arterial wall (e.g., Figure 8). ◦ DTI Tractography: Critical for mapping white matter pathways (e.g., arcuate-SLF, optic radiation) to predict surgical outcomes (Figures 9 & 10). ◦ Contrast Cross-Reactivity: A prior reaction to gadolinium does not predict a future reaction to iodinated contrast; they are not cross-reactive. ◦ MRI Signal Intensities (Table 434-3): ◦ T1W: CSF (Low), Fat (High), Brain (Low), Edema (Low) ◦ T2W: CSF (High), Fat (High), Brain (Medium), Edema (High) ◦ FLAIR (T2): CSF (Low), Fat (High), Brain (Medium), Edema (High)
Reference Tables¶
TABLE 434-1 Guidelines for the Use of CT, Ultrasound, and¶
Harrison's 22e, p.3383
| CONDITION | RECOMMENDED TECHNIQUE |
|---|---|
| Hemorrhage | |
| Acute parenchymal | CT, MR |
| Subacute/chronic | MRI |
| Subarachnoid hemorrhage | CT, CTA, lumbar puncture → angiography |
| Aneurysm | Angiography > CTA, MRA |
| Chronic subarachnoid blood | MR with SWI |
| Ischemic infarction | |
| Hemorrhagic infarction | CT or MRI |
| Bland infarction | MRI with diffusion > CT, CTA, angiography |
| Carotid or vertebral dissection | MRI/MRA> CTA |
| Vertebral basilar insufficiency | CTA, MRI with DWI MRA |
| Carotid stenosis | CTA, MRA > US |
| Suspected mass lesion | |
| Neoplasm, primary or metastatic | MRI ± contrast |
| Infection/abscess | MRI ± contrast |
| Immunosuppressed with focal findings | MRI ± contrast |
| Vascular malformation | MRI ± angiography |
| White matter disorders | MRI |
| Acute demyelinating disease | MRI ± contrast |
| Dementia | MRI > CT, contrast if mass |
| Trauma | |
| Acute trauma | CT |
| Shear injury/chronic hemorrhage | MRI + SWI |
| Headache/migraine | MRI > CT |
| Seizure | |
| First time, no focal neurologic deficits | MRI > CT |
| With neurologic deficit, or immunocompromised or cancer |
MRI ± contrast > CT |
| Partial complex/refractory | MRI |
| Cranial neuropathy | MRI ± contrast |
| Meningeal disease | MRI ± contrast |
| Spine | |
| Low-back pain | |
| No neurologic deficits | MRI or CT after >6 weeks |
| With focal deficits | MRI > CT |
| Spinal stenosis | MRI or CT |
| Cervical spondylosis | MRI, CT, CT myelography |
| Infection | MRI ± contrast CT |
| Myelopathy | MRI ± contrast |
| Arteriovenous malformation | MRI ± contrast angiography |
| 434 | Neuroimaging in Neurologic Disorders William P. Dillon |
TABLE 434-2 Guidelines for Premedication of Patients with Prior Contrast Allergy 13 h prior to examination: 7 h prior…¶
Harrison's 22e, p.3387
- 13 h prior to examination:
- Prednisone, 50 mg PO or methylprednisolone, 32 mg PO
- 7 h prior to examination:
- Prednisone, 50 mg PO or methylprednisolone, 32 mg PO
1 h prior to examination:
Prednisone, 50 mg PO
Diphenhydramine, 50 mg intravenously, intramuscularly, or by mouth (optional) - Immediately prior to examination:
- Benadryl, 50 mg IV (alternatively, can be given PO 2 h prior to exam)
If a patient is unable to take oral medication, 200 mg hydrocortisone IV for
each dose of oral prednisone may be used
If a patient is allergic to diphenhydramine in a situation where
diphenhydramine would otherwise be considered, an alternate antihistamine
without cross-reactivity may be considered, or the antihistamine portion of the
regimen may be removed
Accelerated IV Premedication
1. Methylprednisolone sodium succinate (e.g., Solu-Medrol) 40 mg IV
or
Hydrocortisone sodium succinate (e.g., Solu-Cortef) 200 mg IV immediately,
and then every 4 h until contrast medium administration.
plus
Diphenhydramine 50 mg IV 1 h before contrast medium administration. This
regimen is usually 4–5 h in duration.
2. Dexamethasone sodium sulfate (e.g., Decadron) 7.5 mg IV immediately, and
then every 4 h until contrast medium administration
plus
Diphenhydramine 50 mg IV 1 h before contrast medium administration. This
regimen may be useful in patients with an allergy to methylprednisolone and
is also usually 4–5 h in duration.
3. Methylprednisolone sodium succinate (e.g., Solu-Medrol) 40 mg IV or
hydrocortisone sodium succinate (e.g., Solu-Cortef) 200 mg IV
plus
Diphenhydramine 50 mg IV, each 1 h before contrast medium administration.
This regimen, and all other regimens with a duration <4–5 h, has no evidence
of efficacy. It may be considered in emergent situations when there are no
alternatives.
TABLE 434-3 Some Common Intensities on T1- and T2-Weighted MRI Sequences
| IMAGE | TR | TE | SIGNAL INTENSITY | |||
|---|---|---|---|---|---|---|
| CSF | FAT | BRAIN | EDEMA | |||
| T1W | Short | Short | Low | High | Low | Low |
| Long | Long | High | High | Medium | ||
| FLAIR (T2) | Long | Long | Low | High | Medium | High |
TABLE 434-4 Common Contraindications to Magnetic Resonance Imaging Cardiac pacemaker or permanent pacemaker leads…¶
Harrison's 22e, p.3390
- Cardiac pacemaker or permanent pacemaker leads
Internal defibrillatory device
Cochlear prostheses
Bone growth stimulators
Spinal cord stimulators
Electronic infusion devices
Intracranial aneurysm clips (some but not all)
Ocular implants (some) or ocular metallic foreign body
McGee stapedectomy piston prosthesis
DuraPhase penile implant
Swan-Ganz catheter
Magnetic stoma plugs
Magnetic dental implants
Magnetic sphincters
Ferromagnetic inferior vena cava filters, coils, stents—safe 6 weeks after
implantation
Tattooed eyeliner (contains ferromagnetic material and may irritate eyes)