Image contrast
Contrast is the difference in brightness between the light and dark areas of a picture. An MRI image also has a contrast, the areas with high signal are bright on the image and the areas with low signal are dark on the image. The areas with intermediate signal are usually gray. Tissues with a large transverse component of magnetisation give a high signal due to the large signal amplitude. Tissues with a small transverse component of magnetisation give a low signal due to the small signal amplitude.
In MRI, contrast in the image is obtained through three mechanism i.e. T1 recovery, T2 decay and proton density. The image contrast depends on how much we allow each process to happen.
T1 contrast and weighting
The T1 time of a tissue is the time it takes for the excited spins to recover and be available for the next excitation. Let us take an example of fat and CSF in the human body. According to the larmor equation, the frequency of hydrogen atoms in CSF is higher than that of the hydrogen in fat. Therefore, hydrogen atoms in the fat recover more rapidly along the longitudinal axis than CSF. As the T1 time of fat is shorter than CSF, the fat vector realigns with B0 faster than that of CSF. Therefore, the longitudinal magnetisation of fat is larger than CSF.
After a certain TR, the next RF excitation pulse is applied and this will flip the longitudinal magnetisation of fat and CSF towards the transverse plane. As there was more longitudinal magnetisation in fat before the RF pulse, the net magnetization vector of the fat spins can be flipped into the X-Y plane more easily as compared to CSF. This means the contribution from CSF to the overall signal will be too small in comparison to fat. Therefore, fat has a high signal and appears bright on a T1 contrast image and the low signal CSF will be dark. This is called T1 weighted imaging.
Repetition Time (TR) and T1 Weighting.
Repetition time (TR) is the length of the relaxation period between two excitation pulses and therefore, it is crucial for T1 contrast. TR controls how far each vector can recover before it is excited by the next RF pulse. For T1 weighting, the TR must be short enough so that neither fat nor CSF has sufficient time to fully return to B0. If the TR is too long then both fat and CSF will fully recover to the longitudinal magnetisation. In that case, the difference in T1 contrast can not be demonstrated in the image.
Short TR → strong T1 weighting
Long TR → low T1 weighting
For T1 weighting we should chose a short TR.
Tissues with a short T1 appear bright because they regain most of their longitudinal magnetization during the TR interval and produce a stronger MR signal.
Tissues with a long T1 appear dark because they do not regain much longitudinal magnetization during the TR interval and produce a weaker MR signal.
A typical T1-weighted spin echo (SE) sequence is acquired with a TR/TE of 400/15 msec.
Table below shows T1 relaxation times for various tissues,
BRAIN
Gray matter
White matter
Tumours
Meningioma
Glioma
Oedema
MUSCLE
Normal tissue
Tumours
Carcinoma
Fibrosarcoma
Rhabdomyosarcoma
Oedema
BREAST
Fibrotic tissue
Adipose tissue
Tumours
Carcinoma
Adenocarcinoma
Fibroadenoma
LIVER
Normal tissue
Tumours
Hepatoma
Chirrosis
BONE
Normal marrow
Osteosarcoma
KIDNEY
Normal tissue
Tumours
LUNG
Normal tissue
Tumours
PANCREAS
Normal tissue
Tumours
SPLEEN
Normal tissue
T1 (ms) 1.5 T
921
787
1073
979
959
1090
868
1083
1046
1011
1173
1488
868
259
976
923
1167
1195
493
905
1077
438
732
973
652
907
829
826
513
1448
782
T2 contrast and weighting
The T2 time determines how quickly an MR signal fades after excitation. Let us take an example of fat and CSF again. The T2 decay occurs as a result of energy transfer between spins. The energy exchange in the hydrogen atoms in fat is more efficient than in water. As a result, hydrogen in the fat loses transverse magnetization more rapidly than in CSF. The T2 time of fat(80ms) is shorter than the CSF(200ms). Therefore, the transverse magnetisation of fat decays faster. The magnitude of transverse magnetisation in CSF is large. It therefore appears very bright as compare to fat in T2 contrast image.
Echo Time (TE) and T2 Weighting.
Echo time (TE) is the interval between application of the excitation pulse and collection of the MR signal. If a short echo time is used (25ms), the signal differences between tissues are small because T2 relaxation has only just started and there has only been little signal decay at the time of echo collection. The resulting image has low T2 weighting. If a long echo time is used (100ms), the signal differences between tissues will be large. Tissues with a short T2 have lost most of their signal and appear dark on the image while tissues with a long T2 produce a stronger signal and thus appear bright. This is the reason why cerebrospinal fluid (CSF) with its longer T2 (200ms) is brighter on T2-weighted images as compared to fat(80ms).
