Neuroimaging in Neurologic Disorders¶
Chapter 434 | Part 13: Neurologic Disorders
KEY CLINICAL POINTS¶
- CT is the primary study for acute stroke, trauma, and subarachnoid hemorrhage, while MRI is preferred for demyelinating diseases and subtle CNS lesions.
- MRI is more sensitive than CT for detecting CNS lesions, with diffusion-weighted imaging (DWI) being the most sensitive for acute ischemic stroke.
- Contrast agents (iodinated for CT, gadolinium for MRI) carry risks of nephrotoxicity, allergic reactions, and nephrogenic systemic fibrosis (NSF) in renal failure.
- MRA and CTA are non-invasive alternatives to conventional angiography for vascular imaging, with reduced radiation exposure.
- Premedication with corticosteroids and antihistamines is recommended for patients with prior contrast allergies.
1. DEFINITION & OVERVIEW¶
Neuroimaging encompasses CT, MRI, and other modalities to diagnose neurological disorders. CT is rapid and effective for acute hemorrhage, trauma, and calcifications, while MRI provides superior soft tissue contrast for CNS pathologies. Advanced techniques like DWI, SWI, and MRA enhance diagnostic specificity.
Table 434-1 Guidelines for the Use of CT, Ultrasound, and MRI¶
| CONDITION | RECOMMENDED TECHNIQUE |
|---|---|
| Hemorrhage | Acute parenchymal CT, MR; Subacute/chronic MRI |
| Subarachnoid hemorrhage | CT, CTA, lumbar puncture fi angiography |
| Aneurysm | Angiography > CTA, MRA |
| Ischemic infarction | Hemorrhagic infarction CT or MRI |
| Bland infarction | MRI with diffusion > CT, CTA, angiography |
| Carotid or vertebral dissection | MRI/MRA > C,TA |
| Vertebral basilar insufficiency | CTA, MRI with DWI MRA |
| Carotid stenosis | CTA, MRA > US |
| Suspected mass lesion | MRI ± contrast |
| Neoplasm, primary or metastatic | MRI ± contrast |
| Infection/abscess | MRI ± contrast |
| Immunosuppressed with focal findings | MRI ± contrast |
| CONDITION | RECOMMENDED TECHNIQUE |
|---|---|
| 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: 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 |
1.1 Imaging Modalities¶
Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are the cornerstone techniques. CT is preferred for acute stroke, trauma, and calcifications, while MRI excels in detecting demyelination, tumors, and subtle white matter abnormalities. Functional MRI (fMRI) and diffusion tensor imaging (DTI) provide insights into neural connectivity and tissue microstructure.
1.2 Role in Diagnosis¶
Imaging guides diagnosis of stroke, tumors, infections, and vascular malformations. Early detection of acute ischemic stroke with DWI and characterization of subacute hemorrhage with SWI are critical for treatment decisions.
2. COMPUTED TOMOGRAPHY (CT)¶
CT is rapid and effective for acute stroke, trauma, and calcifications. It provides high-resolution osseous detail, making it ideal for trauma and spinal evaluation. Contrast-enhanced CT improves detection of vascular abnormalities and BBB disruption.
Table 434-2 Guidelines for Premedication of Patients with Prior Contrast Allergy¶
| 13 h prior to examination | 7 h prior to examination | 1 h prior to examination | Immediately prior to examination |
|---|---|---|---|
| Prednisone, 50 mg PO or methylprednisolone, 32 mg PO | Prednisone, 50 mg PO or methylprednisolone, 32 mg PO | Prednisone, 50 mg PO | Benadryl, 50 mg IV (alternatively, can be given PO 2 h prior to exam) |
2.1 Technique¶
CT uses x-ray attenuation to generate cross-sectional images. Helical CT with multi-detector arrays enables rapid, high-resolution imaging. Contrast agents (iodinated) enhance vascular structures and detect BBB disruption.
2.2 Indications¶
CT is first-line for acute stroke, trauma, subarachnoid hemorrhage, and calcifications. It is also used for evaluating spinal trauma, intracranial hemorrhage, and detecting foreign bodies.
2.3 Complications¶
Radiation exposure is minimal for routine brain CT (2–5 mSv). Contrast nephropathy risk is higher in patients with renal insufficiency. Allergic reactions to iodinated contrast occur in ~0.04% of cases.
3. MAGNETIC RESONANCE IMAGING (MRI)¶
MRI provides superior soft tissue contrast and is preferred for CNS pathologies. Techniques like DWI, SWI, and FLAIR enhance detection of ischemia, hemorrhage, and demyelination. Gadolinium-based contrast agents improve lesion characterization.
