Nuclear magnetic resonance (NMR) is a subject that is taught in a variety of different formats in medical school, so students and healthcare professionals may receive different teaching standards. There are several different types, viewing planes, and a wide variety of pathologies associated with viewing.
These complexities can make students intimidated by MRI interpretation. Here, we provide a brief overview of the fundamentals, approach, and interpretation of MRI. While we can't make you a radiologist, we can help you appear less confused when one calls for one.
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Why do we need to use MRIs?
In general, MRI is used less frequently than plain X-rays and CT scans. They are often reserved forsuperior visualization of soft tissue. Magnetic resonance imaging is particularly useful in patients with suspected neurological or musculoskeletal pathology; however, it can also be used in many other specialties. MR images take a little longer to acquire and are more expensive. Magnetic resonance imaging is contraindicated in patients with ferromagnetic metal implants or foreign bodies.1Claustrophobic patients should also be considered.
How do they work?
MRI machines work by exploiting the interaction of the magnetic field, hydrogen ions, and radio frequency (RF) pulses. When you place a patient in a strong magnetic field, their hydrogen ions align themselves in the direction of the magnetic field. The application of an RF pulse will change the alignment direction of these hydrogen ions. When the RF pulse is turned off, these ions will try to realign with the magnetic field again and release a signal. The strength of this signal depends on the type of tissue (fat, muscle, water) in which the hydrogen ion is found.1
Using these principles, you can tune the machine to detect signals of various ranges and planes of magnetization; this is where "weighted image" comes into play. when it comes to viewing the images, they are known as "fat suppression" sequences.2
MRI can also be used as a dynamic imaging tool. For example, the diffusion of water molecules can be studied with diffusion-weighted imaging (DWI), or the macroscopic movement of blood can be studied using MR angiography techniques.
MRI images and sequences
There are many factors that go into producing a final MR image. Different combinations of these will be useful for different clinical presentations, but here are some examples of common images and sequences:
- T1 and T2 weighted images (T1WI and T2WI)
- I am the ADC
- TO MIX
… and many others.
T1 and T2 weighted images
T1 and T2 images show different tissues depending on the timing of the RF pulses. Between the two, the main differences you should be aware of are:
- T1–ONEthe fabric is shinygordo
- T2 - TWOthe fabrics are shiny:gordomiagua(Second World War–Cstick isCbata me T2)
- T1 is the most "anatomical" image (Figure 1). On the other hand, cerebrospinal fluid (CSF) is shiny on T2 due to its water content.
- T2 is usually the most used, but T1 can be used as a reference for anatomical structures or to distinguish between water signals and shiny fat.
Additional features of T1/T2-weighted images
The fat signal can be suppressed to allow better visualization of pathology within and around anatomical structures, particularly edema. This is useful in adrenal tumors or bone marrow pathology where the image will appear homogeneous with the surrounding tissue due to the fat content.
Gadolinium enlarges vasculature (ie, arteries) or pathologically vascular tissues (eg, intracranial metastases, meningiomas). This process involves injecting 5 to 15 mL of contrast intravenously, with images taken soon after. Gadolinium appears bright in the signal, allowing detection of detailed abnormalities (eg, intracranial pathologies). Typical intracranial abscesses have a “ring enhancement” pattern, whereas metastases enlarge in a homogeneous manner. Meningiomas will have homogeneous enhancement after contrast, but will also have a “dural tail”, meaning that the lesion appears continuous with the dura (Figure 2).4
Inversion Recovery (IR) Sequences
These types of images are T1 and T2 manipulations. They nullify certain types of tissue based on their inversion times, preventing tissues such as fat and CSF from showing up as bright signals. This is useful to identify pathological signs. 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 such a way that fat (and any other material with similar signals) is cancelled. Unlike fat-suppressed imaging, however, STIR cannot be used with gadolinium contrast.4As discussed above, fat can make interpretation of edematous areas and bone marrow difficult. Figure 3 shows how this nullified fat signal can help identify edema due to fractures.
Fluid Attenuated Inversion Recovery (FLAIR)
FLAIR is also similar to T2, but the CSF signal is null. 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 edema after stroke, and pathology in other conditions where CSF may interfere with interpretation (Figure 4).1
Diffusion Weighted Image (DWI) and Apparent Diffusion Coefficient (ADC)
DWI is an imaging modality that combines T2 images with water diffusion. With DWI scans, ischemia can be visualized within minutes of its onset (Figure 5). This is because the DWI has a high sensitivity to water diffusion, so it detects the physiological changes that occur immediately after a stroke.
The ADC should be used in conjunction with the DWI to confirm that there is true diffusion restriction and not simply a T2 "glow". The table below explains the main differences between the two.
Measures abnormal diffusion of water (restricted diffusion) but ALSO combines it with T2 image
Structures that are bright on a T2 image may shine on DWI images.
