CT-MRI

Mapping the Body

Functional MRI is based on increased local perfusion to the brain during increased electrical activity

Dr Dayananda Lingegowda Consultant Radiologist Narayana Hruduyalaya Bangalore

Many diseases manifest with changes only in function without any anatomical alteration. Functional MRI is an imaging technique to map the functional status of an organ, which sets it apart from the routine radiological imaging studies. Functional MRI not only points out functional abnormalities, but also improves our understanding of various organs, their functioning and furthermore, helps in therapeutic decision making. Even though it is commonly used in imaging of the brain, it has found wide utility in other body part imaging like the lung, liver, pancreas and heart.

Functional MRI of Brain

Functional MRI is based on increased local perfusion to the brain during increased electrical activity. As a result of increased flow, there is an increase in oxyhaemoglobin levels and proportionate reduction in deoxyhaemoglobin levels. However, this increase in the blood oxyhaemoglobin content is preceded by an initial drop. This occurs because of over-compensation secondary to perfusion. As oxyhaemoglobin is diamagnetic and deoxyhaemoglobin is paramagnetic, there is an alternation in the magnetic field. Blood Oxygen Level Dependent (BOLD) signal can thus be visualised using the T2* gradient echo sequences with deoxyhaemoglobin acting as an endogenous contrast agent during FMRI acquisition.

Technique: A fast MRI imaging sequence is required to demonstrate brain function with a real-time neuro-anatomical localisation. T2 * gradient ECHO EPI sequences are very sensitive to magnetic inhomogeneity due to blood oxygen levels. This imaging sequence exploits the magnetic property of oxy and deoxyhaemoglobin. It is a very fast MRI sequence with a low spatial resolution as compared to anatomic MRI images. Low spatial resolution of FMRI can be compensated by super imposing anatomical MRI images over these images. The imaging protocol varies from institution to institution. Patients are positioned in the MRI scanner as for a routine head scan. In a typical functional imaging, three sets of images are acquired. The initial set of images are acquired during rest (baseline scan), which is followed by images acquired during neuronal activation (task images). The last set of images are acquired without any activation (post-stimulation scan).

FMRI Paradigms

It depends on the clinical question and location of the tumor. For example, to investigate motor function self-triggered movements such as finger movements, toe movements and fist clinching are used. For language function, auditory and / or visual stimulations are used. Patients are expected to respond to questions during auditory stimulations whereas visual stimulation task is performed using check boards.

Clinical application: This is pertaining fundamental neuro-physiological studies of the brain and pre-operative localisation of motor and speech related regions. Functional MRI localises the important areas pre-operatively and non-invasively so that an optimal balance between complete tumour resection and minimal functional deficit can be sought precisely while surgical planning. A distance of 10 mm and more between the functional cortex and the tumor is found to be associated with significantly lesser risk of loss of function in the post-operative period.

Psychiatry: Functional MRI has greatly improved our understanding of many psychiatric disorders. Hyperactivity in anterior cingulate cortex is associated with depression whereas a lesser activity in the same site is seen in a patient with schizophrenia. It has also been observed that with therapy cingulate cortex activity returns to normal levels.

Epilepsy: Localisation of primary motor cortex, somatosensory cortex and lateralisation of language function helps in function preserving surgeries. A good co-relation is observed between intraoperative corticography and functional MRI. Decreased intensity of frontal and temporal region activation is observed in patients with Alzheimer's disease.

• Monitoring the recovery of cognitive functions during rehabilitation from stroke and head injury.
• Assessment of behavioural therapy outcomes.

Advantages of functional MRI:

• Non-invasive techniques with no risk of radiation.
• High-spatial resolution.
• No radio-active isotope injection is required.
• Both anatomical and functional studies can be done in a single sitting.

Disadvantages of functional MRI:

• It is an indirect measure of neural activity.
• Poor temporal resolution.
• Difficult to perform functional MRI in uncooperative patients, like Alzheimer disease, psychiatric patients. MRI sequences used in FMRI are very sensitive to patient's movements.
• General contraindications for MRI also hold for functional MRI.

Functional MRI of Lungs

Functional MRI of lung is useful in non-invasive assessment of lung function in patients with chronic obstructive pulmonary diseases and asthma. Hyper-polarised gas MRI is used to detect the ventilation and perfusion. It plays a promising role in cases of asthma, COPD, cystic fibrosis, paediatric lung diseases and lung transplant cases.

Functional MRI of Liver

Apparent Diffusion Coefficient (ADC) is used in the evaluation of neuroendocrine liver metastasis following chemoembolisation. Studies have shown an increase in ADC values following chemoembolisation.

Functional MRI of Pancreas

Intravenous injection of secretin stimulates the secretion of bicarbonate and pancreatic exocrine fluid. Dynamic images obtained after secretin injection increase the sensitivity and specificity of MRCP. Secretin stimulated MRI of pancreas is useful in the evaluation of sub-clinical chronic pancreatitis. Pancreatic duct outlet obstructions and ductal disruptions are evaluated better on secretin assisted MRCP.

Functional Imaging of Heart

The entire spectrum of cardiac diseases can be imaged using Cardiac Magnetic Resonance (CMR). It has advantage over other modalities like high contrast and spatial resolution. The abilities of tissue characterisation, assessment of functions and accurate measurements of blood flow across pulmonary and systemic vasculature is much superior as compared to echocardiography.

Ultra fast gradient echo images can give dynamic images of heart. CMR accepted has goal standed investigations for diagnosis and management of cardiomyopathies. Cine CMR gives high quality images for evaluation of morphology and functions in cardiomyopathy patients without any geometrical assumption. Left ventricular dimensions and regional wall motions can be very accurately assessed using functional MRI. SSFP sequence has superior spatial and temporal resolution allows improve endocardial border detection. Using myocardial tagging technique, myocardial regional wall motions can be detected with high reproducibility and minimal inter observer variations.

Functional MRI of Joints

Many patients experience the joint abnormality with specific position or during load bearing. Static images obtained during the routine MRI may not be able to demonstrate the abnormality. The functional information can be obtained using kinematic MRI, in which the relative alignment of the anatomical structures is studied through a specific range of motion for a given joint.

Protocols and techniques. In general, kinematic MRI protocol are divided into three primary types:

• The incremental/ passive positioning.
• The active movement.
• Active movement against resistance. Typically T1-weighted spin echo, fast spin echo or gradient echo pulse are used in functional studies of the joint.

Clinical Applications

Ankle joint: Kinematic MRI is very useful in assessment of tibio-talar rotation, evaluation of partial tears of the tendons and ligaments, determination of loading areas of the talar dome and assessment of subtalar instability. Peroneal tendon subluxation.

Cervical spine: Kinematic MRI is used to evaluate the position dependent occipital-cervical changes, cord narrowing, occult subluxation or other forms of functional pathology. Indications include evaluation of spinal stenosis, postoperative cases and suspected instability.

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