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 | |
| FLAIR (T2) | Long | Long | Low | High | Medium | High |
3.1 Technique¶
MRI uses hydrogen proton relaxation times (T1 and T2) to generate images. T1-weighted (T1W) images show fat and hemorrhage, while T2-weighted (T2W) images highlight edema and fluid. FLAIR sequences suppress CSF signal to detect periventricular lesions.
3.2 Contrast Agents¶
Gadolinium-based contrast agents (GBCAs) alter T1 relaxation times. Macrocyclic agents (Group 2) are preferred over linear agents (Group 1) to minimize NSF risk. Gadolinium deposition in the brain is dose-dependent and occurs in patients with renal insufficiency.
3.3 Applications¶
MRI is optimal for detecting acute ischemic stroke (DWI), demyelinating diseases (T2W), tumors, and infections. Functional MRI (fMRI) and diffusion tensor imaging (DTI) assess neural connectivity and white matter integrity.
4. CONTRAST MEDIA AND ALLERGIC REACTIONS¶
Contrast agents (iodinated for CT, gadolinium for MRI) are critical for lesion characterization but carry risks of nephrotoxicity, allergic reactions, and NSF. Premedication with corticosteroids and antihistamines is recommended for high-risk patients.
4.1 Contrast Nephropathy¶
Risk is highest in patients with preexisting renal insufficiency (eGFR <30 mL/min/1.73m²). Hydration and avoidance of nephrotoxic drugs reduce risk. Nonionic contrast agents are preferred over ionic ones.
4.2 Allergic Reactions¶
Allergic reactions to iodinated contrast occur in ~0.04% of cases, while gadolinium-related anaphylaxis is rare. Premedication with corticosteroids and antihistamines reduces risk. Severe reactions (e.g., anaphylaxis) are rare but require immediate intervention.
4.3 Nephrogenic Systemic Fibrosis (NSF)¶
NSF is a rare complication of gadolinium-based contrast in patients with renal failure. Linear GBCAs (Group 1) carry higher risk than macrocyclic agents (Group 2). Avoiding contrast in patients with eGFR <30 mL/min/1.73m² is recommended.
5. OTHER IMAGING TECHNIQUES¶
Advanced techniques like MRA, PET, and MR neurography enhance diagnostic specificity. PET is used for metabolic assessment, while MR neurography detects peripheral nerve abnormalities.
5.1 Magnetic Resonance Angiography (MRA)¶
MRA is non-invasive for evaluating vascular abnormalities. Time-of-flight (TOF) and phase-contrast techniques visualize blood flow without contrast. Gadolinium-enhanced MRA is preferred for detailed vascular imaging.
5.2 Positron Emission Tomography (PET)¶
PET assesses metabolic activity using radiotracers like FDG. It is useful for detecting tumors, infections, and neurodegenerative diseases. PET-CT fusion improves localization of metabolic abnormalities.
5.3 MR Neurography¶
MR neurography detects peripheral nerve abnormalities, including entrapment and inflammation. Fat-suppressed sequences highlight inflamed nerves, aiding in diagnosing radiculopathy and neuropathies.
6. SPECIAL CONSIDERATIONS¶
Imaging protocols must consider patient-specific factors like renal function, pregnancy, and pediatric use. Radiation exposure and contrast risks are minimized with appropriate techniques and premedication.
6.1 Renal Function¶
Avoid iodinated contrast in patients with eGFR <30 mL/min/1.73m². Use nonionic contrast and hydrate patients with renal insufficiency. Gadolinium is contraindicated in patients with severe renal failure due to NSF risk.
6.2 Pregnancy and Pediatrics¶
Avoid radiation in pregnancy unless essential. Use MRI over CT for pediatric patients to minimize radiation exposure. Adjust contrast doses based on weight and renal function in children.
6.3 Radiation Safety¶
Limit CT use in children and pregnant patients. Use low-dose protocols and consider MRI/ultrasound as alternatives. Radiation exposure from CT is generally low but should be minimized when possible.
10. KEY POINTS & CLINICAL PEARLS¶
- Use CT for acute stroke, trauma, and calcifications; MRI for demyelination and subtle CNS lesions. 2. Premedicate high-risk patients with corticosteroids and antihistamines before contrast. 3. Avoid iodinated contrast in renal failure; use nonionic agents. 4. MRI is preferred over CT for spinal cord and brain parenchyma evaluation. 5. Monitor for contrast nephropathy and allergic reactions in high-risk patients.