Diffusion-weighted imaging of the abdomen and pelvis

25 Pages·2016·3.84 MB·English

Diffusion-weighted imaging of the abdomen and pelvis: do
the pitfalls overcome the value of real restriction?
Poster No.: C-1555
Congress: ECR 2016
Type: Educational Exhibit
Authors: 1 2 1 1 2
M. Lima , M. Correia , Â. Marques , I. Oliveira , Z. Seabra ;
1 2
Lisboa/PT, Lisbon/PT
Keywords: Education and training, Artifacts, Technical aspects, Physics, MR-
Functional imaging, MR-Diffusion/Perfusion, MR, Pelvis, Abdomen
DOI: 10.1594/ecr2016/C-1555
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Learning objectives
• Review the basic principles of diffusion-weighted imaging and its importance
in the evaluation of abdominal and pelvic pathology.
• Point out the main pitfalls of this technique, namely the ones that can cause
false positives, false negatives and both.
• Show examples of these pitfalls for the different organs of abdomen and
pelvis.
Background
Diffusion-weighted imaging (DWI) is based on the detection of the microscopic motion
of water molecules in the intra and extracellular spaces and vessels. [1] Free water
molecules are in constant random motion, which is named Brownian motion. On the
other hand, when within the cellular microenvironment, their motion is conditioned by their
interaction with cellular compartments (namely the cell wall and intracellular organelles).
In fact, restriction in the diffusion of water molecules is directly proportional to the
cellularity of a tissue. [2]
This technique is performed by sensitizing an imaging sequence with two opposed
gradients, both strong, which would cancel each other out in the absence of molecular
motion. [3] These two opposed gradients are incorporated into a single-shot spin-echo
(SE) T2-weighted sequence, with each on either side of the 180º refocusing pulse. The
spins that have moved will be subject to a different field strength during the second pulse
and so will not return to their initial state but will undergo a total phase shift. [2] This can
be detected by a signal loss on the resulting image. [3] In contrast, all spins that remain
at the same location along the gradient axis during the two pulses will return to their initial
state and thus will not lose signal (restricted diffusion). [2] Several factors influence the
amount of signal loss, such as: the quantity of moving molecules, their respective speed
of movement and the strength of the diffusion-sensitizing gradients, which is indicated
by the b value. [3]
2
DWI is performed with at least two different b values, including a b value of 0 sec/mm
2
and a higher b value of 500-1000 sec/mm , which depends on the body region that we
are studying. [1]
Highly mobile water molecules, such as those within vessels, will show signal loss with
2
small b values (50-100 sec/mm ). This will result in "black blood images" (Fig. 1 on page
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3 and Fig. 2 on page 4). In highly cellular tissues, water movement is restricted,
and so the water molecules within tissue will retain their signal even at high b values
2
(500-1000 sec/mm ) (Fig. 3 on page 5). [1]
The degree of water motion is described quantitatively as the ADC value. It represents
the average diffusional path of water molecules over a specific time interval. [4] It is
calculated during the postprocessing and this calculation requires at least two different
b values. A higher number of b values will improve the accuracy of ADC, at the cost of
increased scanning time. [1,2] Areas with restricted diffusion will show lower ADC values
(Fig. 4 on page 6 and Fig. 5 on page 7). [4]
Recent developments in the Magnetic Resonance (MR) field have allowed a significant
reduction in DWI scan time [4]. It can actually take less than a minute and so DWI
sequences can be added to a MR protocol without significant delay in the conclusion of
the exam. Another advantage of this technique is that it does not need the administration
of exogenous contrast material. [2] This aspect allows its use in patients with renal
insufficiency and contrast agent allergy. [4]
DWI is therefore an important technique, namely in the field of oncology, for detection of
primary tumor and metastases. However, there are many pitfalls that can mislead DWI
interpretation. In this work, our main objective is to review those pitfalls. For a better
organization and easier comprehension, we divided them into pitfalls that may result in
false positives, pitfalls that may result in false negatives and the ones that can originate
both.
