Principles of MRI magnetic resonance imaging Dr Mohammed Bader Hassan FIBMS,DMRD

Objectives By the end of this lecture the student should be able to 1.calrify the basics of MRI physics 2.define major MRI sequences (T1&T2) 3.identify the MRI advantages and contraindications 4.identify the major MRI studies such is angiography, MRCP and myelography

Basic facts the human contain a predominant of hydrogen atoms (in water ,fat ,and other tissues) The hydrogen atom contain a proton only The proton is a particle with a positive charge Each proton have a spin movement The moving charge is electrical current which have magnetic field So each proton have own magnetic field

P = No Net Magnetization

What happen when the body inside the magnetic field 1.the proton (small magnets )organize the selves with the external magnets 2.a precession movement begin (just like when you hit the top of spin )it is a wobble movement This precession have the specific frequency according to type of atom and magnetic field (so it is a well known frequency)

how can we get a picture from MRI It is difficult to measured aligned magnets inside a river of magnetic field so we should shift this magnetic field We send a radio wave in the same frequency of the precession of the proton (radio frequency )resonance!

Measuring Tissue Magnetization: For an energy transfer to occur the RF Pulse must be at the same frequency as the precessing Mo. ie The Resonance Frequency. Much like a tuning fork experiment. A B C D E B

Measuring Tissue Magnetization: B C D E B

Measuring Tissue Magnetization: B C D E B

Measuring Tissue Magnetization: B C D E B

Measuring Tissue Magnetization: B C D E B

What happen after radiofrequency is sent The radio frequency should be in the same frequency of the precession of proton so it can transfer energy to proton The energy which is picked up by proton will make the proton in higher energy this will make the longitudinal magnetic vector of the body is zero And it will synchronized the precession of the protons which will move in the same direction & will give moving transverse vector

Moving transverse vector

What happen after the radiofrequency is off The energy will be loosed from the spinning proton and longitudinal vector appear again in a specific time according to tissue component. The synchronization (the transvers vector will be loosed in specific time according to tissue component

T1 Relaxation

T1 Relaxation z' T1=the time which is needed for most of the longitudinal vectored to re appear y' x' T1 relaxation is regrowth of the longitudinal magnetization

T1 Relaxation z' y' x' T1 relaxation is regrowth of the longitudinal magnetization

T1 Relaxation z' y' x' T1 relaxation is regrowth of the longitudinal magnetization

T1 Relaxation z' y' x' T1 relaxation is regrowth of the longitudinal magnetization

T1 Relaxation z' y' x' Mlong = M0 T1 relaxation is regrowth of the longitudinal magnetization

T2 Decay The time which is needed to disappear of most of the transverse vectored is T2 - + + z' z' - - + y' y' x' x' T2 relaxation is dephasing of transverse magnetization

Decay of transvers vectoreT2 Transverse vector

MR SIGNAL Collected by a coil Encoded through a series of complex techniques and calculations (magic?) Stored as data Mapped onto an image matrix

T1 Relaxation: With no other signal considerations this would mean that the protons with short T1 time would contribute the highest signal on the image. Pixel containing protons with short T1 time. (eg Fat)

T2 Decay: With no other signal considerations this would mean that the protons with a long T2 time would contribute the highest signal on the image. Pixel containing protons with long T2 time. (eg Water)

Time from the application of one RF pulse to another RF pulse TR - REPETITION TIME Time from the application of one RF pulse to another RF pulse TE - ECHO TIME Time from the application of the RF pulse to the peak of the signal induced in the coil

Image formation The image will depend on T1,T2, and amount of proton within the tissue (spin density ) . The fatty tissues is loosing energy quickly so it will re appear its longitudinal vector rapidly ( high T1 signal) Fatty tissue will loss synchronization quickly Water loosing energy and synchronization slowly so it will appear bright at T2 and black on T1 Solid material will not give a signal on t1 and t2 (signal void ) air will appears signal void because lack of H .

