Alternative Imaging

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Alternative Imaging

• Chapter 21

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Introduction

• Other modalities have assumed prominence in
  imaging field, especially for diagnosis of
  diseases that are often difficult to see on
  radiographs.
• Imaging techniques are complementary to
  radiography.

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Ultrasonography

• Seen in veterinary medicine since the 1980s.
• Can provide information about organ architecture
  independent of organ function.
• Helpful in debilitated or young patients in which
  contrast agents or exploratory surgery is
  contraindicated.

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Technical Aspects

• Ultrasound beam is created by a piezoelectric
  crystal that oscillates at several million Hertz
  per second (mHz) within a tranducer (probe).
• When sound wave interacts with tissues of the
  body, it is reflected and echo is received by the
  tranducer.
• Ultrasound machines display images in real time.

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Terminology

• Acoustic impedance- Relationship between density
  or stiffness of tissue and the velocity of sound
  within the tissue. Differences in acoustic
  impedance of adjacent tissues determine the
  intensity of the reflected sound.
• Attenuation- Reduced intensity of radiation
  caused by absorption or scattering, or both,
  during passage through tissue. Sound is also
  attenuated as it passes through tissue and the
  intensity is reduced.

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More Terminology

• Distant enhancement- Ultrasound artifact.
  Increased sound intensity beyond a fluid-filled,
  anechoic area, created by absence of attenuation
  of the sound beam as it passes through the fluid.
• Acoustic Shadowing- Ultrasound artifact.
  Echo-free zone created distal to the imaged organ
  when sound waves hit a highly reflective tissue
  that prevents sound from being transmitted to
  greater depths.

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And more terminology.

• Echogenicity-Intensity of reflected echoes.
• Anechoic- No echoes are detected, and the area is
  black. Typically associated with fluid-filled
  structures such as the urinary bladder.
• Hypoechoic- A few echoes are detected, and the
  area is low-level gray compared with adjacent
  tissues. Usually seen with solid homogenous
  tissues or complex fluid containing cells such as
  blood.

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And one more.

• Hyperechoic- Echoes produced are brighter than in
  surrounding tissue.

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Clinical Applications

• Echocardiography
• Used to evaluate cardiac disease.
• Two modes
• M-mode (motion mode)- information is displayed as
  depth versus time on graph.
• B-mode (brightness mode)- Intensity of returning
  echoes is expressed as brightness in the display.
• No specific preparation, chest wall is clipped,
  gel applied and animal is restrained.
• Must work between ribs of chest wall.
• Difficult in deep chested animals.

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• Echocardiography continued.
• Long axis view- Echocardiographic image showing
  the heart from base to apex in a longitudinal or
  sagittal plane.
• Short axis view- Echocardiographic image showing
  the heart in transverse plane.
• Doppler shift-Difference between transmitted and
  received sound frequencies. The greater the
  Doppler shift, the greater the flow velocity.

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Abdominal Ultrasound

• Patient must be fasted for 12 hours to reduce the
  amount of intestinal gas.
• Full urinary bladder is optimal for scanning the
  bladder or prostate.
• Liver and biliary tract
• Radiographs are superior to ultrasound for
  assessing liver volume.
• Ultrasound-guided biopsy or fine-needle
  aspiration is often performed in conjunction with
  liver scanning. (Sedation of anesthesia is
  required).

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• Spleen
• Normal spleen is elliptic, flat, and smoothly
  contoured.
• Pancreas
• Normal pancreas is narrow, smoothly marginated
  and hypoechoic.
• Pancreatitis is most common indication for
  scanning.

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Splenic Mass in a dog
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Normal Pancreas of a cat
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• GI tract
• Can be difficult due to variable amounts of gas
  within the lumen, which reflect sound and prevent
  imaging of deeper structurs.
• Can be used to identify masses, assess bowel
  peristalsis, foreign bodies, and confirm
  intussusception.

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Intussusception in a dog
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• Kidneys
• Normal kidney has hyperechogenic capsule.
• Ultrasonography is used when kidneys may not be
  visualized on radiographs, or to assess location
  and distribution of disease in enlarged kidneys.
• Does not assess kidney function unless doppler
  technique is applied.
• Helpful to identify fluid-filled, cystlike
  lesions or solid masses.
• Biopsy usually needed to confirm diagnosis.

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Lateral Kidney view in a dog
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• Adrenal glands
• Normal adrenal glands are small and located in
  the perirenal fat medial to the cranial pole of
  each kidney.
• Left adrenal gland has dumbbell shape while right
  adrenal gland is more triangular.
• In hyperadrenocorticism, both glands become
  enlarged but there is no change in shape.
• In cases of masses, usually on one side and
  alters the shape of the gland.

