ARUN'S RADIOLOGY

Tracheal stenosis in MRI

2012-06-15T04:50:00.000-07:00

Non contrast MR angio.....

2012-02-14T22:41:00.000-08:00

NATIVE (Non-contrast MRA of ArTerIes and VEins)

syngo NATIVE is a contrast-free MR angiography technique for visualizing the vessels of the body. The package contains protocols tailored for use in different body regions (e.g. renal arteries, peripheral vessels). Inline Subtraction and Inline Maximum Intensity Projection (MIP) further simplify the workflow.

Clinical Applications

Alternative to contrast-enhanced MRA examinations when there is contraindication to use of Gadolinium.

Additional Information

For non-contrast MR Angiography Siemens currently offers two techniques under the category of syngo NATIVE —syngo NATIVE TrueFISP and syngo NATIVE SPACE. syngoNATIVE TrueFISP is based on the TrueFISP (True Fast Imaging with Steady state Precession) sequence, which is a balanced steady state gradient echo technique. The contrast mechanism for syngo NATIVE TrueFISP comes from the preparation of the imaging volume with a spatially selective inversion pulse, resulting in the suppression of stationary tissue within the imaging volume and suppression of signal from blood in the imaging volume (e.g. venous blood). Blood which flows into the imaging volume during the inversion time has the same high-signal characteristics exhibited in that TrueFISP. The contrast is further enhanced by the suppression of the background by the inversion pulse. The sequence can be made selective for arteries or veins by appropriate positioning of the inversion pulse, which can be positioned independently from the imaging volume. The sequence accommodates 3D, 2D, breath-hold, syngo PACE (Prospective Acquisition CorrEction) navigated and respiratory triggered approaches depending on clinical environment.

The syngo NATIVE SPACE technique is a modified variable flip angle 3D Turbo Spin Echo sequence in which the contrast mechanism for visualizing vessels is based on the difference in intravascular signal between maximal and minimal flow during the cardiac cycle. The subtracted image is calculated Inline and with Inline MIP generation, instantaneous clinical results are produced for completely non-invasive MR angiography, further improving workflow at the scanner. The syngoNATIVE SPACE technique can also accommodate multi-phase imaging that enables dynamic angiography — for example, in the lower legs.

curtsy to

https://www.medical.siemens.com.

A confusing bleed ???

2012-01-16T05:13:00.000-08:00

A Patient Present with

LOWER ABDOMEN PAIN ---- SINCE 2007

USG 29/12/11--> RIGHT OVARY SHOWS A CYST WITH MULTIPLE LOW LEVEL INTERNAL ECHOES
? INFECTIONS / HEMORRHAGE IN THE CYST
BULKY UTERUS MEASURING 10.1X6.3X5.3 CM
T1 AXIAL(OBL)

-----------------------------------------------------------

T2 AXIAL(OBL)

----------------------------------------------------------
STIR AXIAL(OBL)

---------------------------------------------------------
T1 FAT SATURATED AXIAL(OBL)

Is it a bleed(Hemorrhage )???

If yes ->Why STIR Showing Hypo-intense signal in that lesion ???

If No -> Why T1 FS showing Hyper-intense signal
in that lesion???

CT ABDOMEN -ANATOMY

2011-10-28T09:05:00.000-07:00

watch an academic review of CT abdomen

Top 10 Unfortunate X-Rays

2011-10-28T08:02:00.000-07:00

Hi friends see 10 unfortunate x-rays,it is amazing......

2011-10-27T20:48:00.001-07:00

Shoulder Joint

Elbow joint

Hip Joint

"Neurofibroma"

2011-10-27T20:37:00.001-07:00

"Neurofibroma"
---Patients History
--->young age patient comes for MRI KNEE(Left)
----->C/o Pain and small & hard swelling on popliteal fossae

See from where it starts .............

Neurofibroma is a benign nerve sheath tumor in the peripheral nervous system
Read more about Neurofibroma

"Trigonocephaly"

2011-10-27T20:35:00.000-07:00

"Trigonocephaly"

MRI is better to demonstrate Trigonocephaly (Without radiation exposure)

See Trigonocephaly shown in MR-SSD..............