Short TE → low T2 weighting
Long TE → strong T2 weighting
Tissues with a short T2 appear dark on T2-weighted images.
Tissues with a long T2 appear bright on T2-weighted images.
A T2-weighted fast spin echo (FSE) MR image can be acquired with a TR/TE of 3000/100 msec.
Table below shows T2 relaxation times for various tissues,
BRAIN
Gray matter
White matter
Tumours
Meningioma
Glioma
Oedema
MUSCLE
Normal tissue
Tumours
Carcinoma
Fibrosarcoma
Oedema
BREAST
Fibrotic tissue
Adipose tissue
Tumours
Carcinoma
Adenocarcinoma
Fibroadenoma
LIVER
Normal tissue
Tumours
Hepatoma
Chirrosis
BONE
Normal marrow
Osteosarcoma
KIDNEY
Normal tissue
Tumours
LUNG
Normal tissue
Tumours
SPLEEN
Normal tissue
T2 (ms) 1.5 T
101
92
121
103
111
113
47
87
82
65
67
49
84
80
94
81
60
43
84
84
45
106
85
58
83
79
68
782
Proton density weighting.
The proton density (PD) is the number of excitable spins per unit volume.Proton density determines the maximum signal that can be obtained from a given tissue. The image contrast in PD images is not dependent on T1 or T2 relaxation. The signal we receive is completely dependent on the amount of protons in the tissue. Less protons means low signal and appear as dark areas on the image whereas more protons produce a lot of signal and will be bright on the image. Proton density can be enhance by minimizing the effect of T1 and T2 contrast. If you go back to the example of fat and CSF, a long TR allows both fat and CSF to fully recover there longitudinal magnetisation and therefore reduces the T1 weighting and a short TE does not give fat or water time to decay and therefore reduce the T2 weighting.
Short TE → diminish T2 weighting.
Long TR → diminish T1 weighting.
A typical PD weighted spin echo (SE) sequence is acquired with a TR/TE of 2500/15 msec.
PD sequences are very useful for evaluating structures with low signal intensities such as the bones or connective tissue structures (ligaments and tendons). Proton density weighting is often used for high-resolution imaging. SE sequences are preferred over FSE sequences for PD imaging as SE images are less prone to distortion. PD sequences are mainly used for the imaging of brain, spine, and musculoskeletal system.
Table below shows Proton density(%) for various tissues,
Tissue
Gray matter
White matter
CSF
Meningioma
Metastasis
Fat
Proton density(%)
85
70
100
90
85
100
Magnetization Transfer contrast (MTC).
Apart from the water protons, biological tissues also contain a specific pool of protons bound in macromolecules (normally proteins and membranes). These macromolecular protons cannot be directly visualized because of their very short T1. Macromolecular protons have a wider range of Larmor frequencies than the water protons. Therefore, these macromolecular protons can also be excited by RF pulses with frequencies slightly different from the Larmor frequency of hydrogen protons. It is possible to selectively excite a tissue with a large pool of macromolecular protons without directly affecting the protons in free water. Repeated delivery of the magnetization transfer pulse saturates the magnetization of the macromolecular protons and it is transferred to free protons nearby. This process is associated with a drop in signal that depends on magnitude, concentration of macromolecules and their interaction with free water, and is known as magnetization transfer contrast. The magnetisation is transferred by cross-relaxation and chemical exchange. Magnetization transfer pulses are used in spin-echo or gradient-echo sequences to produce additional signal suppression of tissue water.
Aplications
This phenomenon can be useful in some imaging techniques and therefore named as magnetization transfer imaging. MTC imaging is used in cartilage imaging to improve contrast between synovial fluid and cartilage. As synovial fluid contains only a few bound protons, it shows only a little magnetization transfer while cartilage contains a large proportion of bound protons and shows large magnetization transfer. In brain scanning MTC technique improves the detection of gadolinium-enhancing lesions. Another use of magnetization transfer is in 3D time-of-flight MR angiography. In time-of-flight MR angiography, contrast between normal, stationary tissue and flowing blood is provided through T1 saturation of stationary-tissue water by repeated excitation pulses. Additional suppression of the stationary-tissue water with use of magnetization transfer pulses enables smaller vessels to be visualized.
