General MRI Terms
Signal intensity:
In MRI the terms low, intermediate and high signal intensity are used. Depending on the scan protocol, tissue is imaged as white (= high signal intensity), as a gray tone (= intermediate signal intensity) or as dark gray/black (= low signal intensity).
In MRI the terms low, intermediate and high signal intensity are used. Depending on the scan protocol, tissue is imaged as white (= high signal intensity), as a gray tone (= intermediate signal intensity) or as dark gray/black (= low signal intensity).
Gradient:
An additional magnetic field that may be added manually to the MRI device's magnetic field. This creates an additional subdivision in the ‘total’ magnetic field. Gradient coils are used among other things to determine the location of the protons in the X, Y and Z axis.
Sequence:
Combination of radiofrequent pulses and gradients (= ‘added magnetic fields’), which together constitute the building blocks for an MRI series. For instance, the terms ‘T1 weighted sequence’ and ‘T2 weighted sequence’ are used.
MRI Sequences
- T1 weighted sequence
- T2 weighted sequence
- PD weighted sequence
- Gradient & spin echo sequence
- Fat suppression
- MRI contrast
- Diffusion weighted image
- In/out-of-phase
Contrast differences are required to distinguish normal anatomy from pathology. Contrast is improved when two adjacent areas have high and low signal intensities. There are many different MRI sequences (>100) and all attempt to optimize tissue contrast.
Each MRI image consists of a T1 component and a T2 component (see also Relaxation section). It is possible to switch off most of one of either components, creating a T1 weighted or T2 weighted image respectively. A special form is the proton density (PD) weighted image. This sequence enables the visualization of the number of protons per volume. In order to achieve this, both the T1 and T2 components must be switched off.
The following briefly describes some commonly used MRI sequences.
Each MRI image consists of a T1 component and a T2 component (see also Relaxation section). It is possible to switch off most of one of either components, creating a T1 weighted or T2 weighted image respectively. A special form is the proton density (PD) weighted image. This sequence enables the visualization of the number of protons per volume. In order to achieve this, both the T1 and T2 components must be switched off.
The following briefly describes some commonly used MRI sequences.
T1 weighted sequence
The contrast created in the image is determined in particular by the difference in T1 relaxation times between fat and water. Fat has a high signal intensity (white) and water a low signal intensity (black).
Why does fat have a high signal intensity on a T1 weighted image?
Answer: fat has a shorter T1 relaxation time than water.
Explanation: by its short T1 relaxation time, fat will recover quicker from longitudinal magnetization (Z axis). When a second radiofrequent pulse of 90 degrees is given, water will not have fully recovered in the longitudinal plane. After the second pulse, fat will make a larger deflection than water and create more transversal magnetization (fig. 10). As a reminder, signals can be received and processed only in the transversal plane. The higher the transversal magnetization of the tissue, the more signal is received. When radiofrequent pulses are repeated, fat will contribute more to the final MRI image and therefore be represented as a high signal (white).
Why does fat have a high signal intensity on a T1 weighted image?
Answer: fat has a shorter T1 relaxation time than water.
Explanation: by its short T1 relaxation time, fat will recover quicker from longitudinal magnetization (Z axis). When a second radiofrequent pulse of 90 degrees is given, water will not have fully recovered in the longitudinal plane. After the second pulse, fat will make a larger deflection than water and create more transversal magnetization (fig. 10). As a reminder, signals can be received and processed only in the transversal plane. The higher the transversal magnetization of the tissue, the more signal is received. When radiofrequent pulses are repeated, fat will contribute more to the final MRI image and therefore be represented as a high signal (white).
In practice, T1 weighted images are used mostly to evaluate normal anatomy.
Remember that only a few structures have a high signal intensity (= white) on a T1 weighted image: fat, blood, gadolinium (= contrast), melanin, protein (e.g. high-protein cysts). A high signal can also be seen in specific MRI artifacts and accumulation diseases (not discussed further in this course).
Water and collagenous tissue (ligaments, tendons, scars) have a lower signal intensity on a T1 weighted image (fig. 11).
Remember that only a few structures have a high signal intensity (= white) on a T1 weighted image: fat, blood, gadolinium (= contrast), melanin, protein (e.g. high-protein cysts). A high signal can also be seen in specific MRI artifacts and accumulation diseases (not discussed further in this course).