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Normal Left Adrenal Gland in Dog
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• Prostate
• Normal prostate has homogeneous echogenicity and
  fine texture.
• Indicated for cases of prostatomegaly, signs of
  lower urinary tract disease, constiplation, and
  caudal abdominal pain.
• Can not differentiate between benign hyperplasia
  and neoplasia so biopsy would be recommended.

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• Urinary Bladder
• Normal bladder contains anechogenic urine.
• Bladder is smooth and wall is uniform in
  thickness.
• Indicated with potential lower urinary tract
  disease, stones, masses, etc.
• Urethral masses are not visible because are
  hidden by pelvis.

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Bladder wall tumor in dog
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• Reproductive Tract
• Normal reproductive tract in nonpregnant animals
  is not normally seen.
• Indications are to diagnose pregnancy, pyometra,
  stump granuloma, or ovarian neoplasia.
• Pregnancy detection is 30 days after breeding.

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Cervical cancer in a dog.
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Early Pregnancy in dog
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Late Pregnancy in Dog
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• Eyes
• Can place transducer on cornea.
• Can visualize components of the eye and scan for
  intraocular masses or hemorrhage.
• Extremities
• Focused primarily on equine limb below the carpus
  and tarsus.

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Computed Tomography (CT)

• One of the most expensive diagnostic tests in
  veterinary medicine.
• Advantage is can acquire information not
  available from radiographs, contrast studies, or
  ultrasound examination.
• Indications are for central and peripheral
  nervous system diseases of the brain, spinal
  cord, and lumbosacral spine.
• Also useful for obscured masses in the
  mediastinum, axillary region, and retroperitoneal
  space.

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Technical Aspects

• CT uses x-rays and computers to produce images
  that show anatomy in cross section.
• Allows for visualization of structures in
  sagittal, dorsal, transverse, and oblique planes
  without superimposition artifact from fat, ribs,
  spine, pelvis, or any organs that may mask detail
  on a survey radiograph.
• Consists of a movable bed or cradle on which
  patient lies and a gantry that contains the x-ray
  tube and detectors.
• Tube and detectors can be moved 360 degrees.
• Generally requires general anesthesia.

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CT Terminology

• Pixels (picture elements)- Tiny squares making up
  the image matrix represent voxels.
• CT number- Number converted to gray scale in the
  final image, which represents the attenuation of
  the x-ray beam in tissue within a voxel. The
  number is also referred to as a Hounsfield
  number, named for the inventor of CT scanning.

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Clinical Applications

• Skull
• Radiographs are generally not extremely
  diagnostic of this region.
• Lesions can be easily seen by CT.
• Indications for skull CT are seizure, blindness,
  vestibular signs, and change in disposition.

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• Spine
• CT is helpful with myelography to define lesions
  on spine.
• Method of choice for imaging spine caudal to
  L4-5.
• Can allow visualization of intervertebral disk
  potrusion.

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• Extremities
• Assess coronoid processes in dogs.
• Scan both elbows because may be bilateral.
• May also be used to visual meniscal disease,
  osteochondrosis, and sequestra.
• Thorax
• Indicated with pulmonary and mediastinal masses,
  and can evaluate metastasis.

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• Abdomen
• Liver, gallbladder, stomach, small intestines,
  pancreas, spleen, adrenal glands, kidneys,
  ureters, urinary bladder, prostate, ovary, colon,
  and major vessels are easily identified on CT
  scans.
• Useful for canine adrenal masses.

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Nuclear Scintigraphy

• Noninvasive imaging procedure that uses a small
  amount of radioactive material administered
  intravenously, transcolonically, or by aerosol
  insufflation.
• Do provide physiologic information about the
  function of specific organs.

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Technical Aspects

• Radioactive isotope that emits predominantly
  gamma rays.
• Gamma camera detects the gamma emissions from the
  radionuclide and forms a black and white image of
  the selected organ printed on x-ray film.

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Terminology

• Labeled Compounds- A compound whose molecule is
  tagged with a radionuclide.
• Radiopharmaceutical- A radioactive drug that can
  be administered for diagnostic or therapeutic
  purposes.
• Half-life (t1/2)- Time in which the initial
  activity of a radionuclide is reduced to one
  half. Biologic half-life includes exertion, as
  well as the characteristic half-life of the
  isotope.
• Target organ- The organ intended to be imaged and
  expected to receive the greatest concentration of
  administered radioactivity.

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Clinical Applications

• Thyroid
• Evaluates for hyperthyroidism.
• Image shows a blackened area in the involved lobe
  of the thyroid gland.
• Bone
• Indicated for lameness that cannot be localized
  by physical exam.
• Liver
• Indicated in patients with a small liver or
  evidence of a liver mass, decreased liver
  function, biliary outflow obstruction, or
  abnormal hepatic blood flow.
• Looking for portosystemic shunt.

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