MRI Scan - CT Scans - Comparison

Since the mid-1980's, magnetic resonance imaging (MRI) has been replacing CT as the method of choice for many imaging applications. In MRI, the patient is placed in a chamber and exposed to radio waves in the presence of a powerful magnetic field. This treatment causes the nuclei of certain atoms in the brain/body to emit signals as they align and realign within the magnetic field. Different tissues of the body produce different signals, depending on the mixture of elements they contain. MRI scanners translate the signals into three-dimensional images that can delineate specific structures within the brain and body.

In an MRI machine, the patient lies inside a large, cylinder-shaped magnet. Radio waves 10,000 to 30,000 times stronger than the magnetic field of the earth are then sent through the body.

This affects the body's atoms, forcing the nuclei into a different position. As they move back into place they send out radio waves of their own. The scanner picks up these signals and a computer turns them into a picture. These pictures are based on the location and strength of the incoming signals.

CAT scanner (computerised tomography scanner. The first modern development in brain/body imaging was the introduction in 1973 of computed tomography (CT), a refinement of standard x-ray technology. CT uses x-ray cameras that rotate around the patient's head to measure differences in tissue density across thin slices or sections of bodily tissues. A series of images are usually taken, representing serial slices or sections, from the base of the brain to the crown. A computer converts data on relative tissue density into images that can be displayed on a monitor or transferred to x-ray film. The ability of CT and other tomographicmethods to visualize discrete slices of tissue eliminates confusing shadows from adjacent overlying structures. A basic ct scanner machine is like a donut a lot smaller than a mri scanner.

A computer can use this information to work out the relative density of the tissues examined, changing images into numerical information for us to use in treatment planning for radiotherapy. Each set of measurements made by the scanner is, in effect, a cross-section through the body. The computer processes the results, displaying them as a two-dimensional picture shown on a monitor.

Computed tomography and magnetic resonance imaging can each represent a three-dimensional "slice" of the body anatomy, showing more detail than a conventional X ray. Functional imaging techniques permit scientists to detect changes in blood flow and energy metabolism. Such techniques include magnetic resonance spectroscopy, magnetic resonance imaging MRI, single-photon emission computed tomography, and positron emission tomography. In addition, electroencephalography records the spontaneous electrical activity of the brain, and magnetoencephalography measures and displays the magnetic field that surrounds the head in association with the electrical activity.

CT is significantly more sensitive to gradations of tissue density than a conventional X ray. With CT images, the most contrast occurs among bone, brain tissue, and cerebrospinal fluid (CSF). Bone appears bright, CSF appears dark, and brain tissue appears somewhat in between. Modern CT scanners produce fine-grained resolution, permitting some differentiation between white and gray matter. CT imaging is not limited to the brain; abdominal organs that are virtually invisible to standard X rays--such as the pancreas and adrenal glands--are visualized routinely using CT. Unfortunately, even with high-resolution scanners, a CT image is almost always less precise than it appears. The image comprises a matrix of tiny picture elements, each of which represents the average tissue density of a three-dimensional "slice" of brain that may be up to 10 mm thick. This phenomenon, known as volume averaging, limits the resolution of all current neuroimaging.

With an MRI scan it is possible to take pictures from almost every angle, whereas a CAT scan only shows pictures horizontally. There is no ionizing radiation (X-rays) involved in producing an MRI scan. MRI scans are generally more detailed, too. The difference between normal and abnormal tissue is often clearer on the MRI scan than on the CT scan

Radiotherapy treatment is routinely done via using simulation of the treatment which is images done via a X ray machine. In some specialist hospitals as of royal marsden hospital in Sutton, Surrey now have a Ct scanner within the treatment Planning unit. A simulation uses a scaled down version of a treatment machine that can take regular X-rays. While on the simulator will outline the exact treatment areas, or fields and take x-rays to insure accuracy. This procedure may take some time. Once the simulation is complete some calculations and measurements need to be done. These calculations will determine the dose to be delivered and the length of time the patient will need to be treated for. Whether it a palliation or radical treatment. The quality of conformal therapy treatment directly depends on the precise determination of the volume of the organs at risk (OAR) and of the gross tumor volume (GTV), thus allowing an application of a different dose for each organ using the intensity modulation. Explaining why 3D is more vital to better treatment planning. The CT provides the electronic density of the tissues, thus allowing a precise calculation of the dose distribution. But sometimes its inability to visualize the tumor or to detect certain anatomical structures makes necessary its registration with the MRI. The advantages of Ct being used for treatment planning each voxel has an associated Hounsfield number linked to the tissue density. we should be aware that, when contrast agents are used, they could modify the average electronic density of the soft tissues and especially the lungs (due to being filled with air when inflation occurs of the patient breathing) CT slices that are used by the treatment planning system for dose calculation as well as to create the high quality DRR for each beam. These serve as reference images for comparison with the portal image taken at the time of the radiotherapy treatment. CT drawback is its inability to differentiate between residual tumor or fibrosis and a recurrence after radiotherapy or chemotherapy. Using the functional techniques, like PET or SPECT, a differential diagnosis should be established adequately. Ct- MRI registration system, The treatment planning systems use both volumes of voxels. The CT is used as a base for the dosimetric calculations and the DRR creation, while the registered MRI is used in the definition of the contours of organs at risk and GTV. The main advantage from a RadioTherapy point of view is that the registered images keep their specificity. The advantages using MRI for treatment planning, The spatial distortions with respect to the magnet isocenter are small. Some sequences like weighted T1, T2 (better contrast), or FLAIR (better definition of extent of lesion) are more useful than others in pinpointing the tumoral mass. Compared to CT, it allows a better definition of the anatomical structures and macroscopic tumors, mainly for cerebral, head and neck, pelvic locations. Nevertheless if one compares the size of the GTV obtained with each modality, in some locations GTVs defined with MR images are greater than GTVs defined with CT images and the reverse could be true in other locations - Excellent in some brain tumors that infiltrate the neighboring bone structures. One major obstacle with MRI within treatment planning is that you cannot convert MR images into numerical data to work out the dosages and calculations to plan the radiotherapy treatment. Verfication purposes Using a signal measured at the exit detector, delivery verification computes the energy fluence directed toward the patient. This information can be used to monitor the delivery and help to shut down the RT unit when delivery errors are detected- Research is being done at present time for MR within verification purposes and is another obstacle when using MR for treatment verification as cannot convert images into numerical data, so at present MR cannot be used for RT verification where as Ct would verify the treatment being planned when imaging the patient during treatment. Research is showing more likelihood of bringing new radiotherapy planning treatment systems and fusing MRI with CT as this solves the obstacles being found with MR at present time. The fusion replaces the bone-corresponding voxels in the registered MRI, with the corresponding CT voxels, all the others remaining unchanged. By trying to use the modulated transfer function to translate the Hounsfield numbers in electronic densities, needs an electronic densities abacus that is structure-dependant. The registration can be done:

- manually, taking into account at least three points, in the body

- semi-automatically, in which only one point should be indicated

- automatically, done by pattern recognition of the bone structures and skin surface. This method offers the best registration because it uses more points and has a faster convergence.

References

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