MRIs: T1 & T2??

T1 and T2 weighted images in MRI
From the medical point of view the T1 and T2 weighted images are used to highlight different constituents of various tissues.
T1 weighted images highlight only fat containing tissues in the body.
T2 weighted images are timed to highlight both fat and water.

One easy mnemonic to remember this:
  • T1- 1 constituent is highlighted: Fat.
  • T2- 2 highlighted constituents: Fat and Water.
Note that the fat appears bright on both T1 and T2 images.

Q:How do we differentiate fat from water based tissue on a T2 image since both are highlighted?

A: Compare it with the T1 image. Fat would appear white on both the images (subcutaneous fat in the above images) whereas water based tissue would appear dark on T1 and get highlighted on T2 images( CSF in the above photos).

The T1 weighted image is useful to delineate the normal anatomy whereas certain pathologies become easily appreciable with the T2 images.


The above images are from a patient of multiple myeloma. The T2 image(above) helps to better appreciate the bony mets of the myeloma which understandably have higher water content and get highlighted.

Q: What is meant by T1 weighted and T2 weighted images?

 Magnetic resonance imaging (MRI) makes use of the magnetic properties of protons which are present in water molecules, and therefore in all body tissues.

These protons are randomly oriented in the body.
The extremely powerful magnetic field of an MRI scanner aligns these protons along the field.
Radiofrequency pulses are subsequently applied which excite the protons.

Moments after the removal of the radiofrequency pulses the protons “relax” giving of their own radiofrequency signal.

This relaxing of the protons occurs by two methods: T1 Longitudinal relaxation ("spin-lattice") and T2 transverse relaxation( "spin-spin"/"slow dephasing")

During the process of T1 relaxation, protons reorient back to their longitudinal position in the magnetic field created. Fat quickly realigns its longitudinal magnetization and therefore appears bright on a T1 weighted image.

Conversely, water has much slower longitudinal magnetization realignment after an RF pulse and therefore, water has low signal and appears dark. 

During the process of T2 relaxation, we look at something else: protons dephasing where spin becomes desynchronized.(Imagine a ball being spun and then returning to it's original position radially). Protons in water dephase slowly and this phenomenon is exploited in T2 weighted images.

and finally, a word about FLAIR:

Fluid Attenuated Inversion Recovery images in a way are inverse T2 images. The image from the free fluid, for example CSF, is suppressed instead of being highlighted. FLAIR images are particularly important to detect subtle changes  at the periphery of the hemispheres and in the periventricular region close to CSF. One condition in which this is used commonly is Multiple sclerosis where periventricular white matter demyelination can be detected.
                                                         

The area highlighted by the white arrow in the T2 image may be mistaken for CSF.The FLAIR image darkens the CSF and hence the remaining white hyperintense area is pathological.


Written by Pranav Hinduja







Comments

Post a Comment

Popular posts from this blog

The Icebreaker: Contraction Alkalosis

Image
The Icebreaker: What is Contraction alkalosis ?? The word 'contraction' in contraction alkalosis refers to fluid contraction or in other words, fluid loss . So when the body loses fluid (typically seen with diuretics) it tries to get all that fluid back. Q: How can the body do this ?   A: By increasing the serum sodium concentration. (increased sodium concentration --> increased osmolarity of serum --> increase in intravascular volume) Q: How does the body do it ? A:1) The decreased blood volume is sensed as decreased blood pressure felt by the kidneys at the glomerulus ie. decreased renal perfusion pressure. There are modified smooth muscle cells in the afferent arteriole that detect this change called JG cells . They secrete Renin in response to the low pressures.  2) By the diagram shown below Renin activates an axis that gives rise to Angiotensin II and aldosterone.   The Renin-Angiotensin-Aldosterone axis   3) Angiotens
Read more

Dexametha-Zone

Image
Q: What is the importance of Low Dose and High Dose Dexamethasone Suppression Tests? A: Let's begin with the HPA (Hypothalamus-Pituitary-Adrenal) axis: Cortisol suppresses the release of both CRH from the hypothalamus as well as ACTH from the pituitary. Low Dose Dexamethasone Test: The Overnight DST which is a type of Low Dose DST is used to rule out a hypercortisolism state or the diagnosis of Cushing’s syndrome. It is based on the principle that in a normal individual a low dose of dexamethasone is sufficient to suppress the ACTH production and hence the cortisol levels. Procedure: Overnight Dexamethasone test : 1 mg of Dexamethasone is given at 11pm and the cortisol level is measured at 8am. Failure to suppress the cortisol level to less than 50 nmol/L signifies Cushing's syndrome. LDDST : 0.5mg Dexamethasone q6h for 2 days and then the plasma cortisol is measured. Plasma cortisol more than 50 nmol/L is considered abnormal. Any abnormalit
Read more

Are SGLT2 inhibitors renoprotective?

Image
SGLT2 Inhibitors and Renoprotection Q: “How do SGLT2 inhibitors help in chronic kidney disease?” A: The renoprotective effect of SGLT2 inhibitors is still being extensively studied, but the results have been very promising. Let’s start with discussing what SGLT2 inhibitors are. SGLT 2 inhibitors SGLT2 inhibitors (eg dapagliflozin, canagliflozin, empagliflozin etc) are a class of drugs that inhibit reabsorption of glucose in the kidney (by inhibiting the sodium-glucose transport protein 2 - ie, SGLT2) and therefore lower blood sugar. Because of their use in lowering blood glucose levels, they are regularly used in the management of type 2 diabetes mellitus (and increasingly in type 1 diabetes as well). In addition to glycemic control, they have been shown to provide significant cardiovascular benefit in patients with type 2 diabetes. Mechanism Sodium Glucose cotransporters (SGLTs) are proteins that are found in different tissues that play an important role in mai
Read more