Impression :- Trigonocephaly -Premature closure of metopic suture

Susceptibility artifacts and Bandwidth

2011-10-04T04:54:00.000-07:00

Hi friends see the difference in Susceptibility artifacts when changing bandwidth......

Diffrence between SE and GRE in brain post contrast study

2011-07-28T05:44:00.000-07:00

See the diffrence between SE and GRE images ,

The intensity profile shows more than 50% signal intensity lose ...

so don't trust GRE post contrast study . do at least one SE series in brain post contrast study....

Parallel acquisition technique in MRI - PAT

2011-04-25T05:01:00.000-07:00

Parallel imaging is the one of the advanced techniques recently developed in MRI. It is based on the spatial information inherent in phased array Radio frequency coils and reduces the scan time. In PAT the number of sampled K –space line is reduced, often by a factor of two or greater there by significantly shortening the acquisition time .It can be used in FSE (or TSE) and SS FSE (or HASTE) to improve spatial resolution by reducing ETL. It is used in 90 % of the sequences in higher Tesla machines. The limitations of PAT include reduced SNR and reconstruction artifacts.

History of PAT

The introduction of multiple overlapping small coils replace the large coils,as it cover the same volume as the large coil. when the signals from individual coils are combined, the noise is substantially reduced and SNR is signiﬁcantly improved. the introduction of multiple overlapping small coils leads to the introduction of phased-array Coils.

The phased array coils operate typically as receive only coils. In that case, in the MRI device implemented body coil act as thetransmitter and sends the radio frequency energy to generate the excitation pulses. State-of-the-art array coil systems include the use of 4 (up to 32) coils with separate receivers. This method is often referred to as a phased array system, although the signals are not added such that the signal phase information is included. The use of phased array coils allows the decreasing of the number of signal averages, which shortens the scan time by high SNR and resolution.

Theory –(basic)

The basis of these techniques is the concept that acquisition time is proportional to the number of phase-encoding lines in a Cartesian acquisition. Increasing the distance between phase-encoding lines in k-space by a factor R while keeping the spatial resolution ﬁxed reduces the acquisition time by the same factor. This also decreases the ﬁeld of view (FOV), resulting in aliasing or wraparound artifact .In parallel imaging, the spatial dependence of the phased-array coil elements is used to remove or prevent the aliasing in Fourier transform. Different vendors uses verity of techniques based on PAT .

The different vendors and their techniques are shown in a table

fig-2

Figure 2. Example of the SENSE technique. Reference image of a phantom (a), obtained with the

body receiver coil. Sum-of-squares image (b) from two individual coil elements. Spins at spatial locations closer to the coil elements (arrowheads) exhibit higher signal levels than those that are farther away (arrow). A corresponding reference k-space containing 20 phase-encoding views is shown at left. FT = Fourier transform. Low-resolution full-FOV calibration images (c and d) for each coil, which were obtained to calculate the complex sensitivity proﬁles of each coil element. Sensitivities (s1 and s2) of each coil element are illustrated at two arbitrary points. Reduced-FOV SENSE images (e and f) from each coil element contain extensive wraparound artifacts along the superior-to-inferior phase-encoding axis. a1 and a2 represent the signal intensities of the aliased images that correspond to the arbitrary points chosen in c and d. Diagram of the unaliased image (g) with desired pixel intensities p1 and p2. These intensities can be calculated because the sensitivities of the individual coil elements and the pixel intensities of the aliased images are known. Unaliased image without (h) and with (i) intensity correction, which represents the SENSE-reconstructed image.

