Fundamentals of mri

Presented by :

Nisha karna

M.Sc. 1^st year

NISHA KARNA

History

• Dr Isidor Robi( Nobel) , discovered NMR in the late 1930’s
• Bloch and Prucell were awarded the Nobel prize for physcs in 1952 for the discovery of NMR, and is widely used in assessing complex chemical compounds.

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• Prof Peter Mansfield wa awarded Nobel prize in 2003 for his discoveries in MRI ( with Prof Paul C Lauterbur)

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Introducton

• Several processes must be completed to produce magnetic resonance images, including image acquisition and image formation.
• The processes include nuclear alignment, radio frequency excitation, spatial encoding and image formation, and the hardware required to complete such processes includes:

• a magnet – for nuclear alignment

• a radio frequency source – for RF excitation

• a magnetic fi eld gradient system – for spatial encoding

• a computer system – for the image formation process and user interface

• An image processor – to convert signals into images

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Introduction

• The basic principles of MRI form the foundation
• Thera are essentially 2 ways of explaining the fundamentals of MRI: classically and via quantum physics.
• Using classical theory, MRI is explained of using the concepts of mass, spin and angular momentum on a large or bulk scale.
• Quantum theory operates at a much smaller sub atomic scale and refers to the energy levels of the protons, neutrons and electrons.

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Atomic structure

• All things are made of atoms.
• The most abundant atom in the human body is hydrogen, but there are other elements such as oxygen, carbon, and nitrogen.
• The atom consists of a central nucleus and orbiting electrons
• Atoms are characterized in two ways.

• The atomic number is the sum of the protons in the nucleus. This number gives an atom its chemical identity.

• The mass number or atomic weight is the sum of the protons and neutrons in the nucleus.

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Motion in the atom

Three types of motion are present within the atom :

• Electrons spinning on their own axis

• Electrons orbiting the nucleus

• The nucleus itself spinning about its own axis.

• The principles of MRI rely on the spinning motion of specific nuclei present in biological tissues.

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Motion in the atom

• A nucleus has no spin if it has an even atomic and mass number,
• However, in nuclei with an odd number of protons, an odd number of neutrons, or an odd number of both protons and neutrons, the spin directions are not equal and opposite, so thenucleus itself has a net spin or angular momentum.

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MR active nuclei

• MR-active nuclei are characterized by their tendency to align their axis of rotation to an applied magnetic field.
• MR-active nuclei have a net electrical charge (electric field) and are spinning (motion), and, therefore, automatically acquire a magnetic field.

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MR active nuclei

• Important examples of MR-active nuclei, together with their mass numbers are listed :
• • 1H (hydrogen)
• • 13C (carbon)
• • 15N (nitrogen)
• • 17O (oxygen)
• • 19F (fluorine)
• • 23Na (sodium).

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The hydrogen nucleus

• Protium is the most commonly used MR-active nucleus in MRI.
• It has a mass and atomic number of 1, so the nucleus consists of a single proton and has no neutrons.
• It is used because:
• hydrogen is very abundant in the human body
• solitary proton gives it a relatively large magnetic moment.
• A large gyromagnetic ratio

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Alignment

• Classical theory uses the direction of the magnetic moments of spins (hydrogen nuclei) to illustrate alignment.

• Parallel alignment: Alignment of magnetic moments in the same direction as the main B0 field (also referred to as spin-up).

• Antiparallel alignment: Alignment of magnetic moments in the opposite direction to the main B0 field (also referred to as spin-down)

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Alignment

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Alignment

• Quantum theory uses the energy level of the spins (or hydrogen nuclei) to illustrate alignment.
• Low-energy nuclei do not have enough energy to oppose the main B0 field.
• These are nuclei that align their magnetic moments parallel or spin-up
• High-energy nuclei do have enough energy to oppose the main B0 field.
• These are nuclei that align their magnetic moments antiparallel or spin-down to the main B0 field in the classical description

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Alignment

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Zee man interaction

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Alignment

• It is the magnetic moments of hydrogen nuclei that align with B0 not hydrogen nuclei themselves.