Images for this section:
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Fig. 1: The hepatic veins show low signal intensity on DWI (b50) - "black blood image"
© Centro Hospitalar de Lisboa Central - Hospital de Santo António dos Capuchos -
Lisboa/PT
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Fig. 2: The left branch of portal vein and the inferior vena cava show low signal intensity
on DWI (b50) - "black blood image"
© Centro Hospitalar de Lisboa Central - Hospital de Santo António dos Capuchos -
Lisboa/PT
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Fig. 3: Rectal cancer shows high signal intensity even at very high b values (b1000)
© Centro Hospitalar de Lisboa Central - Hospital de Santo António dos Capuchos -
Lisboa/PT
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Fig. 4: Adrenal metastasis from renal cell carcinoma shows very high signal intensity on
DWI (b800) and low ADC values
© Centro Hospitalar de Lisboa Central - Hospital de Santo António dos Capuchos -
Lisboa/PT
Fig. 5: Anal canal cancer shows high signal intensity on DWI (b800) and low ADC values
© Centro Hospitalar de Lisboa Central - Hospital de Santo António dos Capuchos -
Lisboa/PT
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Findings and procedure details
As we have already seen, restricted diffusion results in high signal intensity at high b
2
values (500-1000 sec/mm ), depending on the tissue, and low ADC values. However,
there are several pitfalls that may influence these characteristics.
Pitfalls that may result in false positives
T2 Shine-Through
DWI uses a T2 weighted sequence and so the signal intensity of the tissue will depend
not only on the degree of attenuation after applying the motion-probing gradient pulses,
but also on the T2 signal. [2] Because of that, tissues with very long T2 relaxation times,
like cysts or fluid in the gallbladder, will show a high signal intensity on DWI, similar to
that seen on conventional T2 weighted MR images (Fig. 6 on page 11, Fig. 7 on page
11, Fig. 8 on page 12 and Fig. 9 on page 12). [1] The peripheral zone of the
prostate may also show high signal intensity due to T2 shine-trough (Fig. 10 on page
13). [2]
The high signal intensity of T2 shine-through is seen even at high b values (from 0 to 500
2
sec/mm ), despite some signal loss in mobile water molecules. [1,4] This is one of the
reasons why DWI and ADC maps must be evaluated in conjunction. [5] On ADC map,
true restriction will appear as an area of low signal intensity, while T2 shine-through will
appear as an area of high signal intensity. [2]
Restricted diffusion in normal structures and nonmalignant lesions
Restricted diffusion is not observed only in malignant lesions. In fact, it is associated with
a number of factors, namely the increased cell membrane density due to hypercellularity,
expansion of the intracellular compartment and increased viscosity of a fluid. [2,4]
• Highly cellular benign lesions
Restricted diffusion can be observed in a non-malignant lesion with high cellular density.
In the liver, for example, it may be seen in highly cellular benign lesions such as adenoma
and focal nodular hyperplasia (Fig. 11 on page 13 and Fig. 12 on page 13). [2]
In the prostate gland, benign hyperplastic nodules may also show restricted diffusion
with low ADC values. [2] In the female pelvis, restricted diffusion may be seen in normal
proliferative endometrium and normal ovaries and may also be associated with fibrosis
(Fig. 13 on page 14).
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Restricted diffusion can also be seen in normal bowel mucosa. [4]
Regarding the urinary system, the intravesical segment of urachal remnant has high
signal intensity on DWI and therefore can be interpreted as bladder cancer. Although
inflammatory processes of the bladder wall (such as interstitial cystitis) usually show high
signal intensity on DWI and also high ADC values, they can sometimes show decreased
ADC values and mimic malignancy. Benign wall thickening secondary to benign prostatic
hyperplasic is another potential confounder. [6]
• Slow-flowing blood and cystic nonmalignant lesions with other high-
viscosity contents
Unlike water molecules within the vessels, which are highly mobile, slow-flowing blood
within a region may show high signal intensity in DWI, like highly cellular lesions. A
typical example of it is the case of hemangiomas, which demonstrate high signal intensity
2
at b values like 500 sec/mm and may also show low signal in ADC map (Fig. 14
on page 14). [1] It can also occur for hemorragic cysts (Fig. 15 on page 15).