Applications of PAT

MR Angiography- fig-3
Dynamic Contrast-enhanced Imaging - fig-4
Fast SE Imaging - fig-5
Cardiac Imaging - fig-6
Breast Imaging - fig 7
MR Enterography and Colonography - fig 8

Limitations of PAT

Limitations of parallel imaging include reduced SNR and reconstruction artifacts. It is important to consider these limitations when deciding when to employ these techniques.

fig-3

Figure 3-Partial volume maximum intensity projection images from renal MR angiography of a patient with ﬁbromuscular dysplasia, obtained without (a) and with (b) SENSE (acceleration factor = 2, applied in the inplane right-left phase-encoding direction). In the standard image, early ﬁlling of the left renal vein limits visualization of the left renal artery. Venous contamination is eliminated in the SENSE image, which was obtained with the acquisition time reduced by half. Collecting system activity in the SENSE image is due to excretion of contrast material from the ﬁrst non-SENSE acquisition

Figure 4 Axial arterial phase (a, b) and portal venous phase (c, d) fat-saturated 3D SPGR images of a patient

with a metastatic neuroendocrine tumor. The images were obtained with SENSE (acceleration factor _ 2, applied in

the anteroposterior phase-encoding direction), with a and c acquired slightly superior to b and d. SENSE was used

to achieve both excellent temporal and excellent spatial resolution. In this case, achieving optimal timing and adequate

coverage in the arterial phase was crucial because many of the early enhancing hepatic and osseous metastases

could not be visualized on images from subsequent phases.

Figures 5) Coronal single-shot fast SE image obtained without (a) and with (b) SENSE (acceleration

factor _ 2). Although the SENSE image has slightly diminished SNR, it also has slightly less blurring. (13) Maximum

intensity projection image from 3D fast-recovery fast SE MR cholangiopancreatography. SENSE (acceleration

factor _ 2) was used to reduce the acquisition time sufficiently so that an adequate volume could be obtained within

a comfortable breath hold.

Figure 6. Short-axis cardiac steady-state free precession cine images (in end diastole) obtained

from apex to base by using SENSE (acceleration factor = 2). Three sections were acquired per

breath hold, allowing the total acquisition time to be considerably shortened.

Figure 7. Sagittal image of a patient with residual breast carcinoma after an excisional biopsy, obtained with a dynamic contrast-enhanced 3D fat-saturated SPGR sequence. Use of

SENSE (acceleration factor = 2) allowed coverage of both breasts with adequate spatial resolution and adequate temporal resolution.

Figure 8. Coronal contrast-enhanced fatsaturated 2D SPGR image of a patient with

Crohn disease shows mild thickening and enhancement of the terminal ileum. SENSE (acceleration factor 2) was used to reduce the acquisition time and thereby decrease motion artifact caused by peristalsis

Note-PAT-Parallel Acquisition Technique, FSE-Fast Spine Echo, TSE-Turbo Spine Echo,

SS FSE-Single Shot Fast Spine Echo, HASTE-Half Acquisition study Time Echo, SNR-signal to Noise Ratio, ETL-Echo Train Length, FT = Fourier transform, SPGR-Spoiled Gradient Recalled Acquisition, Crohn disease –it is an inflammatory disease of the intestines

MR Cysto-colpo-proctography

2011-03-16T05:24:00.000-07:00

It is a pelvic floor imaging method in MRI (Cysto-Bladder,colpo-uterus ,procto-rectum )

Protocol

•Axial, coronal and sagittal T2 whole pelvis

•Sagittal midline T2 -3 images

–At rest

–Squeezing/Clenched

–Straining/Bearing down

–

•Sagittal T2 Bearing down 15 images

–

•Sagittal T2 single slice 1/second during evacuation

–

•Sagittal T2 post evacuation Valsalva

Ce-MR angiography

2011-02-23T06:34:00.000-08:00

Ce-MR angiography Con..

2011-02-23T06:32:00.000-08:00

Cardiac mri- planning & slice anatomy

2010-10-11T10:21:00.000-07:00

Cardiac mri- planning & slice anatomy

Cardiac mri&slice anatomy
View more presentations from arunmtek.

Cardiac MRI

2010-06-08T21:08:00.001-07:00

Cardic Mri

Cardiac MRI

View more documents from arunmtek.

Artifacts in MRI

2010-06-08T20:37:00.001-07:00

A Short list of mri artfacts

Artifacts in MRI

View more presentations from arunmtek.

