Tuesday, January 29, 2019

A Primer on Diffusion Weighted Imaging

Diffusion weighted imaging (DWI) is a sequence used in MRI studies to gain additional information about the anatomy based on the diffusion of water molecules. Understanding the physics of is important to help understand its applications, especially in neuroradiology and body imaging.

The Physics of Diffusion

Molecular diffusion, including of water, in tissues is not free but involves interactions with many other substances. It also depends on the temperature and chemical properties of adjacent materials. This is associated with Brownian motion, which is the random movement of particles in a fluid as they interact with each other.

DWI assumes that a molecule of water is free to diffuse or move in any direction, known as isotropic diffusion. To assess motion, the homogeneity of the field is varied linearly by a pulsed field gradient. Molecules that move more freely in between pulses lose signal more rapidly.

In reality, the motion of the molecule is affected by several factors, including the strength of the gradient pulse, the duration, the field strength, and more. To simplify the coefficients of the diffusion, these values were combined into a single scalar known as a b factor and multiplied by the attributed diffusion coefficient (ADC), to account for the additional interactions in biological tissues.

Ultimately, when images are generated, they are weighted by the diffusion process, with areas of higher diffusion and larger b factor showing more attenuation (ie, appear darker). On MRI studies, an ADC map is provided, which is essentially the DWI map without the T2 weighing that is inherent in it. In other words:

ADC = DWI - T2 Weighing

The ADC map can be used to correlate the findings on DWI. To actually form the DWI image, the machine first runs the b=0 (also noted as b0). Then, various diffusion gradients are applied, typically along each axis x, y, and z. The echo signal is acquired, and then recombined to from a trace image. The trace is what is commonly referred to as DWI.  Specifically:

The aforementioned process generates four sets of images: a T2* b=0 image and three diffusion-weighted images (one for each X, Y and Z direction) with the T2* signal attenuated according to how easily water can diffuse in that direction. 
These images can then be combined arithmetically to generate maps that are devoid of directional information (isotropic): isotropic diffusion-weighted images (what we usually refer to as DWI) and ADC maps. 
To generate the isotropic DWI maps, the geometric mean of the direction-specific images is calculated.  
The ADC map, in contrast, is related to the natural logarithm (ln) of the isotropic DWI divided by the initial T2* signal (b=0). These can either be calculated directly from the isotropic DWI images or by finding the arithmetic mean of ADC values generated from each directional diffusion map.


DWI in Neuroradiology

In neuroradiology, the main use for DWI is to assess for acute ischemia. Increased signal is seen in ischemic tissue a few minutes after arterial occlusion, as extracellular water moves into the intracellular space, which "restricts" its motion causing cytotoxic edema. The ADC value decreases, bottoming out about 1-4 days after the acute insult. Over time, the ADC normalizes first. DWI may remain bright due to T2 shine through from edema in the surrounding tissues. Over time, the DWI normalizes as well.

Case courtesy of Dr Gagandeep Choudhary, Radiopaedia.org. From the case rID: 12093
However, not all the restricts is ischemia. Other entities that show restriction include:

  • Vascular: Posterior reversible encephalopathy (PRES)
  • Neoplastic: Lymphoma, gliomas, epidermoid, choroid plexus xanthogranuloma
  • Infectious: Abscess, empyema
  • Traumatic: Hematoma, diffuse axonal injury (DAI)
  • Toxic/Metabolic: Carbon monoxide poisoning, drugs
  • Demyelinating: Acute disseminated encephalomyelitis (ADEM), multiple sclerosis, delayed post anoxic encephalopathy

DWI in Body Imaging

In body imaging, DWI can be useful for assessing malignant disease. By evaluating the relative attenuation of signal intensity from lower (b=0) to higher b values, tissue characterization becomes possible. As Koh et al note in AJR, the more cystic or necrotic portion of a tumor will show greater signal attenuation at higher b values because water diffusion is less restricted. The more cellular components will continues to show high signal.

From Koh and Collins, AJR:188, June 2007.
However, care has to be taken not mistake intrinsic high T2 signal for restricted diffusion (ie "T2 shinethrough"). As an example, a gallbladder will remain bright on DWI because of this effect. Non-malignant lesions can also show increased signal, such as hemangioma, but this will be bright on T2 as well.

