A proton density (PD) weighted MRI image visualizes the number of protons in tissues. Tissues with few protons appear dark, while those with many protons appear bright. On a PD weighted image, fat has a bright signal intensity, but not as bright as on a T1 weighted image. Fluid has an intermediate signal intensity, rather than the high intensity on a T2 weighted image. A PD weighted image is useful for evaluating meniscal tears in the knee due to the contrast between CSF and pathology.

1. proton density PD weighted mri image

2. PD weighted image
The proton density (PD) weighted image visualizes the number of
protons per volume.
Tissues with few protons have low signal intensity, tissues with
many protons have high signal intensity.

3. PD weighted image
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.

4. PD weighted image
A PD weighted image is used among other things to evaluate
meniscal tears in the knee
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.

5. MRI image appearance
The easiest way to identify PD weighted images is to
compare the fluid against the fat signal.
Fluids normally appear as grayish white, almost similar
appearance as the fat in the body
https://mrimaster.com/characterise%20image%20pd.html

6. Tissues and their PD appearance
Bone marrow: - equal to or higher than that of muscle (fatty marrow is
usually bright)
Fat – bright (slightly darker than the fat signal in T1 images)
Fluids – bright (darker than the fluid signal in T2 images)
Moving blood- dark
Muscles-gray
Bone – dark
Air - dark
https://mrimaster.com/characterise%20image%20pd.html

7. Use
Useful for extremity imaging (e.g. ankle, knee, elbow
shoulder and hips)
Can be useful in thighs, lower legs, upper arms and
forearms imaging
https://mrimaster.com/characterise%20image%20pd.html

8. On spin-echo (SE) imaging, the repetition time (TR) and
the echo time (TE) are used to control image contrast
and the "weighting" of the MR image. There are many
misconceptions about what the term "weighting" actually
means, so a separate Q&A is devoted to that very
subject. For now we will not specifically define what we
mean by "T1-weighting", "T2-weighting", or "PD (proton-
density) weighting" other than to say that it implies that
image contrast is significantly affected by T1, T2 or [H].
http://mriquestions.com/image-contrast-trte.html

9. •The traditional model in SE imaging to which the main
question refers considers four combinations
of TR and TE values:
Short TR/Short TE → T1-W
•Long TR/Short TE → PD-W
•Long TR/Long TE → T2-W
•Short TR/Long TE → not used
The exact reference ranges are not generally specified,
but usually "long" TR or TE means 3-5x T1 or T2
respectively, while "short" implies TR or TE << T1 or T2.
http://mriquestions.com/image-contrast-trte.html

10. http://mriquestions.com/image-contrast-trte.html

11. An MRI sequence in magnetic resonance imaging (MRI) is a
particular setting of pulse sequences and pulsed field gradients,
resulting in a particular image appearance.[1]
A multiparametric MRI is a combination of two or more sequences,
and/or including other specialized MRI configurations such
as spectroscopy.[2][3]
https://en.wikipedia.org/wiki/MRI_sequence#Spin_echo

12. Group Sequence Abbr. Physics Main clinical distinctions
E
x
a
m
p
l
e
Spin echo
T1 weighted T1
Measuring spin–lattice
relaxation by using a
short repetition time (TR)
and echo time (TE).
•Lower signal for more water content,[4] as
in edema, tumor, infarction, inflammation, i
nfection, hyperacute or
chronic hemorrhage.[5]
•High signal for fat[4][5]
•High signal for paramagnetic substances,
such as MRI contrast agents[5]
Standard foundation and comparison for
other sequences
T2 weighted T2
Measuring spin–spin
relaxation by using long TR and
TE times
•Higher signal for more water content[4]
•Low signal for fat[4] − Note that this only
applies to standard Spin Echo (SE)
sequences and not the more modern Fast
Spin Echo (FSE) sequence (also referred to
as Turbo Spin Echo, TSE), which is the most
commonly used technique today. In
FSE/TSE, fat will have a high signal.[6]
•Low signal for paramagnetic substances[5]
Standard foundation and comparison for
other sequences
Proton density
PD
Long TR (to reduce T1) and
[7]
•Joint disease and injury.[8]High signal
[9]

13. Group Sequence Abbr.
Spin echo
T1
weighted
T1
T2
weighted
T2
Proton
density
weighted
PD

14. Physics Main clinical distinctions
Measuring spin–lattice
relaxation by using a
short repetition
time (TR) and echo
time (TE).
•Lower signal for more water content,[4] as
in edema, tumor, infarction, inflammation, infection, hyperacute or
chronic hemorrhage.[5]
•High signal for fat[4][5]
•High signal for paramagnetic substances, such as MRI contrast agents[5]
Standard foundation and comparison for other sequences
Measuring spin–spin
relaxation by using
long TR and TE times
•Higher signal for more water content[4]
•Low signal for fat[4] − Note that this only applies to standard Spin Echo
(SE) sequences and not the more modern Fast Spin Echo (FSE) sequence
(also referred to as Turbo Spin Echo, TSE), which is the most commonly
used technique today. In FSE/TSE, fat will have a high signal.[6]
•Low signal for paramagnetic substances[5]
Standard foundation and comparison for other sequences
Long TR (to reduce T1)
and short TE (to
minimize T2).[7]
•Joint disease and injury.[8]High signal from meniscus tears.[9] (pictured)

15. Group Sequence Example
Spin echo
T1 weighted
T2 weighted
Proton density weighted

16. Proton density (PD)- weighted images are created by
having a long repetition time (TR) and a short echo time
(TE).[36] On images of the brain, this sequence has a more
pronounced distinction between gray matter (bright)
and white matter (darker gray), but with little contrast
between brain and CSF.[36] It is very useful for the
detection of joint disease and injury.[37]
https://en.wikipedia.org/wiki/MRI_sequence#Spin_echo