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Net magnetization vector

• Magnetic moments of hydrogen spins are constantly changing their orientation because, due to Zeeman interaction, they are always moving between high- and low-energy states.
• As there is a larger number aligned parallel, there is always a small excess in this direction that produces a net magnetic moment (NMV)

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Net magnetization vector

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Precesssion

• Each hydrogen nucleus spins on its axis
• The influence of B0 produces an additional spin or wobble of the magnetic moments of hydrogen around B0.
• This secondary spin is called precession and causes the magnetic moments to circle around B0.

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Precessional frequency

• The precessional frequency is governed by the larmor equation:

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ωo = ƴ.Bo , where ƴ = gyromagnetic ratio

ωo= precessional frequency

Bo = external magnetic field

• The gyromagnetic ratio expresses the relationship between angular momentum and the magnetic moment of each MR-active nucleus.

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Precessional frequency

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Precessional phase

• Phase refers to the position of magnetic moments on their precessional path around Bo at any moment in time.
• Out of phase or incoherent means that magnetic moments of hydrogen are at different places on the precessional path at a moment in time.
• In phase or coherent means that magnetic moments of hydrogen are at the same place on the precessional path at a moment in time.

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Resonance

• Resonance is a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.
• Resonance is achieved by transmitting an RF pulse called an RF excitation pulse.
• For resonance to occur, the frequency of the RF excitation pulse must equal the Larmor frequency of magnetic moments of the hydrogen nuclei.

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Resonance

• From the classical theory perspective, application of the B1field in a plane at 90° to B0, termed the transverse plane or x–y-axis, causes magnetic moments of the spins to precess around this axis rather than about the longitudinal plane or z-axis.
• Another consequence of the RF excitation pulse is that the magnetic moments of the spin-up and spin-down nuclei move into phase with each other.
• The RF excitation pulse gives energy to hydrogen nuclei and causes a net increase in the number of high-energy, spin-down nuclei and it results in in the movement of NMV from its alignment.

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Resonance

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Flip angle

• The angle to which the NMV moves of its alignment is called flip angle
• The magnitude of the flip angle depends on the amplitude and duration of the RF excitation pulse.
• Usually, the flip angle is 90°, i.e. the NMV is given enough energy by the RF excitation pulse to move through 90° relative to B 0 .
• As the NMV is a vector, even if flip angles other than 90 are used, there is always a component of magnetization in a plane perpendicular to B0.
• B0 is termed as longitudinal palne
• And the plane at 9o to Bo is termed as the transverse plane.

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MR Signal

• Because of resonance, in-phase or coherent magnetization precesses in the transverse plane.
• Acc. to Faradays law, a changing ,magnetic field causes movement of charged particles i,.e. electros
• This flow of electrons is a current, and if a receiver coil is placed in a mag field, a voltage generated by this current is induced in the receiver coil.
• This is signal and is produced when coherent magnetization cuts across the coil

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The FID signal

• When the RF excitation pulse is switched off, the NMV is influenced only by B0, and it tries to realign with it.
• the hydrogen nuclei lose energy given to them by the RF excitation pulse.
• The process by which hydrogen loses this energy is called relaxation.
• The amount of magnetization in the longitudinal plane gradually increases – this is called recovery
• At the same time but independently, the aount of magnetization in the transverse palne decreases – this is called decay.
• The induction of decaying voltage is called the free induction decay (FID) signal .

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Relaxation

• During relaxation, hydrogen nuclei give up absorbed RF energy to surrounding spin- lattice and the NMV religns with B0.
• And at the same time but independently the nuclei lose coherency due to dephasing spin-spin interaction.
• The recovery of longitudinal magnetization is caused by a process termed as T1 recovery.
• The decay of transverse magnetization is caused by a process termed T2 decay

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T1 recovery

• T1 recovery is caused by the nuclei giving up their energy to the surrounding environment or lattice, and it is termed spin lattice relaxation
• The rate of recovery is an exponential process, with a recovery time constant called the T1 relaxation time
• This is the time it takes 63% of the longitudinal magnetizati on to recover in the tissue

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T2 decay

• T2 decay is caused by the magnetic fields of neighbouring nuclei interacting with each other.
• It is termed spin - spin relaxation and results in decay or loss of coherent transverse magnetization
• The rate of decay is also an exponential process, so that the T2 relaxation time of a tissue is its time constant of decay
• It is the time it takes 63% of the transverse magnetization to be lost (37% remains)time constant of decay.

