Advantages of MRI over CT
The choice of neuroimaging modality always depends on the clinical presentation and required speed. However, MRI has several key advantages:
- Soft tissue contrast. MRI provides exceptional visualization of normal anatomy and more accurate, early detection of pathologies.
- Water sensitivity. In the early stages of parenchymal lesions, water content increases, which is clearly captured on MRI.
- Hemorrhage evaluation. Adipose tissue and subacute hemorrhage produce a hyperintense signal on T1-weighted images due to shortened T1 relaxation time. This is useful for the dynamic monitoring of parenchymal hematomas and ruling out occult tumors or vascular malformations.
- Ideal for the posterior cranial fossa. Unlike CT, where the bones of the skull base create streak artifacts (streak artefacts), MRI flawlessly visualizes the brainstem and cerebellum.
MRI is also the gold standard for symptoms of spinal cord dysfunction and nerve compression (evaluating intervertebral discs, the spinal axis, and bone marrow).
Intraoperative and planning applications
Intraoperative MRI is actively used not only to assess the volume of the resected tumor, but also for navigation in the treatment of movement disorders (MR-guided focused ultrasound, laser, or radiofrequency ablation).
A huge role is played by preoperative planning using functional MRI (fMRI) and MR tractography (DTI).
Example 1: Arteriovenous malformation (AVM) of the left frontal lobe. The combination of 3T MR angiography (time-of-flight) and tractography with a language task allows visualization of the vascular nidus and displaced white matter tracts. fMRI clearly identifies Broca's area (left hemisphere language dominance) located posterior to the AVM, and occipital lobe activation (patient reading instructions), which helps prevent damage to speech centers.
Example 2: Lung cancer metastasis. A mass was found in a patient with seizures and right-sided weakness. The combination of fMRI and tractography demonstrated the hand motor area and the corticospinal tract running lateral to the tumor. The data were used to select the surgical approach. Intraoperative MRI during craniectomy confirmed successful resection, resulting in the recovery of limb strength.
Safety and study nuances
Thorough screening is mandatory before any scan.
- Contraindications: Ferromagnetic clips, implants, certain pacemakers. Risks include heating, object displacement, and battery malfunction. Some devices are safe only at a field strength of 1.5T.
- Anxiety and claustrophobia: May require sedation or anesthesia.
- Time limitations: MRI takes longer than CT. For critically ill patients, ultrafast methods such as EPI (echo-planar imaging) or single-shot FSE are used, at the expense of reduced spatial resolution.
- Pregnancy: Fetal MRI (feto-MRI) is acceptable in high-risk pregnancies after evaluation by a radiologist and obstetrician. Contrast administration is strictly avoided.
Contrast enhancement utilizes gadolinium (Gd)-based agents, which shorten T1. Important: contrast is not used in renal failure due to the risk of nephrogenic systemic fibrosis. Additionally, gadolinium may accumulate in the dentate nuclei of the brain.
Common artifacts
Long scan times make MRI vulnerable to distortions. The main ones are:
| Artifact type | Cause | How to eliminate |
|---|---|---|
| Motion artifact (Motion) | Patient movement (most pronounced with prolonged phase encoding) | Fast sequences, motion correction |
| Pulsation artifact (Pulsation) | Arterial pulsation or CSF flow (frequently in the thoracic spine near the aorta) | Changing the phase/frequency encoding direction |
| Aliasing (Wrap-around) | Structures extend beyond the field of view (FOV) and "wrap around" onto the image | Increasing the FOV or phase encoding steps |
Briefly: answers to topical questions
What are the advantages of MRI over CT in neuroimaging?
MRI has the following advantages over CT in neuroimaging:
- better visualization of normal anatomy and pathological processes due to superior soft tissue contrast;
- earlier and more accurate detection of pathologies;
- high intrinsic water sensitivity;
- high contrast for adipose tissue and subacute hemorrhage, which appear hyperintense on T1-weighted images.
When examining the posterior cranial fossa, MRI allows visualization of the brainstem and cerebellum without the streak artifacts from the skull base bones commonly encountered on CT.
Which structures and tissues produce a hyperintense signal on T1-weighted MRI images?