T1 WEIGHTING T2 WEIGHTING A short TR and short TE will result in a T1 weighted image Excellent for demonstrating anatomy T2 WEIGHTING A long TR and long TE will result in a T2 weighted image Excellent for demonstrating pathology MANY OTHER DIFFERENT TYPES OF IMAGES THAT COMBINE ABOVE AND INCLUDE OTHER PARAMETERS

Relative Relaxation Rates Time Color in image Solid (bone ,stone ) Long black short Black semi-solid (muscle ) intermediate liquid long dark bright Fat Dark

T1-, -, and T2-weighted images

Brain Tumor Imaging T1-weighted Sagittal What’s changed between these images? T1-weighted Axial T2-weighted Axial

STIR = Short Tau Inversion Recovery FAT

STIR = Short Tau Inversion Recovery FAT

STIR = Short Tau Inversion Recovery FAT

STIR = Short Tau Inversion Recovery FAT

STIR = Short Tau Inversion Recovery FAT

STIR = Short Tau Inversion Recovery FAT 90 deg Very Small transverse component = small signal

STIR = Short Tau Inversion Recovery Fat & water

STIR = Short Tau Inversion Recovery Fat & water

STIR = Short Tau Inversion Recovery Fat & water

STIR = Short Tau Inversion Recovery Fat & water

Fat & water STIR = Short Tau Inversion Recovery

STIR = Short Tau Inversion Recovery Fat & water 90 deg Water will have signal. Fat will not!

Advantage of MRI Non ionizing radiation Multiplanar images (cross section , saggital and coronal views ) The ability of imaging vessels without contrast (MR angiography ) Have a good soft tissue contrast

Contraindication of MRI Patient with pacemaker Patient with bullet injury or ferromagnetic F.B ,or surgical clip (because of heat and missile effect ) Pregnancy especially first trimester Claustrophobia reported that between 1 % and 10 % of patients experience some degree of claustrophobia which in the extreme cases results in their refusal to proceed with the scan

bioeffect No known or expected harmful effects on humans using field strengths up to 10 Tesla Currently pregnant women are normally excluded from MRI scans during the first trimester although there is no direct evidence to support this restriction The most invasive MR scans involve the injection of contrast agents (e.g. Gd-DTPA). noise

DISADVANTAGES OF MRI Expensive Long scan times Audible noise (65-115dB) Isolation of patient (claustrophobia, monitoring of ill patients) Exclusion of patients with pacemakers and certain implants

THE CHANGING MAGNETIC FIELDS CAN DO DAMAGE TO: Monitoring equipment Infusion pumps Credit cards Cellular telephones Any electronic device

THE FOLLOWING ARE (USUALLY*) OKAY: Gold Silver Digital watches Eyeglass frames Snaps/zippers fastened to clothing Dental work

Special MRI studies MR angiography :non-invasive imaging of the vascular tree , In normal circumstances flow effects cause unwanted artefacts, but in MRA these phenomemna are used advantageously ,

Direction of Blood Flow.

Direction of Blood Flow.

Direction of Blood Flow.

Direction of Blood Flow.

Overall signal from Voxel is reduced due to Phase Loss. Direction of Blood Flow.

Vascular Techniques 2D ToF Blood flowing into the slice is fully magnetized so when it receives an RF pulse it is flipped into the transverse plane and gives a high signal. Arrows represent Mo Vector

Vascular Techniques 2D ToF Stationary tissue receives continual RF pulses that arrive before the proton can relax back to Bo. This causes Saturation of the stationary tissue and therefore less signal from the stationary tissue. 2D ToF sequences utilize short TR’s and high flip angles. Eg: 33ms & 70deg.

Vascular Techniques 2D ToF Blood flowing into the slice is fully magnetized so when it receives an RF pulse it is flipped into the transverse plane and gives a high signal. Arrows represent Mo Vector Venous Flow Direction

By placing an additional 90deg RF pulse in the path of the venous blood we can saturate its signal before it enters the imaging slice. This is referred to as a Pre-Saturation Pulse. Venous Flow Direction

Vascular Techniques 2D ToF The resultant image shows flowing blood with bright signal and the stationary tissue as a “saturated” signal. The venous blood is saturated in the same manner as stationary tissue and contributes no signal.

Vascular Techniques 2D ToF By acquiring many individual slices long vessels may be covered easily. The resulting slices are then combined and viewed as a maximum intensity projection (MIP) in any orientation.

Vascular Techniques 2D ToF The resultant image may be rotated, paged and manipulated to demonstrate the optimum viewing plane. (With IVI or AW software packages)

MRCP Magnetic resonance colangio pancreaticography: Heavily T2 weighted imaged for biliarry and pancreatic tree No need for contrast No need for endoscopy Non invasive method

Normal duct stone

MR mylogarphy Heavily T2 weighted image ,used for imaging spinal canal and theca sac , with out the need for injection of contrast in the thecal sac ( non invasive method )

QUESTIONS?????

Thank you