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Pulse timing parameters

• A pulse sequence consists of several time periods: the main one are
• The repetition time (TR) is the time from the application of one RF pulse to the application of the next RF pulse for each slice and is measured in milliseconds (ms).
• The TR determines the amount of longitudinal relaxation that is allowed to occur between the end of one RF pulse and the application of the next.
• TR thus determines the amount of T1 relaxation that has occurred when the signal is read.

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Pulse timing parameters

• The echo time (TE) is the time from the application of the RF pulse to the peak of the signal induced in the coil and is also measured in ms.
• The TE determines how much decay of transverse magnetization is allowed to occur.
• TE thus controls the amount of T2 relaxation that has occurred when the signal is read.
• The application of RF pulses at certain repetition times and the receiving of signals at predefined echo times produce contrast in MRI images.

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Basic pulse sequence

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IMAGE WEIGHTHING AND CONTRAST

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• All clinical diagnostic images must demonstrate contrast between normal anatomical features and between anatomy and any pathology.
• One of the main advantages of MRI compared with other imaging

modalities is the excellent soft tissue discrimination of the images.

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Image contrast

• The factors that affect image contrast in diagnostic imaging are usually divided into two categories.:
• Intrinsic contrast parameters are those that cannot be changed because they are inherent to the body ’s ti ssues.

• Extrinsic contrast parameters are those that can be changed.

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Image contrast

Intrinsic contrast parameters are:

• T1 recovery time
• T2 decay time
• Proton density
• Flow
• ADC

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Image contrast

Extrinsic contrast parameters are:

• TR
• TE
• Flip angle
• TI
• Turbo factor/ ETL
• B value

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Image Contrast

• An MR image has contrast if there are areas of high signal (white on the image) and areas of low signal (dark on the image).
• A tissue has a high signal if it has a large transverse component of coherent magnetization at time TE.
• If there is a small component of transverse coherent magneti zati on, the amplitude of the signal received by the coil is small, resulti ng in a dark area on the image.
• Images obtain contrast mainly through the mechanisms of T1 recovery, T2 decay and proton or spin density.

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Image Contrast

• The proton density of a tissue is the number of mobile hydrogen protons per unit volume of that tissue.
• T1 and T2 relaxation depend on three factors:

1. The inherent energy of the tissue : If the inherent energy is low, then the molecular lattice is more able to absorb energy from hydrogen nuclei.

• This is especially important in T1 relaxation processes, which rely on energy exchange between the hydrogen nuclei and the molecular lattice (spin lattice).

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Image Contrast

2. How closely packed the molecules are: in tissues where molecules are closely spaced, there is more efficient interaction between the magnetic fields of neighboring hydrogen nuclei.

• This is especially important in T2 decay processes.

3. H ow well the molecular tumbling rate matches the Larmor frequency of hydrogen: If there is a good match between the two, energy exchange between hydrogen nuclei and the molecular lattice is efficient.

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Relaxation in different tissues

• Generally, the two extremes of contrast in MRI are fat and water
• Fat molecules contain atoms of hydrogen arranged with carbon and oxygen, which are closely packed together and their molecular tumbling rate is slow.
• Whereas, in water, the molecules are spaced apart and their molecular tumbling rate is fast.
• Hydrogen in fat recovers more rapidly along the longitudinal axis than water and loses transverse magnetization faster than in water.
• Subsequently, fat and water appear differently in MR images.

