Theory of MRI
Relies on radiofrequency emissions from atoms & molecules in tissues exposed to a static magnetic field.
The atom that produces the MRI signal is the hydrogen atom (proton) which behaves like a small magnet when exposed to the magnetic field.
The hydrogen atom is mainly found in water & fat. Other atoms with an odd number of electrons could be used, but hydrogen is most prevalent, & good for showing oedema
- patient is placed in the scanner, which has a very strong & constant magnetic field of 1.5 Tesla or more. The magnetic field is graded slightly in three dimensions by use of magnetic field gradient coils. These coils are responsible for the loud clanging noise in the MRI
- magnetic field aligns any free hydrogen atoms along the magnetic field – known as longitudinal magnetization. The atoms “precess” – spin like tops – around a net longitudinal vector. Some atoms are antiparallel to the vector. A very slightly larger number of atoms is parallel to the vector, because this is a lower energy state than being antiparallel
- A magnetic (radiofrequency) pulse is then applied at right angles to the longitudinal field, via a small coil that is placed over the part being examined – known as transverse magnetization. The pulse is at a frequency that causes energy to be transmitted efficiently to the protons (this frequency is determined by the Larmor equation)
- free hydrogen atoms line up with the transverse field, & are all in phase with each other
- When the transverse field is turned off, the atoms return to the longitudinal orientation, emitting energy as they do so. Different atoms do this at different rates, & become out of phase
- This energy induces a voltage in a receiver coil, & is transformed via Fourier mathematics to an image
- energy can be recorded at different times
- Short times to record give good anatomical images (T1)
- Longer times give lower image resolution, as the energy degrades quickly
Technical issues
TR Repetition time – time between repeated Rf (transverse) pulses
TE Echo time – time between the imparted Rf pulse & recording of generated pulses.
T1 relaxation time – the rate of recovery of longitudinal magnetization
T2 relaxation time – decay of transverse magnetization
- T1 constant refers to the time taken for the atoms to return to the longitudinal orientation. The T1 constant is defined as the time taken for 63% of the atoms to return to the longitudinal orientation, & is always longer than the T2 constant
- T2 constant reflects the time taken for the atoms to fall out of phase & their generated signal to decay. The T2 constant or relaxation time is defined as the time required for 63% of the signal to disappear
- Varying the TR & TE enables T1 or T2 qualities to be emphasized
- As the time to repetition is ↓, not all tissues become fully relaxed (that is, back to their pre stimulated state). Fat has a short T1 time so with short T1 intervals will appear hyperintense
- Longer T1 intervals allow all tissues to relax
- Short TE images, which are insensitive to T2 differences in the tissues, merely reflect the abundance of protons & so are commonly called proton density-weighted images
- Long TE images are sensitive to the T2 differences in tissues, & are called T2 weighted. In tissues with few free water molecules, the hydrogen atoms quickly fall out of phase. In tissues with lots of free water molecules (e.g. synovial fluid) the time taken to fall out of phase is longer
Sequences in MRI
T1
This shows anatomy.
Short TE(<20) / Short TR (<1000)
Can be either SE (spin echo) or GE (gradient echo)
This is the signal that is enhanceable with gadolinium. Gadolinium will be detected in tissues where there are leaky capillaries & space into which the gadolinium can ooze.
T2
This shows pathology.
Long TE (>60-100) / Long TR (>1000)
Can be spin echo, turbo or fast spin echo (TSE or FSE) or gradient echo (T2*, GE GRE, FLASH)
T2 can be fat satted (this converts fat signal from white to black & makes water more obvious)
Proton density images are routinely produced while making T2 images.
STIR
The “fat satting workhorse”.
Can’t be enhanced with gadolinium.
Highly sensitive to water which makes it a good screening tool.
Proton density
Intermediate TE(20-40) / Long TR(>1200)
Routinely obtained with standard T2 weighted spin echo images.
Very black & white & therefore ideal for looking at the anatomy of structures such as ligaments, capsules & tendons which are black
Gradient Echo
Short relaxation
Short excitation time
Fat & water are intermediate signals
Excellent for physeal & articular cartilage
Gadolinium
Administration of gadolinium acts to dramatically shorten T1 (↑ the signal) of tissues which are vascularized, have leaking capillaries & where there is space for oozing to occur.
It can differentiate solid tumour from cysts & oedematous tissue from fluid collections.