Water and collagenous tissue (ligaments, tendons, scars) have a lower signal intensity on a T1 weighted image (fig. 11).
The spearhead of T1 weighted imaging is visualizing normal anatomy, particularly the musculoskeletal system (fig. 12). When the signal intensity of the fat-containing bone marrow (high on T1!) is replaced by a low signal intensity, beware of bone marrow edema or bone marrow infiltration (fig. 13/14).
T2 weighted image
Characteristic of a T2 weighted image is the high signal intensity of water. Pathology is often associated with edema/fluid and therefore a T2 sequence is very suitable to detect pathology (fig. 15).
As in a T1 weighted image, air and calcifications have very low signal intensity.
Tip: you are unsure whether you are seeing air or calcifications? Try to find the structure on an X-ray/CT scan!
Tip: you are unsure whether you are seeing air or calcifications? Try to find the structure on an X-ray/CT scan!
Tip: always look for fluid-filled structures (CSF, gallbladder, bladder) to decide whether you are looking at a T1 or T2 weighted image. Fluid has a high signal intensity on T2 weighted images. Note: fat is a less reliable marker to distinguish between T1 and T2 (especially in view of the development of new types of MRI sequences).
PD weighted image
The proton density (PD) weighted image visualizes the number of protons per volume. To achieve this, both the T1 and T2 components are switched off as fully as possible (for the sake of convenience, technical background is not discussed in this course).
Tissues with few protons have low signal intensity, tissues with many protons have high signal intensity.
Fat has a relatively high signal intensity, however, not as high as in a T1 weighted image. Fluid has an intermediate signal intensity rather than the high signal intensity as in a T2 weighted image.
A PD weighted image is used among other things to evaluate meniscal tears in the knee (fig. 17).
Additionally, a PD sequence can be useful in e.g. brain MRI to evaluate gray/white matter pathology. Explanation: as opposed to a T2 weighted image, a PD clearly distinguishes between gray and white matter (gray matter has a higher signal intensity than white matter). The distinction between CSF and pathology is difficult on a T2 weighted image; both have a high signal. The contrast between CSF (intermediate signal intensity) and pathology (high signal intensity) will be better on a PD weighted image.
Tissues with few protons have low signal intensity, tissues with many protons have high signal intensity.
Fat has a relatively high signal intensity, however, not as high as in a T1 weighted image. Fluid has an intermediate signal intensity rather than the high signal intensity as in a T2 weighted image.
A PD weighted image is used among other things to evaluate meniscal tears in the knee (fig. 17).
Additionally, a PD sequence can be useful in e.g. brain MRI to evaluate gray/white matter pathology. Explanation: as opposed to a T2 weighted image, a PD clearly distinguishes between gray and white matter (gray matter has a higher signal intensity than white matter). The distinction between CSF and pathology is difficult on a T2 weighted image; both have a high signal. The contrast between CSF (intermediate signal intensity) and pathology (high signal intensity) will be better on a PD weighted image.
Gradient & spin echo sequence
‘Gradient’ and 'spin echo’ are common terms in an MRI context. Importantly, this is a technique that can be used on a T1, T2 and PD sequence. The gradient & spin echo technique may be regarded as two large families in which multiple variations are possible.
In summary: the gradient technique has a shorter scan time than the spin echo technique and is used among other things for angiography, brain, heart, abdomen and functional MRI. A significant drawback of the gradient technique is susceptibility to artifacts (hemoglobin in blood and prosthesis/osteosynthesis material).
Spin echo technique is an alternative option. The traditional spin echo was commonly used because of its many applications. Nowadays the spin echo has been developed into a faster sequence; the fast spin echo (FSE) and the single shot fast spin echo (SSFSE). Scan time has now been reduced to a few minutes, resulting in fewer movement artifacts. Despite the fact that the spin echo is not as fast (as the gradient), the technique is used frequently because of its image quality. The fast spin echo sequences are used frequently to image the abdomen (e.g. MRCP), pelvis (urogenital) and musculoskeletal system (especially in prosthesis material!).
Practical tip:
- gradient is a good choice to detect blood products.
- spin echo technique has fewer unwanted artifacts in prosthesis and osteosynthesis material.