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Relaxation in different tissues

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T1 recovery in fat

• Since T1 recovery occurs due to nuclei giving up their energy to the surrounding environment.
• Fat has a low inherent energy and can easily absorb energy into its lattice from hydrogen nuclei.
• And also the slow molecular tumbling allows the recovery process to be rapid.
• The NMV of fat realigns rapidly with B0 so the T1 time of fat is short

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T1 recovery in water

• Water has a high inherent energy and cannot easily absorb energy into its lattice from hydrogen nuclei.
• Molecular mobility is high, resulting in less efficient T1 recovery
• The NMV of water takes longer to realign with B 0 and so the T1 time of water is long

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T2 decay

• In fat :
• As the molecules are packed closely together and therefore spin – spin interactions are more likely to occur in fat, resulting in short T2 time of fat
• In water:
• As the molecules are spaced apart and spin – spin interactions are less likely to occur, resulting in long T2 time of water

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T2 decay

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Fig: T2 decay in fat

T2 decay

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Fig: T2 decay in water

T1 contrast

• T1 contrast means that image contrast is derived from differences in the T1 recovery times of the tissues rather than any other mechanism.
• The T1 recovery time of fat is much shorter than that of water, so the fat vector realigns with B0 faster than the water vector.
• Fat are brighter on T1 images ad water is hypointense

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T2 contrast

• The T2 time of fat is shorter than that of water, so the transverse component of magnetization of fat decays faster.
• Since the transverse magnetization of water is large it appears bright on T2 contrast image.
• Such images are called T2 weighted images.

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T1 and T2 relaxation time

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Proton density contrast

• Proton density contrast refers to differences in signal intensity between tissues that are a consequence of their relative number of mobile hydrogen protons per unit volume.
• Tissues with a high proton density have a large transverse component of magnetization and are hyperintense.
• Proton density contrast is always present and depends on the patient and the area under examination.

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Weighting

• All the intrinsic contrast parameters listed above affect contrast so it is possible to obtain images of mixed aapereance.
• To demonstrate T1, T2, or proton density weighting, specific values of TR and TE should be selected.

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T1 weighting

• A T1-weighted image is one where contrast depends predominantly on the differences in the T1 recovery times between fat and water
• TR controls the amount of T1 contrast.

• For T1 weighting, the TR must be short and the TE must also be short

• T1-weighted images are used to show anatomy and pathology post contrast enhancement.

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T1 weighting

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T2 weighting

• A T2 weighted image is one where the contrast predominantly depends on the differences in the T2 times between fat and water
• TE controls the amount of T2 weighting.
• For T2 weighting the TE must be long .

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T2 weighting

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Proton density weighting

• To achieve proton density weighti ng, the eff ects of T1 and T2 contrast must be diminished so that proton density weighti ng can dominate.
• Long TR and short TE gives the PD weighted images

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PD weighting

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T2* Decay

• When the RF excitation pulse is removed, the relaxation and decay processes occur immediately.
• T2 * decay is the decay of the FID following the RF excitation pulse.
• This decay is faster than T2 decay since it is a combinatiion of two effects:
• T2 decay itself
• dephasing due to magnetic field inhomogeneities.

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T2* Decay

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Pulse sequence

• Spin echo pulse sequence :

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Spin echo pulse sequence : �

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GE pulse sequence

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Encoding

• After the signal is produced the system must be able to locate signal spatially in 3 dimensions, so that each signal could be positioned at correct point on the image.
• It is achieved by gradients.
• There are 3 gradient coils situated within the bore of the magnet, named acc. to the axis along which they act.

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Gradients

• T he Z gradient alters the magneti c fi eld strength along the Z - ( long ) axis of the magnet.
• The Y gradient alters the magnetic field strength along the Y - (vertical ) axis of the magnet.
• The X gradient alters the magnetic field strength along the X - (horizontal ) axis of the magnet.

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Gradients

• Gradients can be used to either dephase or rephase the magnetic moments of nuclei as in GE seq.
• Gradients also perform:

1. Slice selection – locating a slice within the scan plane selected.

• Spatially locating (encoding) signal along the long axis of the anatomy – this is called frequency encoding .
• Spatially locating (encoding) signal along the short axis of the anatomy – this is called phase encoding .

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Slice selection

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Frequency encoding

• Once the slice selection has been done, the signal coming from it must be encoded along both axis of the image.
• Encoding along long axis of the anatomy.