Other hemorrhagic lesions, like hematomas (Fig. 16 on page 16), and post-biopsy
hemorrhage, namely in the prostate gland, can also show restricted diffusion. [2] In the
female pelvis, endometrioma, hematosalpinx and hematometra are also examples of this
phenomenon. [4]
Not only blood restricted to a cavity can show restricted diffusion, but also other high-
viscosity contents within cysts, like proteinaceous, keratinous or mucinous contents.
Abscesses are a typical example and can occur in the different organs of abdomen and
pelvis. [2,4].
• Lymph nodes
Normal lymph nodes may show intermediate to high signal intensity on DWI due to the
high cellularity of lymphoid tissue. Furthermore, ADC values overlap between benign and
malignant lymph nodes (Fig. 17 on page 17). [6]
• Fat and Melanin
Fat and melanin can also show restricted diffusion and act as potential confounders. [5]
Pitfalls that may result in false negatives
T2 Blackout effect
T2 blackout effect refers to low signal intensity seen on T2 weighted DWI and ADC
maps, which results from a signal dropout that is seen, for example, in calcifications
or fibrotic tissues. This may be seen in lesions with an important fibrotic component,
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like uterine leiomyomas (Fig. 18 on page 17) and ovarian fibromas/fibrotechomas,
Brenner tumors and cystadenofibromas. Calcified peritoneal implants, like the ones from
ovarian cancer, may also be difficult to detect due to T2 blackout effect from dense
calcifications. [4]
Absence of restriction in some tumors
Cystic and low-cellular well-differentiated malignant tumors may not show restricted
diffusion. In the liver, this can happen with well-differentiated hepatocellular
adenocarcinoma. [2] Mucinous (Fig. 19 on page 18) and cystic tumors, like ovarian
cystadenocarcinomas, may also be overlooked. [4] Mucinous peritoneal implants may
also be missed due to their T2 shine-through effect. [5]
Tumor necrosis, which constitutes an indicator of poor differentiation in a neoplasm,
facilitates diffusion. [4] In fact, within necrotic areas there are fewer cells and defective
cell membranes, which allow free movement of water molecules. [2] Therefore, tumor
necrosis acts as a confounding factor in ADC calculation. Correlation between DWI and
anatomic sequences should be made to avoid cystic and necrotic areas at ROI placement
(Fig. 20 on page 18). [4]
Pitfalls that may result in both false positives and false negatives
Echo-planar-related artifacts: susceptibility artifact (for example, due to air and
metallic devices) and image distortion (ghosting)
DWI uses an echo-planar sequence, which is particularly susceptible to magnetic field
inhomogeneities. [4] In abdominopelvic imaging, susceptibility artifacts arise specially
from air in the lung bases and bowel (Fig. 21 on page 19) , but also from metallic
devices, like surgical clips and metallic stents (Fig. 22 on page 19 and Fig. 23 on page
20). [2] They are amplified at 3 Tesla imaging. [4] Susceptibility artifacts may obscure
important findings and thus hide a malignancy, but can also mimic one. For example,
gas in the small intestine, colon and rectum may potentially cause image distortion with
abnormally high signal of adjacent bladder walls on DWI, which can be interpreted as a
flat-type cancer (Fig. 24 on page 21).[6]
SE sequences of the abdomen are frequently degraded by respiratory motion artifact.
[7] During the phase-encoding direction, it can create "ghost" images. Because of that,
generated signal is spread throughout the image, thus creating a potential error in ADC
values. [2] Ghosting is particularly problematic in the presence of ascites (Fig. 25 on page
22 and Fig. 26 on page 22). [8] It can be diminished by increasing the speed of
image acquisition (using single-shot echoplanar imaging) and balancing the field of view,
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