Patients with mass disease must have gadolinium to prove that they have a tumour rather than a cyst.
Articular cartilage
“Currently the most sensitive clinical application for cartilage evaluation” is the T1 weighted fat suppressed three dimensional spoiled gradient echo sequence (Chung 2001).
Proton density is also very sensitive for evaluating cartilage.
Advantages of MRI
- Superior soft tissue contrast
- Assess physiology as well as anatomy
- Lack of ionizing radiation
- Multiplanar capabilities
Who can’t be done?
Definitely
- Cardiac pacemakers
- Dorsal column stimulators
- Prosthetic heart valves
- Intra-cranial vascular clips
- Metallic intra-ocular foreign bodies
- Those with vascular clips anywhere that are less than two weeks old unless they are known to be MRI compatible
Probably shouldn’t
- Unstable patients requiring continuous monitoring or resuscitation
- Patients with artificial eyes
- Patients who are too large or heavy
- Those in severe pain
It is not known whether or not MRI harms fetuses. It does cause a definite ↑ in temperature.
Appearance of structures
| T1 T2 STIR T1 + gad |
| Fat White Grey Black No change |
| Water Grey White White+++ No change |
| Trauma Grey White White+++ White |
| Tumour Grey White White+++ White |
| Inflammation Grey White White+++ White |
| Infection Grey White White+++ White rim/grey centre |
| Fibrosis Grey White White/black White |
| Scarring Grey/black Grey/black Black No change |
| Ligaments Grey/black Grey/black Black No change |
| Bone/Ca2+ Black Black Black No change |
| Haemorrhage Grey Black |
Points in interpreting MRIs
- MRI can’t differentiate trauma from tumour from inflammation
- Spinal stenosis – T2 weighted images of the spine over emphasize the severity of osseous stenosis
- If a patient has positive neurology & a negative MRI was obtained, the study should be repeated using gadolinium
MRI characteristics of lesions
- Soft tissue sarcomas
- T1 – medium intensity
- T2 – high intensity
- Chondrosarcoma
- T1 – low intensity
- T2 – high intensity (calcification shows as low intensity)
- Spine: disc vs. scar
| T1 | T2 | |
| Scar | Decreased | Increased |
| Free fragment | Increased | Increased |
| Extruded fragment | Decreased | Decreased |
Approach to Knee MRI
ACL injuries
- Normal ACL has high signal near its tibial insertion due to intrasubstance fat
- Ruptures are normally proximal; may be empty wall sign at femoral insertion onto medial aspect LFC
- Follow ACL down to tibial insertion on all images
- Characteristic bone bruise pattern is posterolateral aspect of tibia & anterolateral femur, due to forward subluxation of the tibia. It is seen in 92% of patients, but is only valid in adults, as around 28% of paediatric patients can have this finding without ACL tear
- ACL should be oblique, if it is vertical consider posterior subluxation of the tibia due to PCL injury
PCL injuries
- Runs obliquely, so will not all be seen on one image
- In kids it avulses distally, often with a bone block
- In adults it tears proximally, often with a bone block
- Normally uniformly black with a low signal. Any ↑ in signal on T1 or T2 weighted images is abnormal
- If the ACL is torn the normal wave in the PCL is accentuated
MCL injuries
- Look on coronal slices
- Normally appears sharp & well defined, although thin. Inserts into epicondyle so look here & trace it down
- Pathology is thickening & blurring of its margins
Posterolateral corner injuries
- Look for the head of the fibula & insertion of BF. It lies most laterally, & can be identified on a coronal image by the broad muscle expanding proximally
- LCL is thinner, lies inside BF, also arises from the fibular head
- Popliteus is the deepest structure, rounded, inserts into popliteal groove
| T1 weighted | T2 weighted | T1 fat suppressed | Magnetization transfer subtraction image | |
| TR repeat time | 50-500 ms | 2-3 secs | ||
| TE echo time | < 20 ms | > 80 ms | ||
| Cortical bone | Black | Black | Black | |
| Ligaments | Black | Black | Black | |
| Healthy cartilage | Gray | Gray | Gray (good contrast to bone marrow) | |
| Diseased cartilage (chondromalacia) | White | Gray | Gray | |
| Bone Marrow | Gray | White | Black | |
| Normal Fluid | White | Black | Black | |
| Abnormal Fluid (pus) | White | Gray | Black | |
| Muscle | White | Gray | Gray |