A hyperintense signal on T1-weighted MRI images can be produced by:
- adipose tissue;
- subacute hemorrhage;
- areas of hyperintense signal in adenomyosis, corresponding to small punctate hemorrhages within ectopic endometrial tissue;
- endometriomas, i.e., ovarian endometrioid cysts;
- areas of cystic adenomyosis associated with extensive hemorrhage into ectopic endometrium.
What are the absolute contraindications to MRI?
Absolute contraindications for MRI include the presence of incompatible electronic devices and ferromagnetic objects in the patient's body. The following groups of contraindications are distinguished:
- Electronic implantable devices — pacemakers, middle ear implants, insulin pumps.
- Ferromagnetic foreign bodies — metal fragments inside or near the eyeball.
- Metallic implants — ferromagnetic cerebral vessel clips, recent implants.
Modern MR-conditional models of these devices allow the procedure to be performed.
Which pulse sequences are classified as ultrafast MRI methods?
Ultrafast magnetic resonance imaging methods include specialized pulse sequences that significantly reduce scanning time. Clinical practice utilizes:
- Echo-planar imaging (EPI) — an ultrafast technique (T2 or T2*) using an initial radiofrequency pulse and a series of gradient echoes. It is the method of choice for diffusion-weighted MRI.
- Single-shot fast spin-echo (single-shot FSE) — used in specific clinical situations, but often requires a reduction in spatial resolution.
What are the most common artifacts encountered during MRI?
During magnetic resonance imaging, artifacts related to patient motion, tissue physical properties, and technical limitations of the method are most frequently encountered. The main types include:
- Motion artifact (Motion artefact) — occurs due to the duration of the scan, manifesting as blurring or "ghost" images.
- Pulsation artifact (Pulsation artefact) — a variant of motion artifact caused by arterial pulsation or cerebrospinal fluid flow.
- Wrap-around artifact (Wrap-around artefact / aliasing) — superposition of structures extending beyond the field of view.
- Susceptibility artifact (Susceptibility artefact) — distortions at the interface of media with different magnetic susceptibilities.
- Truncation artifact (Truncation artefact) — bright and dark bands at tissue boundaries.
- Chemical-shift artifact (Chemical-shift artefact) — bands at fat-water interfaces.
How to eliminate the aliasing (wrap-around) artifact in MRI scanning?
To eliminate the aliasing (wrap-around) artifact in MRI scanning, methods aimed at correcting data acquisition parameters are used. This artifact occurs when anatomical structures extend beyond the field of view and superimpose onto the opposite side of the image. It can be eliminated in the following ways:
- Increasing the field of view (FOV).
- Increasing the number of phase-encoding steps.
In which brain structures can gadolinium accumulate after multiple contrast-enhanced MRIs?
Gadolinium can accumulate in certain areas of the brain, including the dentate nuclei. The long-term effects of such accumulation are unknown.
How to remember the main MRI artifacts: MPA (Motion, Pulsation, Aliasing). Motion and pulsation blur the image ("ghosts"), while Aliasing "wraps around" the edges (treated by expanding the field of view — FOV).
Class and exam questions
Can MRI be performed in pregnant patients?
Yes, MRI (including fetal MRI) is performed in high-risk pregnancies after weighing the benefits and risks. However, the administration of gadolinium-based contrast agents to pregnant women is generally avoided.
Why is MRI contrast contraindicated in renal failure?
In severe renal pathology, gadolinium can accumulate in the body, causing a rare but severe complication known as nephrogenic systemic fibrosis.
What is the main advantage of MRI in brainstem imaging?
MRI is the method of choice for the posterior cranial fossa because it does not produce streak artifacts from the dense bones of the skull base, which often obscure visualization on CT.
What to do when a wrap-around artifact (aliasing) occurs?
To eliminate this artifact, it is necessary to increase the field of view (FOV) or increase the number of phase-encoding steps.
What else to study on this topic
- Full list of primary and extended indications for brain MRI
- Principles of functional MRI (fMRI) and brain mapping
- Basics of MR tractography (DTI) and white matter integrity assessment
- Contrast physics: how gadolinium affects T1 and T2 relaxation
- MR spectroscopy in evaluating tumor and infectious processes
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