Gre&SE

2010-06-08T05:57:00.001-07:00

Check out this SlideShare Presentation:

Gre&SE

View more presentations from arunmtek.

SIMPLE MRI PHYSICS

2009-10-11T20:45:00.000-07:00

Note:If you got errors on this page please open with Mozilla...................

SIMPLE MRI Physics

Quantum Mechanics

Year -Researcher

1901 -Planck

1905 -Einstein

1913 -Bohr

1922 -Compton
1924 -Pauli

1925 -De Brodlie

1926 -Schroedinger

1927 -Heisenberg

1927 -Davison, Germer and Born

MRI-->Magnetic Resonance Imaging

M is for Magnetic

Ø Atomic structure

Neutron
Proton
Electron

Ø SPIN

Motion within the atom

Ø MR active nuclei:

Ø Why H1?

1000 Billion Billion Atom in adult

Solitary Proton gives it a large magnetic moment

Ø What happen when placed in external magnetic field? (Quantum Theory)

Spin Down

Spin Up

High Energy

NNSS

Low Energy

NSNS

Ø "NMV"

Net Magnetization Vector

Ø PRECESSION

Ø Larmor Equation

Frequency α Magnetic Field

ω α β

Incoherent Magnetic moment of H is

At different place in Precessional Path

Coherent Magnetic moment of H is

Placed in the same place in Precessional Path

M Stands..........

Ø Atomic structure

Ø SPIN

Ø MR active nuclei H1

Ø What happen when placed in external Z magnetic field (Spin Up and Spin Down)

Ø NMV

Ø Precession

Ø Larmor Equation

Ø Coherent

R is for Resonance

Ø Resonance Definition:

Energy transition that occur when object is subjected to frequency the same as its own

Here;

Radio Frequency "RF"

Same Frequency of H nuclei

At 90 degree toB0

Ø 2 things happen at Resonance:

1- Energy Absorption

Increase number of High energy Spin Up nuclei

2- Phase Coherence

NMV precesses in transverse plane at Larmor Frequency

MR Signal

NMV rotates around transverse plane

It passes across Receiver Coil

Inducing voltage in it

RF Removed à Signal decreased à Amplitude of MR Signal decreased

Ø Free Induction Decay "FID":

Free (No RF Pulse)

ID (because of Decay of Induced signal in Receiver Coil)

Ø Radio Frequency "RF”

Ø 2 things happen at Resonance

Ø Energy Absorption

Ø Phase Coherence

Ø Free Induction Decay "FID"

Ø MR Signal

I is for Imaging

Ø Areas of High Signal

Ø Areas of Low Signal

Ø Areas of Intermediate Signal

NMV can be separated in to

Individual Vectors of tissue present in the patient

Such as Fat, CSF & Muscle

Low Signal

High Signal

Intermediate Signal

White

Black

Grey

Small transverse component of magnetization

Large transverse component of magnetization

Ø Gradient Magnets

Used to vary magnetic field in known manner

Each point has slightly different rate of precession & Larmor Frequency.

Variety of signal released by Protons returning to z-plane can used to determine the composition of exact location of each point.

Ø "Gradient Function"

1.Slice selection

2.Frequency encoding

3.Phase encoding

Ø Contrast Mechanisms

Relaxation Process

After removal of RF pulse

Signal induced in Receiver Coil decreased

Why?

NMV recovers and realign to B0this process called "T1 Recovery"
Nuclei loss Precessional coherence or dephase and NMV decay in the transverse planethis process called "T2 Decay"

Ø T1 time is

An intrinsic contrast parameter that inherent to tissue being imaged

Ø T2 Decay is

an intrinsic contrast parameter and is inherent to the tissue being imaged

Ø Repetition Time "TR"

Definition

Time from application of one RF pulse

To the application of the next

(It affects the length of relaxation period

after application of one RF excitation pulse

to the beginning of the next)

Ø Echo Time "TE"

Definition

Time between RF excitation pulse and

collection of signal

(It affects the length of relaxation period

after removal of RF excitation pulse

and the peak of signal received in receiver coil)