In summary: the gradient technique has a shorter scan time than the spin echo technique and is used among other things for angiography, brain, heart, abdomen and functional MRI. A significant drawback of the gradient technique is susceptibility to artifacts (hemoglobin in blood and prosthesis/osteosynthesis material).
Spin echo technique is an alternative option. The traditional spin echo was commonly used because of its many applications. Nowadays the spin echo has been developed into a faster sequence; the fast spin echo (FSE) and the single shot fast spin echo (SSFSE). Scan time has now been reduced to a few minutes, resulting in fewer movement artifacts. Despite the fact that the spin echo is not as fast (as the gradient), the technique is used frequently because of its image quality. The fast spin echo sequences are used frequently to image the abdomen (e.g. MRCP), pelvis (urogenital) and musculoskeletal system (especially in prosthesis material!).
Practical tip:
- gradient is a good choice to detect blood products.
- spin echo technique has fewer unwanted artifacts in prosthesis and osteosynthesis material.
Susceptibility artifact
Magnetic susceptibility means that protons with their own internal magnetization interact with the external magnetic field. In other words: it is the degree to which tissue becomes magnetic as a result of exposure to a magnetic field. When two tissues with different magnetic susceptibilities are close together, local field inhomogeneities may develop. This disruption accelerates dephasing and will eventually lead to loss of signal or distorted images. These susceptibility artifacts develop with metals (depending on the metal type) and natural transitions such as air-tissue (sinuses-brain parenchym) and tissues surrounding bone. Also the blood product hemoglobin may cause susceptibility artifacts on a gradient echo sequence. Despite this undesired phenomenon, it can also be used to characterize lesions. A frequently used gradient echo is the SWI sequence (Susceptibility Weighted Imaging) (fig. 18).
Magnetic susceptibility means that protons with their own internal magnetization interact with the external magnetic field. In other words: it is the degree to which tissue becomes magnetic as a result of exposure to a magnetic field. When two tissues with different magnetic susceptibilities are close together, local field inhomogeneities may develop. This disruption accelerates dephasing and will eventually lead to loss of signal or distorted images. These susceptibility artifacts develop with metals (depending on the metal type) and natural transitions such as air-tissue (sinuses-brain parenchym) and tissues surrounding bone. Also the blood product hemoglobin may cause susceptibility artifacts on a gradient echo sequence. Despite this undesired phenomenon, it can also be used to characterize lesions. A frequently used gradient echo is the SWI sequence (Susceptibility Weighted Imaging) (fig. 18).
Fat suppression
Suppression of fat tissue is one of the many options that can be used in an MRI sequence.
In virtually all abdominal MRI examinations, suppressing the fat tissue signal is advisable. The created low signal intensity of fat then contrasts more strongly with the vessels & pathology (high signal intensity!).
Also in skeletal imaging, it may be useful to make a sequence with fat suppression. Bone marrow contains fat and may mask bone marrow edema on a T2 weighted image.
There are several technical options to suppress fat tissue. Frequently used sequences are the STIR (short-tau inversion recovery) and the SPIR (spectral pre-saturation inversion recovery) sequences. Both are T2 weighted images.
You can also recognize fat suppression by the abbreviation FatSat, meaning Fat Saturation (e.g. T2wFatSat).
Tip: you can easily recognize fat suppression by looking at the subcutaneous fat (fig. 19). When it has a low signal, you are looking at a fat suppression sequence. The technique may be used 'as extra’ in T1, T2 and PD weighted images.
In virtually all abdominal MRI examinations, suppressing the fat tissue signal is advisable. The created low signal intensity of fat then contrasts more strongly with the vessels & pathology (high signal intensity!).
Also in skeletal imaging, it may be useful to make a sequence with fat suppression. Bone marrow contains fat and may mask bone marrow edema on a T2 weighted image.
There are several technical options to suppress fat tissue. Frequently used sequences are the STIR (short-tau inversion recovery) and the SPIR (spectral pre-saturation inversion recovery) sequences. Both are T2 weighted images.
You can also recognize fat suppression by the abbreviation FatSat, meaning Fat Saturation (e.g. T2wFatSat).
Tip: you can easily recognize fat suppression by looking at the subcutaneous fat (fig. 19). When it has a low signal, you are looking at a fat suppression sequence. The technique may be used 'as extra’ in T1, T2 and PD weighted images.
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