Absolute contraindications: patients with implants
The presence of medical devices or metal implants in a patient is considered the main contraindication to MRI. The physical nature of the method does not allow for standard scanning, as the magnetic field distorts the image or may affect the implant material.
In such cases, multislice computed tomography (MSCT) becomes the diagnostic method of choice. However, the presence of metal in the body requires the radiologic technologist and physician to modify the protocol. Scanning settings must strictly account for the implant material to minimize interference:
- Higher X-ray tube voltage (kV) values are applied.
- Special edge-enhancing reconstruction kernels are used.
- Iterative reconstruction is implemented to improve diagnostic accuracy.
Combating artifacts: visualization of stents
A frequent "complication" of quality vascular visualization is the presence of previously placed metal stents (e.g., after balloon angioplasty of aortic coarctation). Conventional MR angiography protocols will produce severe metal artifacts that obscure the vessel lumen.
To successfully overcome such artifacts, specialized scanning programs must be used. The gold standard is the use of high flip angle gradient-echo sequences (high flip angle gradient-echo). This allows for a reliable assessment of the caliber of stented vessels and timely detection of postoperative complications such as recoarctation or pseudoaneurysm.
Alternative solutions in neuroimaging
Magnetic resonance imaging is the "gold standard" for diagnosing tumors, demyelinating diseases, and neuroinfections. However, if MRI is contraindicated or the waiting time exceeds one month, the tactics change radically.
The patient undergoes brain computed tomography, but strictly with intravenous contrast enhancement. Unenhanced CT (without contrast) is not recommended for most pathologies due to extremely low diagnostic yield.
At the same time, the alternative method has its diagnostic limits that the physician must take into account:
- Posterior cranial fossa (PCF): CT does not always provide a clear image of structures due to overlapping artifacts from the massive bones of the skull base.
- Hyperacute period: CT often fails to show ischemic brain lesions (especially in the brainstem) in the first hours of stroke development. In such cases, the absence of changes on CT with clear clinical symptoms makes MRI vital.
Limitations in cardiology: respiration and choice of method
When evaluating congenital heart disease, cardiac MRI (CMR) is preferred over computed tomography provided there are no contraindications. However, the method has technical limitations.
All cine-imaging and morphological assessment sequences require the patient to hold their breath. If the patient is unable to cooperate (or has implants), modern MSCT with volume scanning (high-pitch spiral) comes to the rescue. New scanners minimize motion artifacts, often eliminating the need for general anesthesia and even ECG gating, which is only needed when image quality is non-diagnostic due to cardiac motion.
CT-C: Contrast CT — Compensation. If MRI is contraindicated (due to implants), we compensate by performing CT with contrast and parameter correction (kV and iterative reconstruction).
Class and exam questions
What should be done if a patient with a brain tumor has a contraindication to MRI?
As a replacement, computed tomography with mandatory intravenous contrast enhancement must be performed, as unenhanced CT in this situation is uninformative.
How to reliably visualize a vessel with a metal stent during MRI?
To overcome metal artifacts, high flip angle gradient-echo sequences are used.
How are CT parameters adjusted in patients with implanted devices?
Higher kV values, iterative reconstruction, and edge-enhancing reconstruction kernels are used to improve image quality.
In which cases cannot CT fully replace a brain MRI?
CT is limited in the hyperacute period of ischemic stroke (especially in the brainstem) and in examining the posterior cranial fossa due to bone artifacts.
What else to study on this topic
- Tissue MR characteristics: myocardial T1 and T2 mapping
- Contrast administration protocols and bolus tracking in CT
- Visualization of congenital malformations: coarctation and interruption of the aortic arch
- Normal neuroanatomy of the brain on CT and MRI
- MRI diagnosis of vascular rings and pulmonary artery anomalies
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