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Frequency encoding

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Frequency encoding

• In coronal and sagittal images, the long axis of the anatomy lies along the Z- axis of the magnet and therefore, the Z gradient performs frequency encoding
• In axial images, the long axis of the anatomy usually lies along the horizontal axis of the magnet and therefore, the X gradient performs frequency encoding.

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Phase encoding

• In short axis of the image.
• When the phase encoding gradient is switched on, the magnetic field strength, the precessional frequency and the phase of nuclei along the axis of the gradient is altered.
• The phase encoding gradient is usually switched after the application of the excitation pulse

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Phase encoding

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Phase encoding

• In coronal images the short axis of the anatomy usually lies along the horizontal axis of the magnet, therefore the X gradient performs phase encoding.
• In sagittal images the short axis of the anatomy usually lies along the vertical axis of the magnet, therefore the Y gradient performs phase encoding.
• In axial images the short axis of the anatomy usually lies along the vertical axis of the magnet, therefore the Y gradient performs phase encoding.

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Encoding

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Encoding

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Sampling

• Sampling frequency is the rate at which frequencies are sampled during the acquisition window per second
• The sampling frequency thus determines how many data points can be acquired during the acquisition window
• It therefore also determines the time interval between each sample called sampling interval

sampling interval = 1/sampling frequency.

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Sampling

• In MRI, the sampling frequency is determined by the Nyquist theorem
• The Nyquist theorem states that when digitizing a signal with a range of analogue frequencies , the highest frequency must be sampled at least twice as fast to accurately digitize or represent it.

sampling frequency = 2 × Nyquist frequency

receive bandwidth = 2 × the highest frequency (Nyquist frequency).

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K- space

• K space is a storage device
• When data of each signal positi on are collected, the informati on is stored as data points in the array processor of the system computer.
• The data points are stored in K space .
• K space is a spatial frequency domain, i.e. where information about the frequency of a signal and where it comes from in the patient is stored
• The unit of K space is radians per cm.

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K- space

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K- space

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K- space

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Important facts about K -space

1. K space is not the image : data points in the top line of K space do not result in the top of the image.
1. In fact, every data point contains information from the whole slice.
2. Data are symmetrical in K space: data in top half of K space are identical to those in bottom half.

3. Data acquired in the central lines contribute signal and contrast, while data acquired in the outer lines contribute resolution

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Types of data acquisition

1. Sequential : in this all the data from slice 1 is acquired then go on to acquire all the data from slice 2, might be used for breath-holding techniques.

2. Two dimensional volumetric: in this, one line of k-space is filled for slice one, and then go to fill the same line of k-space for slice 2 etc.

• When this has been filled for all the slices, the next line is filled for all the slices.
• It is most common type of data acquisition.

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Types of data acquisition

3. Three dimensional volumetric: acquires data from an entire volume of tissue rather than in separate slices.

• Volume imaging allows reformatting in any plane
• Volume imaging increases the SNR as a whole volume of tissue is exicted , but also increases the scan time.

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Fast fourier transform

• To produce an image from the acquired data points we need to complete a mathemati cal process called Fast Fourier transform or FFT.
• An MR image consists of matrix of pixels, the no. of which is determined by the no. of lines filled in k space(phase matrix) and the no. of data points in each line (frequency matrix).
• As a result of FFT, each pixel is allocated a color on a grayscale corresponding to the amplitude of specific frequencies coming from the same spatial location as represented by that pixel.

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Summary

• T1 relaxation results in the recovery of longitudinal magnetization due to energy dissipation to the surrounding lattice

• T 2 relaxation results in the loss of coherent transverse magnetization due to interactions between the magnetic fi elds of adjacent nuclei

• A signal or voltage is only induced in the receiver coil if there is coherent magnetization in the transverse plane, that is, in phase

• Fat has short TR and short TE
• T1 weighted images- short TR , short TE

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Summary

• T2 weighted images- long TR, long TE
• PD weighted images- long TR, short TE

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Refrences

• Mri in practice Catherine westbrook, Carolyn kaut roth, John Talbot Wiley Blackwell 4^th and 5^th edition
• Mri made easy M.D. Chavahan ,Govind B, Jaybee Brothers 2^nd edition
• Various websites

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Thank you

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