Mri Superiority in Soft Tissue Imaging Compared to Ct

MRI superiority in soft tissue imaging compared to CT

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Introduction

Magnetic Resonance Imaging (MRI), Magnetic Resonance Tomography (MRT) or NMRI (Nuclear Magnetic Resonance Imaging) refers to a radiology technique which utilizes radio waves, magnetism and a computer in the production of body images structures. The MRI scanner is a tube surrounded by a circular magnet as shown below.

The patient is placed on the movable table shown above which is fitted with a magnet. Consequently, the magnet forms a strong magnetic field with hydrogen atoms protons. These are thus exposed to a radio waves beam that spins various body protons and a faint signal is produced that is identified by the MRI scanner receiver portion after which a computer processes the received information and an image is produced of the specific body part.

The resolution of the images produced by the MRI is very clear, thus use of MRI provides varying information about body structures which are unobtainable when Computed Tomography (CT), standard x- ray or ultrasound are being used. For instance, a joint MRI exam provides comprehensive cartilage and ligaments images. A contrast agent, a magnetically active material, is utilized to show the internal abnormalities or structures more clearly.

Through the use of MRI, physicians are able to monitor treatments and diagnose various medical conditions. These include abnormalities in the abnormalities and injuries in the joints, particular heart problems. The intensity of signals in MRI depends on various intrinsic tissue properties. The relative significance of properties is dependent on precise MRI procedure parameters, which allows for extensive image contrast control.

MRI scan is advantageous compared to CT scan since it uses magnetic field to build pictures of body to build pictures and thus does not have any known effects compared to the CT scan that uses x-rays, MRI also provides higher details than CT scan, RI is also able to change the contrast of images hence highlighting different parts of the tissues and changing the imaging plane without moving the patient something that is not possible when taking a CT scan1.

MRI Physical Principle

The proton which creates the hydrogen atom nucleus has a spin and charge. Hence, it has a magnetic moment. According to the nuclear magnetic resonance semi- classical model, it is thought that the proton is a minute spinning magnet. If there is no external magnetic field, there is a random orientation of the protons. In case (Bo) external magnetic field is applied, for instance by putting the patient in the MRI scanner magnet, protons align against or with the magnetic field, and form around it . The procession frequency is dependent on external magnetic field strength. This is elaborated by Larmor equation, which is considered the principal MRI equation:

W= YB

W= precession frequency

Y=the constant referred to as gyromagnetic ratio

B=external magnetic field strength

If there is an external magnetic field, there are more protons which are aligned in low energy state than against the magnetic field. This leads to net magnetization in the direction. In case a second and much weaker (B1) magnetic field is applied to the 1 field at ninety degrees, and oscillated at the same frequency as the precession frequency (resonant frequency), proton magnetization spirals down in a plane that is perpendicular to external magnetic field. Therefore, there is all protons' magnetization is aligned. The oscillating magnetic field (B1) that is necessary in order to achieve this is referred to as the 900 excitation pulse.

Hence, after applying the resonant frequency' oscillating magnetic field to a patient who has been placed on a magnet, a hydrogen nuclei signal can be identified at a similar frequency. This frequency is dependent on the magnet's strength3

Spin echoes and Gradient

The rotation frequency of proton magnetization is dependent on Bo (external magnetic field' strength). In case the magnetic field subjected to the patient is uniform, but escalates from right to left, the protons' rotation frequency will also intensify from right to left. A non- uniform field like this can be formed through superimposing a minute magnetic field gradient on an external magnetic field. This can be achieved by passing current down positioned gradient or wire coils.

Since the patient's left hydrogen nuclei process slowly compared to those on the right, they begin lagging behind Therefore, hydrogen nuclei at various places in the patient move from the phase together. In addition, magnetization from various regions cancels out. As a result, the detected signal and net magnetization diminishes. However, field gradients reversal through inversion of the current in the gradient coils makes the nuclei on the patient's left to process quickly than the right ones. The signal and net magnetization increases to the maximum whenever the magnetization across a patient is on phase, and consequently decreases again. Refocusing the NMR signal is referred to as gradient echo. The spin echo is regarded as the alternative to a gradient echo for NMR signal regeneration.

Image formation

To be able to generate a patient's image, the NMR signal should be localized in space. To achieve this, only those protons are selected first through a thin slice. Consequently, the signal from the slice is encoded spatially in 2 perpendicular directions. Hydrogen nuclei excitation from patients using static magnetic field needs a magnetic field that is oscillating at a frequency that matches the nuclei precession frequency In case a gradient is created in a magnetic field in a patient, the precession frequency varies at different positions.

Image contrast

The darkness or brightness of a tissue representation on MR images denotes the detected signal intensity. The signal intensity is dependent on various intrinsic tissue MR features including hydrogen nuclei concentration, T1, T2* and T2 relaxation times Variations in the properties result in signal intensity differences, and therefore the difference between the image's various tissues. The relative

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