This post is derived from notes I took during training. Any images are copyright their respective owners.

References:




     

Thursday, December 20, 2018

MRI Hepatic Lesion Differential Diagnosis


This post is derived from notes I took during training. Any images are copyright their respective owners.

Focal nodular hyperplasia (FNH)
  • Demographics: young women 
  • Pathology: localized hyperplastic hepatocyte response to an underlying congenital arteriovenous malformation 
  • MR: fibrous central scar that is T2↑, T1↓ with delayed, enhancing scar 
  • US: internal vascularity in spoke wheel configuration 
  • NM: Uptake ↑ on sulfur colloid scan due to Kupffer cell uptake. HIDA positive. 
  • Differential: Adenoma (photopenic on sulfur colloid)

Hepatic adenoma
  • Demographics: Young women on contraceptives
  • MR: mixture of fat, hemorrhage, necrosis; fat will be T1 bright, drops out on opposed phase imaging; may see internal vascularity / hyperintense capsule; if bleeding, may see hematoma as below 
  • Management: Monitor for size; if greater than 4 cm, treat because of bleeding risk




Hepatic hemangioma

  • Most common benign liver mass, not prone to spontaneous bleeding 
  • US: Homogeneously echogenic with no flow on Doppler. Never see hypoechoic halo. 
  • CT/MR: typically early discontinuous nodular filling, fills centripetally
    if cavernous/large, may have central hypointense scar +/- calcification 
  • NM: delayed blood pool activity on Tc99m RBC has nearly 100% PPV

Hepatocellular carcinoma (HCC)
  • Demographics: older pts 
  • Pathology: associated with cirrhosis; MC primary hepatic malignancy. AFP elevated. 
  • MRI: hypointense but with rapid uptake and early washout. Portal vein invasion. 
  • Differential: regenerative nodules (not hypervascular, no PV invasion), regenerative nodular hyperplasia (associated with Budd-Chiari, resemble FNH) 
  • Treatment: Chemo, Ablation, Surgery 
Fibrolamellar HCC
  • Demographics: young adults, may have h/o hepatitis 
  • CT/MRI: Large, calcified central scar; heterogeneous appearance 
  • Differential: FNH (smaller, older patients, more homogeneous appearance, enhancing scar)


Metastases
  • Children: Neuroblastoma (Stage 4 & 4S), Burkitt, Wilms, AML, Sarcomas 
  • Adults: 
    • Hyperechoic: Colon, RCC, Breast (either), Carcinoid, Chorio 
    • Hypoechoic: Breast (either), pancreas, lung, lymphoma 
    • Calcified: colon (mucinous type), gastric, osteosarcoma (rare) 
    • Cystic: ovarian cystadenoca, GI sarcoma 
    • US: hypoechoic rim (target sign)



References:



     

Tuesday, December 18, 2018

How To Read A Shoulder MRI

Shoulder MRI is a common musculoskeletal imaging exam. The exam is typically ordered for shoulder pain with suspicion of underlying rotator cuff pathology. Contrast is not needed. If there is a concern for labral pathology, an MRI arthrogram may be ordered. The arthrogram requires gadolinium-based contrast to be injected intra-articularly under fluoroscopic guidance prior to the MRI. The steps below are for a routine, non-contrast MRI.

The MRI shoulder is usually acquired in 3 planes (axial, sagittal, and coronal) obliqued to the plane of the scapula. Usually each plane is acquired as PD and T2, with one of the planes being done as a T1 instead of PD.

Prior to looking at the MRI, it is helpful to compare with any prior shoulder x-rays available.