Flip Angle

Angle throw which the NMV moved as result of a RF excitation pulse

Ø Image contrast controlled by:

Extrinsic Contrast parameters:

TR, TE & Flip Angle

Intrinsic Contrast parameters:

T1 Recovery, T2 Decay, Proton Density, Flow & Apparent Diffusion Coefficient

Ø T1 Recovery

Caused by exchange of energy from

nuclei to their surrounding environment or lattice

"Spin Lattice Energy Transfer"

and realign in B0

this occur in exponential process

at different rates in different tissue

NB: Molecules are constantly in motion; Rotational and Transitional

T1 in Fat

T1 in Water

T1 is very short

absorb energy quickly

i.e. nuclei dispose their energy to surrounding fat tissue and return to B0 in very short time

T1 is longer

inefficient at receiving energy

i.e. nuclei take allot longer to dispose energy to surrounding water tissue

Ø Short TR --->T1 contrast (T1 Weighted)

TR 300-600 ms

TE 10-30 ms

Ø T2 Decay

Caused by exchange of energy from one nucleus to another

As result of intrinsic magnetic fields of nuclei interlacing with each other

This energy exchange à loss of coherence or dephasing

and as result NMV decay in transverse plane

T2 is exponential process

occur at different rates in different tissues

Contrast Mechanisms (T2 Decay)

Fat much better at energy exchange than Water

Ø Because this T2 depend on:

1-How closely molecular motion of atoms matches Larmor Frequency

2-Proximity of other spins

So;

Fat's T2 time is very short compared to water

Ø Long TE àT2 contrast (T2 Weighted)

TR 2000 ms

TE 70 ms

Ø Proton Density

Long TR --->Proton density

TR 2000 ms

TE 10-30 ms

Ø Contrast Media

· As Gadolinium

· Local magnetic field fluctuation occur near Larmor frequency

· T1 Relaxation times of nearby protons are reduced

· So they appear brighter in T1 weighted Image

Ø Areas of High Signal, Low Signal, & Intermediate Signal

Ø Gradient Magnets

Ø T1 Recovery

Ø T2 Decay

Ø Contrast parameter

Ø T1, T2&PD Imaging

Ø Contrast Media

ARUN A
MRI TECHNOLOGIST
DR SHAJI'S MRI &
MEDICAL RESEARCH CENTER (P) Ltd.

Artifacts in MRI

2009-09-28T10:29:00.001-07:00

Artifacts in MRI

MRI physics

2009-09-28T10:14:00.001-07:00

MRI physics

INDUSRTIAL RADIOGRAPHY images

2009-01-08T21:05:00.000-08:00

BASIC MRI

2008-12-17T06:04:00.000-08:00

Spin
What is spin? Spin is a fundamental property of nature like electrical charge or mass. Spin comes in multiples of 1/2 and can be + or -. Protons, electrons, and neutrons possess spin. Individual unpaired electrons, protons, and neutrons each possesses a spin of 1/2.
In the deuterium atom ( 2H ), with one unpaired electron, one unpaired proton, and one unpaired neutron, the total electronic spin = 1/2 and the total nuclear spin = 1.
Two or more particles with spins having opposite signs can pair up to eliminate the observable manifestations of spin. An example is helium. In nuclear magnetic resonance, it is unpaired nuclear spins that are of importance.
Properties of Spin
When placed in a magnetic field of strength B, a particle with a net spin can absorb a photon, of frequency . The frequency depends on the gyromagnetic ratio, of the particle.
= B
For hydrogen, = 42.58 MHz / T.
Nuclei with Spin
Nuclei are composed of positively charged protons and uncharged neutrons held together by nuclear forces. Both protons and neutrons have approximatevely the same mass, which is about 1840 times as large as the mass of an electron. Neutrons and protons are referred to collectively as nucleons.
The shell model for the nucleus tells us that nucleons, just like electrons, fill orbitals. When the number of protons or neutrons equals 2, 8, 20, 28, 50, 82, and 126, orbitals are filled. Because nucleons have spin, just like electrons do, their spin can pair up when the orbitals are being filled and cancel out. Almost every element in the periodic table has an isotope with a non zero nuclear spin. NMR can only be performed on isotopes whose natural abundance is high enough to be detected.
Energy Levels
To understand how particles with spin behave in a magnetic field, consider a proton. This proton has the property called spin. Think of the spin of this proton as a magnetic moment vector, causing the proton to behave like a tiny magnet with a north and south pole.
When the proton is placed in an external magnetic field, the spin vector of the particle aligns itself with the external field, just like a magnet would. There is a low energy configuration or state where the poles are aligned N-S-N-S and a high energy state N-N-S-S.
Transitions
This particle can undergo a transition between the two energy states by the absorption of a photon. A particle in the lower energy state absorbs a photon and ends up in the upper energy state. The energy of this photon must exactly match the energy difference between the two states. The energy, E, of a photon is related to its frequency, , by Planck's constant (h = 6.626x10-34 J s).
E = h
In NMR and MRI, the quantity is called the resonance frequency and the Larmor frequency.
Energy Level Diagrams
The energy of the two spin states can be represented by an energy level diagram. We have seen that = B and E = h , therefore the energy of the photon needed to cause a transition between the two spin states is
E = h B
When the energy of the photon matches the energy difference between the two spin states an absorption of energy occurs.
In the NMR experiment, the frequency of the photon is in the radio frequency (RF) range. In NMR spectroscopy, is between 60 and 800 MHz for hydrogen nuclei. In clinical MRI, is typically between 15 and 80 MHz for hydrogen imaging.
CW NMR Experiment
The simplest NMR experiment is the continuous wave (CW) experiment. There are two ways of performing this experiment. In the first, a constant frequency, which is continuously on, probes the energy levels while the magnetic field is varied. The energy of this frequency is represented by the blue line in the energy level diagram.
The CW experiment can also be performed with a constant magnetic field and a frequency which is varied. The magnitude of the constant magnetic field is represented by the position of the vertical blue line in the energy level diagram.
Boltzmann Statistics
When a group of spins is placed in a magnetic field, each spin aligns in one of the two possible orientations.
At room temperature, the number of spins in the lower energy level, N+, slightly outnumbers the number in the upper level, N-. Boltzmann statistics tells us that
N-/N+ = e-E/kT.
E is the energy difference between the spin states; k is Boltzmann's constant, 1.3805x10-23 J/Kelvin; and T is the temperature in Kelvin.
As the temperature decreases, so does the ratio N-/N+. As the temperature increases, the ratio approaches one.
The signal in NMR spectroscopy results from the difference between the energy absorbed by the spins which make a transition from the lower energy state to the higher energy state, and the energy emitted by the spins which simultaneously make a transition from the higher energy state to the lower energy state. The signal is thus proportional to the population difference between the states. NMR is a rather sensitive spectroscopy since it is capable of detecting these very small population differences. It is the resonance, or exchange of energy at a specific frequency between the spins and the spectrometer, which gives NMR its sensitivity.
It is worth noting at this time two other factors which influence the MRI signal: the natural abundance of the isotope and biological abundance. The natural abundance of an isotope is the fraction of nuclei having a given number of protons and neutrons, or atomic weight. For example, there are three isotopes of hydrogen, 1H, 2H, and 3H. The natural abundance of 1H is 99.985%. The following table lists the natural abundances of some nuclei studied by MRI.

The biological abundance is the fraction of one type of atom in the human body. The following table lists the biological abundances of some nuclei studied by MRI.

Spin Packets
It is cumbersome to describe NMR on a microscopic scale. A macroscopic picture is more convenient. The first step in developing the macroscopic picture is to define the spin packet. A spin packet is a group of spins experiencing the same magnetic field strength. In this example, the spins within each grid section represent a spin packet.
At any instant in time, the magnetic field due to the spins in each spin packet can be represented by a magnetization vector.
The size of each vector is proportional to (N+ - N-).
The vector sum of the magnetization vectors from all of the spin packets is the net magnetization. In order to describe pulsed NMR it is necessary from here on to talk in terms of the net magnetization.
Adapting the conventional NMR coordinate system, the external magnetic field and the net magnetization vector at equilibrium are both along the Z axis.
T1 Processes
At equilibrium, the net magnetization vector lies along the direction of the applied magnetic field Bo and is called the equilibrium magnetization Mo. In this configuration, the Z component of magnetization MZ equals Mo. MZ is referred to as the longitudinal magnetization. There is no transverse (MX or MY) magnetization here.
It is possible to change the net magnetization by exposing the nuclear spin system to energy of a frequency equal to the energy difference between the spin states. If enough energy is put into the system, it is possible to saturate the spin system and make MZ=0.
The time constant which describes how MZ returns to its equilibrium value is called the spin lattice relaxation time (T1). The equation governing this behavior as a function of the time t after its displacement is:
Mz = Mo ( 1 - e-t/T1 )
T1 is the time to reduce the difference between the longitudinal magnetization (MZ) and its equilibrium value by a factor of e.
If the net magnetization is placed along the -Z axis, it will gradually return to its equilibrium position along the +Z axis at a rate governed by T1. The equation governing this behavior as a function of the time t after its displacement is:
Mz = Mo ( 1 - 2e-t/T1 )
Again, the spin-lattice relaxation time (T1) is the time to reduce the difference between the longitudinal magnetization (MZ) and its equilibrium value by a factor of e.
Precession
If the net magnetization is placed in the XY plane it will rotate about the Z axis at a frequency equal to the frequency of the photon which would cause a transition between the two energy levels of the spin. This frequency is called the Larmor frequency.
T2 Processes
In addition to the rotation, the net magnetization starts to dephase because each of the spin packets making it up is experiencing a slightly different magnetic field and rotates at its own Larmor frequency. The longer the elapsed time, the greater the phase difference. Here the net magnetization vector is initially along +Y. For this and all dephasing examples think of this vector as the overlap of several thinner vectors from the individual spin packets.
The time constant which describes the return to equilibrium of the transverse magnetization, MXY, is called the spin-spin relaxation time, T2.
MXY =MXYo e-t/T2
T2 is always less than or equal to T1. The net magnetization in the XY plane goes to zero and then the longitudinal magnetization grows in until we have Mo along Z.
Any transverse magnetization behaves the same way. The transverse component rotates about the direction of applied magnetization and dephases. T1 governs the rate of recovery of the longitudinal magnetization.
In summary, the spin-spin relaxation time, T2, is the time to reduce the transverse magnetization by a factor of e. In the previous sequence, T2 and T1 processes are shown separately for clarity. That is, the magnetization vectors are shown filling the XY plane completely before growing back up along the Z axis. Actually, both processes occur simultaneously with the only restriction being that T2 is less than or equal to T1.
Two factors contribute to the decay of transverse magnetization.1) molecular interactions (said to lead to a pure T2 molecular effect)2) variations in Bo (said to lead to an inhomogeneous T2 effectThe combination of these two factors is what actually results in the decay of transverse magnetization. The combined time constant is called T2 star and is given the symbol T2*. The relationship between the T2 from molecular processes and that from inhomogeneities in the magnetic field is as follows.
1/T2* = 1/T2 + 1/T2inhomo.

Rotating Frame of Reference
We have just looked at the behavior of spins in the laboratory frame of reference. It is convenient to define a rotating frame of reference which rotates about the Z axis at the Larmor frequency. We distinguish this rotating coordinate system from the laboratory system by primes on the X and Y axes, X'Y'.
A magnetization vector rotating at the Larmor frequency in the laboratory frame appears stationary in a frame of reference rotating about the Z axis. In the rotating frame, relaxation of MZ magnetization to its equilibrium value looks the same as it did in the laboratory frame.
A transverse magnetization vector rotating about the Z axis at the same velocity as the rotating frame will appear stationary in the rotating frame. A magnetization vector traveling faster than the rotating frame rotates clockwise about the Z axis. A magnetization vector traveling slower than the rotating frame rotates counter-clockwise about the Z axis .
In a sample there are spin packets traveling faster and slower than the rotating frame. As a consequence, when the mean frequency of the sample is equal to the rotating frame, the dephasing of MX'Y' looks like this.
Pulsed Magnetic Fields
A coil of wire placed around the X axis will provide a magnetic field along the X axis when a direct current is passed through the coil. An alternating current will produce a magnetic field which alternates in direction.
In a frame of reference rotating about the Z axis at a frequency equal to that of the alternating current, the magnetic field along the X' axis will be constant, just as in the direct current case in the laboratory frame.
This is the same as moving the coil about the rotating frame coordinate system at the Larmor Frequency. In magnetic resonance, the magnetic field created by the coil passing an alternating current at the Larmor frequency is called the B1 magnetic field. When the alternating current through the coil is turned on and off, it creates a pulsed B1 magnetic field along the X' axis.
The spins respond to this pulse in such a way as to cause the net magnetization vector to rotate about the direction of the applied B1 field. The rotation angle depends on the length of time the field is on, , and its magnitude B1.
= 2 B1.
In our examples, will be assumed to be much smaller than T1 and T2.
A 90o pulse is one which rotates the magnetization vector clockwise by 90 degrees about the X' axis. A 90o pulse rotates the equilibrium magnetization down to the Y' axis. In the laboratory frame the equilibrium magnetization spirals down around the Z axis to the XY plane. You can see why the rotating frame of reference is helpful in describing the behavior of magnetization in response to a pulsed magnetic field.
A 180o pulse will rotate the magnetization vector by 180 degrees. A 180o pulse rotates the equilibrium magnetization down to along the -Z axis.
The net magnetization at any orientation will behave according to the rotation equation. For example, a net magnetization vector along the Y' axis will end up along the -Y' axis when acted upon by a 180o pulse of B1 along the X' axis.
A net magnetization vector between X' and Y' will end up between X' and -Y' after the application of a 180o pulse of B1 applied along the X' axis.
A rotation matrix (described as a coordinate transformation in Chapter 2) can also be used to predict the result of a rotation. Here is the rotation angle about the X' axis, [X', Y', Z] is the initial location of the vector, and [X", Y", Z"] the location of the vector after the rotation.

Spin Relaxation
Motions in solution which result in time varying magnetic fields cause spin relaxation.
Time varying fields at the Larmor frequency cause transitions between the spin states and hence a change in MZ. This screen depicts the field at the green hydrogen on the water molecule as it rotates about the external field Bo and a magnetic field from the blue hydrogen. Note that the field experienced at the green hydrogen is sinusoidal.
There is a distribution of rotation frequencies in a sample of molecules. Only frequencies at the Larmor frequency affect T1. Since the Larmor frequency is proportional to Bo, T1 will therefore vary as a function of magnetic field strength. In general, T1 is inversely proportional to the density of molecular motions at the Larmor frequency.
The rotation frequency distribution depends on the temperature and viscosity of the solution. Therefore T1 will vary as a function of temperature. At the Larmor frequency indicated by o, T1 (280 K ) < href="http://www.cis.rit.edu/htbooks/mri/chap-3/c15-5.htm" target="ANIMATION">
Fluctuating fields which perturb the energy levels of the spin states cause the transverse magnetization to dephase. This can be seen by examining the plot of Bo experienced by the red hydrogens on the following water molecule. The number of molecular motions less than and equal to the Larmor frequency is inversely proportional to T2.
In general, relaxation times get longer as Bo increases because there are fewer relaxation-causing frequency components present in the random motions of the molecules.
Bloch Equations
The Bloch equations are a set of coupled differential equations which can be used to describe the behavior of a magnetization vector under any conditions. When properly integrated, the Bloch equations will yield the X', Y', and Z components of magnetization as a function of time.

INDUSRTIAL RADIOGRAPHY

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Industrial Radiography is the use of ionising electromagnetic radiation to view objects in a way that can't be seen otherwise. It is not to be confused with the use of ionizing radiation to change or modify objects; radiography's purpose is strictly for viewing. Industrial radiography has grown out of engineering, and is a major element of nondestructive testing (NDT). It is a method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation to penetrate various materials