High signal at initial ischemia but decreases after several weeks
Very sensitive (for example, if the CT is normal but a stroke is still suspected)
Measure the diffusion of pure water without T2 combination
Provides confirmation of true restricted transmission in DWI
Low signal at first, but signal builds up over several weeks and stays high
Always use DWI together with ADC
A systematic approach to MRI interpretation
Start by checking the following details:
- Patient details (ie name, date of birth, hospital number)
- Image details (ie date, type)
- Make sure it is the most recent image of the correct patient
- Look for previous cross-sectional images (if available)
See T2-weighted images
Inspect the T2-weighted images:
- Look at each available plane (axial, coronal, sagittal)
- Check for abnormal MRI signs
- Work with the anatomy of the areas you are looking at to make sure nothing is missing or abnormal.
- Comparison of both sides of an image (if possible) can reveal clear areas of abnormal signaling
- Signal shape, size, location, and strength
Compare different MRI imaging sequences
Compare available MRI imaging sequences to help differentiate pathology:
- Comparison of fat-sensitive images (eg, T1) versus water-sensitive images (eg, T2 or STIR) can help differentiate pathologies such as ischemia and inflammation.
- Postcontrast enhancement is useful for vascular pathology or pathologically vascular tissue.
- Know why each type of image is used - this will let you know what you are looking for (for example, for the MR brain, it is useful to look at T2, FLAIR, and DWI/ADC, as this will help distinguish between most differentials).
Compare with other imaging modalities
Compare MRI with other imaging modalities (eg, ultrasound, CT, plain film):
- Can you visualize the pathology in other imaging modalities?
- Plain radiographs can be particularly helpful in evaluating musculoskeletal pathology.
Compare with previous images
Compare current MRI images with previous MRI scans, if available:
- Are the abnormal signs new or old?
- Is there any change in the size/shape/brightness of the abnormal moles?
Consider the clinical context
Finally, place your findings in context with the clinical presentation to establish a radiological diagnosis:
- Are the symptoms acute or chronic?
- How sick is the patient?
- Does the imagined pathology correlate with the symptoms presented?
In this article, we outline the basics of the different types of MRIs, along with key examples. A lot has been covered and a lot hasn't, but this will give you a good understanding of the basics of MRIs.
Okey pointsThey are the following:
- MRIs are a superior imaging modality for visualizing soft tissue.
- T1- and T2-weighted images represent the main types of MR images.
- T1 and T2 images can be adjusted: fat suppression, gadolinium enhancement, and inversion recovery.
- The different sequences tell you what is in the lesion and how it is behaving. With these characteristics, the location of the lesion and the clinical history we can make a diagnosis.
- Anatomy, as in all exams, is essential. MRIs produce a very clear view of structures, so a strong anatomical knowledge is particularly helpful.
- Spend time looking at normal scans. The more familiar you become with what is normal, the easier it will be to see when things are abnormal.
- Always compare both sides of the scan; the pathology is rarely bilateral.
- Be methodical!
Dr. Muiz Shariffuddin
- Westbrook, C., Roth, CK & Talbot, J. MRI in practice. Published in 2005. Available at:[LINK]
- Bitar, R. et al. MR Pulse Sequences: What Every Radiologist Wants To Know But Is Afraid To Ask. Published in 2006. Available at:[LINK]
- Dr Hidayatullah Hamidi. Normal brain MRI shows differences between T1 and T2 images. License:[CC BY-SA].
- Andrew Murphy and others. Magnetic resonance sequences (abstract). Radiopaedia.org, the wiki-based collaborative radiology resource. [Internet] (Accessed: March 21, 2020). Available from: [LINK]
- Associate Professor Frank Gaillard. The meningioma is shown more clearly with gadolinium contrast with a dural tail. License:[CC BY-SA]. Available from: [LINK]
- Dr. Dahlia Ibrahim. STIR shows spinal edema at the L1 vertebra, indicative of a fracture. License: [CC BY-SA].
- Dr Mahmoud Rashed. Multiple sclerotic plaques in periventricular regions and corpus callosum. License: [CC BY-SA].
- Dr Bahman Rasuli. Recent ischemic stroke on the right. License: [CC BY-SA].
A radiologist, a physician specifically trained to supervise and interpret radiology tests such as MRI, will analyze the images and send a signed report to your primary care or referring physician, who will share the results with you.Who interprets the results of an MRI? ›
Imaging scans are read by a diagnostic radiologist, who then provides the information to the physician who ordered the test. And, if they see something that is not just a concern but a medical emergency, they will have a radiologist read your test immediately.Do Neurologists know how do you read MRIs? ›
Neurologists may read their own patients' MRIs. There is a significant benefit in correlating the clinical and imaging findings, but again, findings in other body parts could potentially be missed.How long does it take for a radiologist to interpret an MRI? ›
Results. The radiologist may discuss initial results of the MRI with you right after the test. Complete results are usually ready for your doctor in 1 to 2 days. An MRI can sometimes find a problem in a tissue or organ even when the size and shape of the tissue or organ looks normal.Do MRI techs know results? ›
The technician really won't. It's unlikely to see an MRI technician if you're being scanned. An MRI technician is a specialist who maintains and repairs MRI machines.