MRI uses magnetic fields and radio waves to produce detailed images of internal structures without using ionizing radiation. It provides excellent soft tissue contrast and can visualize injuries and diseases. The core MRI sequences are T1-weighted and T2-weighted images, which show different tissue properties. Additional sequences like fat-suppressed, contrast-enhanced, and inversion recovery images provide further information. A systematic approach is needed to fully interpret MRI scans.

1. BASICS OF MRI

2. • It is a non-invasive method for mapping
internal structure within the body
• Uses non-ionizing electromagnetic radiation
• Employes radiofrequency radiation in the
presence of carefully controlled magentic
fields to produce high quality cross-sectional
images of the body in any plane

3. Indications of MRI
• Diagnosing: strokes; infections of the
brain/spine/CNS; tendonitis
• Visualising: Injuries; torn ligaments –
especially in areas difficult to see like the
wrist, ankle or knee
• Evaluating: Masses in soft tissue; cysts; bone
tumours or disc problems.

4. Contraindications
• The strength of the magnet is 5000 times stronger
than the earth so all metals must be removed.
• People with pacemakers or metal fragments in the
eye cannot have a scan
• There has not been enough research done on babies
and magnetism, so pregnant women shouldn’t have
one done before the 4th month of pregnancy – unless
it is highly necessary.

5. Advantages
• The MRI does not use ionizing
radiation
• Also the contrast dye has a very
low chance of side effects
• ‘Slice’ images can be taken in
many planes
Disadvantages
• Claustrophobia-Patients are in a
very enclosed space.
• Weight and size - There are
limitations to how big a patient
can be.
• Noise - The scanner is very noisy.
• Keeping still - Patients have to
keep very still for extended
periods of time.
• Cost - A scanner is very, very
expensive, therefore scanning is
also costly.
• Medical Contraindications -
Pacemakers, metal objects in body
etc.

6. Use of Hydrogen in MRI
• Hydrogen has an unpaired proton which is positively
charged
• Every hydrogen nucleus is a tiny magnet which
produces small but noticeable magnetic field.
• Hydrogen atom is the only major species in the body
that is MR sensitive
• Abundant in the body in the form of water and fat
• MRI is hydrogen (proton) imaging
• The protons - being little magnets - align
themselves in the external magnetic field like a
compass needle in the magnetic field of the earth.
• May align parallel or anti-parallel

7. T1 Image
• T1 is defined as the time it takes for the
hydrogen nucleus to recover 63% of its
longitudinal magnetization.

8. Types of MRI imagings
• T1WI
• T2WI
• FLAIR
• STIR
• DWI
• ADC
• GRE
• MRA
• MRV
• MRS
• MT
• Post-Gd images

9. T1 Image

10. T2 Image
• T2 relaxation time is the time for 63% of the
protons to become dephased owing to
interactions among nearby protons

12. • Time to Echo (TE) - Time between RF excitation
pulse and collection of signal.
• Repetition Time (TR) - Time between two
excitations is called repetition time
• By varying the TR and TE one can obtain T1WI
and T2WI
• In general a short TR (<1000ms) and short TE
(<45 ms) scan is T1WI
• Long TR (>2000ms) and long TE (>45ms) scan is
T2WI

13. Short TI inversion-recovery (STIR)
sequence
• In STIR sequences, an inversion-recovery pulse is
used to null the signal from fat (180° RF Pulse).
• STIR sequences provide excellent depiction of
bone marrow edema which may be the only
indication of an occult fracture.
• Unlike conventional fat-saturation sequences STIR
sequences are not affected by magnetic field
inhomogeneities, so they are more efficient for
nulling the signal from fat.

14. Fluid-attenuated inversion recovery
(FLAIR)
• First described in 1992 and has become one of the corner stones of
brain MR imaging protocols
• An IR sequence with a long TR and TE and an inversion time (TI)
that is tailored to null the signal from CSF
• In contrast to real image reconstruction, negative signals are
recorded as positive signals of the same strength so that the nulled
tissue remains dark and all other tissues have higher signal
intensities.
• Most pathologic processes show increased SI on T2-WI, and the
conspicuity of lesions that are located close to interfaces b/w brain
parenchyma and CSF may be poor in conventional SE or FSE T2-WI
sequences.
• FLAIR images are heavily T2-weighted with CSF signal suppression,
highlights hyperintense lesions and improves their conspicuity and
detection, especially when located adjacent to CSF containing
spaces

15. T1 Image
• Subacute Hemorrhage
• Fat-containing
structures
• Anatomical Details
T2 Image
• Edema
• Demyelination
• Infarction
• Chronic Hemorrhage

16. T1 and T2 weighted images
T1 and T2 images demonstrate different tissues
based on the timing of the RF pulses. Between the
two, the key differences you need to be aware of are:
T1 – ONE tissue is bright: fat
T2 – TWO tissues are bright: fat and water (WW2 –
Water is White in T2)
T1 is the most ‘anatomical’ image (Figure 1).
Conversely, the cerebrospinal fluid (CSF) is bright in
T2 due to its’ water content.
T2 is generally the more commonly used, but T1 can
be used as a reference for anatomical structures or to
distinguish between fat vs. water bright signals.

17. Fat suppressed
• The fat signal can be suppressed to enable a better view of
pathology in and around anatomical structures –
particularly oedema. This is useful in adrenal tumours or
bone marrow pathology, where the image will appear
homogenous with surrounding tissue due to fat content.
Gadolinium-enhanced
• Gadolinium enhances vasculature (i.e. arteries) or
pathologically-vascular tissues (e.g. intracranial metastases,
meningiomas). This process involves injecting 5-15ml of
contrast intravenously, with images taken shortly
thereafter. Gadolinium appears bright in signal, allowing for
detection of detailed abnormalities (e.g. intracranial
pathologies). Typical intracranial abscesses have a “ring-
enhancement” pattern, while metastases enhance
homogeneously. Meningiomas will have a homogenous
enhancement after the contrast, but will also have a “dural
tail,” meaning the lesion appears continuous with the dura.

18. Inversion recovery (IR) sequences
• These types of images are manipulations of T1 and T2. They
nullify certain tissue types based on their inversion timings,
thereby stopping tissues such as fat and CSF from
appearing as bright signals. This is helpful to identify
pathological signals. The two main types are discussed
below.
Short tau inversion recovery (STIR)
• STIR is based on a T2 image, but the image is manipulated
in a way that results in fat (and any other materials with
similar signals) being nullified. Unlike fat-suppressed
images, however, STIR can not be used with gadolinium
contrast.As previously discussed, fat can make the
interpretation of oedematous areas and bone marrow
difficult. Figure 3 shows how this nullified fat signal can
assist with the identification of oedema due to fractures.
•

20. Fluid attenuated inversion recovery (FLAIR)
• FLAIR is also similar to T2, however, the CSF signal is
nullified. This is particularly useful for evaluating structures
in the central nervous system (CNS), including the
periventricular areas, sulci, and gyri. For example, FLAIR can
be used to identify plaques in multiple sclerosis, subtle
oedema after a stroke, and pathology in other conditions
whereby CSF may interfere with interpretation.
Diffusion-weighted imaging (DWI) and apparent diffusion
coefficient (ADC)
• DWI is an imaging modality that combines T2 images with
the diffusion of water. With DWI scans, ischaemia can be
visualised within minutes of it occurring . This is because
DWI has a high sensitivity for water diffusion, thereby
detecting the physiological changes that happen
immediately after a stroke.

21. A systematic approach to MRI interpretation
• Verify details
• Begin by verifying the following details:
• Patient details (i.e. name, date of birth, hospital number)
• Image details (i.e. date, type)
• Make sure it is the most recent image for the correct
patient
• Look for previous cross-sectional imaging (if available)
• Look at the T2 weighted images
• Inspect the T2 weighted images:
• Look at each available plane (axial, coronal, sagittal)
• Check for abnormal MRI signals
• Work through the anatomy of the areas you are looking at
to make sure nothing is missed/abnormal
• Comparing both sides of an image (if possible) can reveal
clear areas of abnormal signalling
• Shape, size, location, and intensity of the signal

22. • Compare different MRI image sequences
• Compare the available MRI image sequences to help differentiate pathology:
• Comparing fat sensitive images (e.g. T1) vs water-sensitive images (e.g. T2 or STIR)
can help differentiate pathologies such as ischaemia and inflammation.
• Post-contrast enhancement is useful for vascular pathology or pathologically-
vascular tissue.
• Learn why each image type is used – this will enable you to know what you are
looking for (e.g. for MR brain it’s useful to look at T2, then FLAIR, then DWI/ADC, as
this will help distinguish between most differentials).
• Compare against other imaging modalities
• Compare the MRI images to other imaging modalities (e.g. ultrasound, CT, plain
film):
• Can you view the pathology on other imaging modalities?
• Plain films can be particularly useful when assessing musculoskeletal pathology.
• Compare against previous images
• Compare the current MRI images to previous MRI scans if available:
• Are the abnormal signals new or old?
• Are there any changes in the size/shape/brightness of the abnormal signals?
• Consider the clinical context
• Finally, place your findings in context with the clinical presentation in order to
ascertain a radiological diagnosis:
• Are the symptoms acute or chronic?
• How unwell is the patient?
• Does the imaged pathology correlate with the presenting symptoms?

23. Summary
• MRIs are a superior imaging modality for viewing soft tissues.
• T1 and T2 weighted images represent the core types of MR images.
• T1 and T2 images may be adjusted: fat-suppressed, gadolinium-
enhanced and inversion recovery.
• The different sequences tell you what is in the lesion and how it is
behaving. Using these features, the location of the lesion, and the
clinical history, we can make a diagnosis.
• Anatomy, as with all scans, is key. MRIs produce a very clear view of
structures, therefore strong anatomical knowledge is particularly
helpful.
• Spend time looking at normal scans. The more you become familiar
with what is normal, the easier it is to see when things are
abnormal.
• Always compare both sides of the scan – pathology is rarely
bilateral.