T1 vs. T2 MRI Imaging: Guide To Understanding The Primary Difference

Comparing T1 and T2 MRI Imaging Techniques

MRI systems help capture high-quality diagnostic images of patients through the combined use of high-powered magnets and radio waves. The clarity and resolution of the images captured depend on a variety of factors, including the type of MRI system being used. The strength of the magnetic field being produced has a big impact on the images--and there is a range of low-field to high-field MRIs available (there are even ultra high-field MRIs on the market now).

However, the quality of the diagnostic images aren't just dependent on the system, they are also affected by the techniques being used. For example, there are two main types of imaging techniques--T1 and T2--both of which can be effective depending on their use.

MRI Basics

To understand what T1 and T2 techniques are and their differences, you need to have a basic understanding of how MRIs work. An MRI system uses high-powered magnets to scan patients. These magnets produce strong magnetic fields that align the protons contained by the body's water molecules.

The MRI system also produces a radio frequency current, which creates a varying magnetic field. The energy it produces is absorbed by the protons, causing them to flip their rotation. As soon as this field is turned off, the protons will slowly return to their original rotation. This process is known as "precession." As the protons return to their normal rotation, they produce a radio signal that the MRI system's receivers can measure--it's these radio signals that are used to create an image of the part of the body being scanned.

MRI Image Techniques

To create different types of images of the parts of the body being scanned (allowing the doctor to see different areas of the same part), the sequence of radio frequency pulses applied and collected can be varied. For example, repetition time (referred to as "TR") is the time measured between pulse sequences applied to the same slice. Time to echo (referred to as "TE") is the time between the delivery of the RF pulse and the capture of the echo signal.

The tissue being scanned can be characterized by two types of relaxation times, T1 and T2. T1 determines the rate at which the protons return to their normal spin, while T2 determines the rate at which the protons either reach equilibrium or go out of phase with each other. To create T1 weighted images, short TE and TR times are used, while to create T2 weighted images, longer TE and TR times are used.

Why Use Different MRI Sequences?

There are many different components to every part of the body. When attempting to diagnose a certain condition, it might be difficult to spot anything if what you're trying to see is being obscured by other elements of the body. By using different MRI sequences, you can emphasize what you want to look at when viewing an image of a particular body part, making it easier to find what you're looking for. This also means that depending on what part of the body you're looking at, choosing between T1 or T2 techniques may result in more detailed imaging.

T1 Weighted Images

When creating T1 weighted images, different elements will come out as darker or brighter on the scan. The following elements will appear as darker on a T1 weighted scan:

• Increased water (edema, hemorrhage, infarction, infection, inflammation, tumor)
• Rapidly flowing blood
• Ligaments, muscles, tendons
• Abdominal organs
• Fluid
• Cartilage
• Calcium
• Bone

Elements that will appear brighter on a T1 weighted image include:

• Fat
• Blood
• MRI contrast elements (gadolinium)
• Laminar necrosis of cerebral infarction
• Paramagnetic substances (copper, gadolinium, manganese)
• Protein-rich fluids
• High-protein tissue (complex cysts, abscess)
• Melanin
• Subacute hemorrhage

When are T1 Images Preferable?

T1 imaging tends to be more effective for visualizing normal anatomy, such as the musculoskeletal system and brain structure, due to the fact that fat appears very bright and bone marrow contains a lot of fat. This means that T1 imaging can be very useful in diagnosing conditions such as leukemia and multiple sclerosis.

 T2 Weighted Images

The following are the dark elements produced on a T2 weighted image:

• Air
• Calcium
• Rapidly flowing blood
• Ligaments and tendons
• Liver
• Pancreas
• Adrenals
• Cartilage

The following are the brighter elements that can be captured on a T2 weighted image (interesting note: some elements can appear both light gray to dark gray, including the pancreas and liver):

• Fluid
• Bladder
• Bile
• Gallbladder
• Kidneys
• Liver
• Pancreas
• Muscles

When are T2 Images Preferable?

Because water and fluids appear brighter on T2 weighted images, T2 imaging is typically used when looking for areas of inflammation. In general, this also means that T2 imaging tends to be used for pathology; for example, T2 is useful to review the condition of the kidneys or to look for signs of common diseases that affect it.

Choosing The Correct MRI Sequence For The Best Results

When performing an MRI scan, you'll have the option of using either T1 or T2 imaging techniques. The images you capture will highlight different elements of the body. The technique you choose should depend on what part of the body you're scanning and exactly what you're looking for. For example, if you're looking for something in the patient's brain, use the T1 technique; if the patient is complaining about pain in their kidney area and you suspect it to be an issue with their kidneys, use the T2 technique.

Knowing what elements of the body each technique captures most effectively will allow you to make the best choice between T1 and T2 imaging when you perform an MRI scan.