On the MRI, the assessment should include:
  • Long heads of the biceps tendon: Start on the axial sequences and follow it along its course in the bicipital group to its origin at the biceps labral anchor
  • Supraspinatus (SS)
  • Infraspinatus (IS)
  • Teres Minor
  • Subscapularis (SSc)
  • Rotator Interval [2]
  • Labrum: best seen on the axial and coronal sequences; should appear hypointense and symmetric. Better assessed on MR arthrogram. 
  • Coracohumeral ligament (CHL): best seen on the coronals as a hypointense structure arising from the lateral coracoid, with its medial band inserting on the lesser tuberosity, and lateral band, greater tuberosity
  • Inferior glenohumeral ligament (IGHL): also assessed on the coronal
  • Acromioclavicular joint
  • Humerus and other bony structures
  • Spinoglenoid notch
  • Suprascapular notch
  • Quadrilateral space

Shoulder MRI Pearls:
  • Fluid in the subcoracoid bursa is suggestive of a tear [1]
  • Articular side anterosuperior rotator cuff tears (ie SSc and SS tears) can dissect into the CHL
  • IGHL is greater than 4 mm in capsulitis, but this should be a clinical diagnosis ultimately


This post is derived from notes I took during training. Any images are copyright their respective owners.

Revised: 2019-01-25

References:



     

Monday, September 17, 2018

Spondylosis, Spondylolysis, Spondylolisthesis, and Spondylitis

Spondylosis, Spondylolysis, Spondylolisthesis, and Spondylitis are four terms that are easily confused for one another. All refer to specific pathologies of the spine. Hopefully the descriptions and explanation below will help you break down each word and remember the distinctions.

To distinguish each word, ignore the prefix spondyl- , which comes from the Greek spondylos, meaning vertebral body. Focus on the suffixes.

Spondylosis

The suffix -osis here refers to any pathology, but usually is referring to degenerative changes of the spine.

Spondylolysis

The suffix -lysis refers to the breakdown or absences of bone, specifically pars defects, most commonly found in the lower lumbar spine at L5-S1.

Spondylolisthesis

In this context, -listhesis refers to the alignment of spine. For specificity, the term anterolisthesis refers to the more superior vertebral body being more anteriorly positioned relative to the next inferior vertebral body. Retrolisthesis is the reverse.

Spondylitis

As in many other conditions, the suffix -itis refers to inflammatory changes (i.e. arthritis). A common form of spondylitis is ankylosing spondylitis, a HLA-B27 seropositive arthropathy.

Spondylosis, Spondylolysis, Spondylolisthesis, and Spondylitis Explained
Source: Huffington Post 
Hopefully these definitions clear up the confusion among these entities. Remember, focus on the suffix in order to figure out the disease process being discussed.

This post is derived from notes I took during training. Any images are copyright their respective owners.

References:



     

Monday, September 10, 2018

How To Read An Abdomen-Pelvis CT

Abdominal pain is a very common chief complaint, especially among ER patients. While the truism of "look at every organ on every image" applies, this is the reading pattern I have developed for myself over time.
  • Bones - look in sagittal and axial. Make note of any pars defects around L5. For trauma cases, consider looking in coronal as well for femoral and sacral pathology.
  • Lung bases / lower thorax - look in lung windows. Glance at heart/pericardium 
  • Aorta / Retroperitoneum -  look for aneurysms, dissection, and lymphadenopathy
  • Liver / Gallbladder - scroll from inferior to superior, looking at the right hepatic parenchyma and hepatic veins. Then scroll from superior to inferior looking at the left hepatic lobe. Scroll back through the gallbladder and biliary ducts. Lastly, scroll inferiorly from the right and left portal veins to the portal vein down the SMV.
  • Pancreas - after you scroll through the portal veins, scroll back up the pancreas from the uncinate process through the head, body, and tail.
  • Spleen - look from pole to pole.
  • Stomach / Duodenum - look from the gastroesophageal junction through the third portion of the duodenum.
  • Adrenals
  • Kidneys - look in axial and coronal planes to exclude any exophytic lesions.
  • Bladder / Pelvis - scroll down from the kidneys along the ureters to the bladder
  • Colon - look at rectum, sigmoid, descending, transverse, ascending colon, cecum, and appendix in that order. 
The advantage of this scan pattern is that you mostly scroll continuously, avoiding jumps between different areas of the abdomen/pevis. Of course, specific chief complaints will lead to a focus on other particular areas of interest. Compare any findings with any prior comparison studies available. 

This post is derived from notes I took during training. Any images are copyright their respective owners.



References: