TECHNIQUES IN MRI
This technique has been applied to evaluate brain physiology while maintaining anatomic specificity . Ischemic changes in the brain can be detected as early as 15 minutes after arterial occlusion ,and appear as bright areas of increased signal intensity due to decreased water diffusion. This is called ‘diffusion imaging’.
Another technique is ‘perfusion imaging’ following IV injection of a contrast agent. Dynamic imaging during the first passage of the contrast bolus in the brain tissue gives information about the cerebral blood flow (CBF) and cerebral blood volume (CBV).Gadolinium reduces T1 and T2 values of tissues ,resulting in an increased signal on T1W1 images and a decreased signal on T2W1 images.
Normally there is a signal drop in the cerebral cortex by 20-30 percent on perfusion MRI depending on concentration of contrast agent in blood and fractional volume of intravascular space. Time versus concentration curves can be easily mapped giving regional CBF and CBV.
BOLD(BLOOD OXYGEN LEVEL DEPENDENT):
Imaging is based on imaging deoxy-hemoglobin content. While oxyhemoglobin has no substantial magnetic properties , de-oxyhemoglobin , present in the RBCs in veins, is strongly paramagnetic. Neuronal activation is exquisitely coupled with increased regional blood flow; which leads to locally decreased de-oxyhemoglobin concentration.
Interesting differences between the function of the male ad female brains have been noted on fMRI as well as on functional SPECT and PET studies done earlier than fMRI. The language function is highly lateralized in the male brain while being diffusely spread in both the hemispheres in the female brain. Rapid MRI techniques have revolutionised the utility and potential of MRI. Fast MRI opens new areas of applications such as abdominal imaging and pediatric imaging .
The dream of real time MRI comes close to realization with the combination of continuous scanning, online reconstruction and rapid image display with feedback control.
MRA provides a non –invasive technique for imaging vascular anatomy as well as flow measurement. It can be done during parenchymal imaging of the brain and has been widely applied to visualise the extra cranial and the intra cranial circulation, arch of aorta and abdominal aorta with delineation of all major branches and peripheral limb vessels . The improvement in pulse sequence design, hardware design and post processing methods have made it possible to acquire data in as short period with excellent vascular visualisation in a variety of clinical applications such as TIA (transcient ischemic attacks) in the brain.
PHASE CONTRAST MRA:
Spins that are moving in the same direction as the magnetic field gradient develop a phase shift that is proportional to the velocity of the spin. This is the basis of phase contrast MRA. Automated MR injection systems have the ability to precisely deliver contrast media to meet the need of current and emerging MRI techniques. The release of one molar contrast agent introduces opportunities for further exploration of cardiac , peripheral vascular and interventional MRI applications. The combination of very short echo times and breath –hold 3D acquisition has propelled MRA into the forefront of non-invasive vascular imaging.
The obstruction of major veins (cerebral, portal , peripheral) is an important cause of morbidity in patients with cancer, haematological diseases, in-dwelling catheters, arteriovenous dialysis shunts or in liver cirrhosis and Budd Chiari syndrome. Cerebral venous thrombosis is a serious condition often missed by conventional CT. MR venography with a flow-compensated FLASH(fast –low angle shot)sequence at 1.5T has been successful in demonstrating venous obstruction without the need of ionjecting contrast media or the need for venipuncture. Venous obstruction by extrinsic pressure of a tumor are well seen especially in the coronal plane.
DYNAMIC ADAPTIVE MRI:
In a novel approach , information from one scan is used to optimize subsequent data acquisition. Instead of using conventional Fourier Spatial Encoding, wavelet encoding or singular value decomposition (SVD) is used to compute near optimal encoding functions for image upcoding . In the wavelet approach, data acquired at low resolution is processed to predict locations wherein high resolution data is needed and subsequent encoding pulses are chosen appropriately.
Measurement of the magnetic field created by electric currents in the brain by MEG facilities calculation of the source of measured biomagnetic field and thereby localization of epileptogenic foci. The MEG co-ordinate system is defined by the anatomical landmarks, which are also easily identified with MRI, making it possible to align the three dimensional MEG data with the 3D MRI data. The resulting magnetic source images show the spatial relationships between the functional areas provided by MEG and the neighbouring anatomy and pathology both provided by MRI. The same principle of MEG can be applied to the heart. The electric currents in the myocardium creates extrathorasic magnetic fields and the source of these fields may be calculated by the same principles as in MEG .
Advances in imaging technology have enabled virtual endoscopy .The reconstructed 3D image can be rotated ,its layers peeled away and individual organs separated and isolated .The VE creates an illusion that the examiner is actually navigating inside the lumen .Currently available VE include gastroscopy,pancreatoscopy,colonoscopy,ureterorenoscopy,otoscopy,laryngoscopy,bronchoscopy, angioscopy and sinoscopy.VE is totally non invasive though expensive. It shows areas that are difficult to visualise with conventional invasive endoscopy.