Radiology Block 1 Flashcards
What is the sequence from highest to lowest wavelength/strength for ionizing radiation?
Gamma: Shorter, higher freq/energy Xray UV Visible Infrared Microwave Radio: longer, lower freq/energy
Define Scintigraphy
Radioactive tracer used for imaging organ function
What is the use for radionuclide Tc-99m?
Numerous
Most frequently for bone scans
What is the use for radionuclide Ga-67?
Inflammation
Infection
What is the use for radionuclide In-111?
Infection
Tumors
What is the use for radionuclide Ti-201?
Cardiac Perfusion
What are 7 variations of nuclear medicine imaging?
Bone scan Myocardial perfusion HIDA V/Q Thyroid scan PET SPECT
Define Sclerotic
Unhealthy hardeing/thickening of bone
Define Lytic
Destruction of area of bone by disease/cancerous cells
Define Osteomalacia
Bone softening due to insufficient mineralization
What are the Generalized reasons for Inc/Dec bone density?
Inc- Diffuse osteoblastic metastases, osteopetrosis
Dec- Osteoperosis, Hyperparathyroidism, Rickets/osteomalacia
What are the Focal reasons for Inc/Dec bone density?
Inc- Localized osteoblastic metastases, Avascular necrosis, Paget Disease
Dec- Localized Osteolytic metastases, Multiple Myeloma, Osteomyelitis
What are 3 pitfalls to recognizing Fx?
Sesamoids
Accessory ossicles
Unhealed Fx
What are the two types of Comminuted Fx?
Butterfly
Segmental
What are 5 types of healing fractures?
Periosteal reaction Delayed union Nonunion Malunion Pseudoarthrosis
What are the 3 classifications of arthritis?
Hypertrophic
Erosive
Infectious
What are the types of Hypertrophic arthritis?
Primary Secondary Erosive Charcot Ca Pyrophosphate Deposition Disease
How how many Hounsfield units have air, bone, fat, water and soft tissue been assigned?
Air: -1000 HU Bone: 400-600 HU Fat: -40 - -100 Water: 0 Soft tissue: 20 - 100
Radiation causes biological effects at a cellular level in what two ways?
Direct damage
Indirectly forming radicals
What are 4 examples of testing done by CT that are beneficial for provider and PT?
Virtual colon/bronchoscopy Fx Coronary angiography Abdominal pathology Trauma Cardiac calcium
What is a preferred imaging form for women of childbearing age and during pregnancy?
US
What is the imaging study of choice for imaging female pelvis and pediatric PTs?
US
What form of imaging can differentiate cystic verse solid lesions in PTs of all ages?
US
US is the image of choice for what types of PT populations?
Female pelvis Pediatrics Cystic vs solid lesions Non-invasive vasculature Fetal/placental Real-time fluid aspiration and biopsy
What are 4 common evaluation images US is used for?
Cystic vs solid breast masses
Thyroid nodules
Tendons
Newborn spine/brain/hips
What type of imaging are MRIs widely used for?
Neurological
Soft tissue
Masses
Female pelvis
What are 3 uses of interventional radiology?
Iodine contrast used to view anatomy, pathology or position of implants
How are radioisotopes artificially produced?
Neuron enrichment in nuclear reactor or cyclotron
What are two examples of naturally occurring radioisotopes?
Uranium
Thorium
Define SPECT
Single-photon Emission Computed Tomography
Gamma camera rotates around PT capturing 2D images from numerous angles and formed into 3D images by computer
What type of nuclear injection is used during PET scans?
Positron (positive electron) isotope attached to pharmaceutical
When is PET scanning most often used?
Diagnosis and treatment f/u of cancer
Locate hidden metastases
Detect recurrence
Define Flat Plate
Plain film of abdomen w/ PT lying supine
What is the systematic approach to reading abdominal scans?
1- gas pattern
2- etraluminal air
3- abnormal abdominal calcifications
4- soft tissue masses
What is the difference between distended and dilation when reading abdominal CT scans?
Distended- bowel filled w/ normal amounts of air, normal
Dilated- bowel filled with beyond normal amounts of air, abnormal
Why would an increase number of air-fluid levels be seen in a PTs colon?
Recent enema
Anti-cholinergic meds
Anti-peristaltic meds
What is being looked for with a supine abdomen view?
Bowel gas patterns
Masses
Calcifications
What is being looked for with a prone abdomen view?
Gas in retrosigmoid colon
What is being looked for with a upright abdomen view?
Air/fluid levels in bowel
What is being looked for with a upright chest view?
Air/effusions
Pneumonia
Supine view is AKA ?
Scout film
Define Organomegaly
Evaluation of soft tissue structures in abdomen
Define Riedel lobe
Right lobe of liver extending to iliac crest in women
Define FAST
Focused Abdominal Sonogram for Trauma
Portable US for unstable PTs to look for free peritoneal fluid
What is a FAST exam used in place of?
Peritoneal lavage
What are the most commonly injured solid organs from blunt trauma?
Spleen
Liver
Kidney
Bladder
What is the most frequently injured organ from blunt and penetrating trauma?
What is the preferred imaging technique to locate it?
Right lobe of liver
CT w/ contrast
Spleens are usually injured from what mechanism of injury?
What is the imagine method of choice to screen?
Deceleration of unrestrained passengers
CT due to the fact the spleen is the most highly vascular organ
What are the most common causes of kidney injuries?
What is the imaging method of choice?
Motor vehicle accidents
CT w/ contrast
What causes shock bowel injuries?
Blunt abdominal trauma w/ hypovolemia or hypotension
What is the MOA for bladder injuries?
What is the preferred imaging method to detect it?
Pelvic Fx
CT cystogram
What is the MOA of urethral injuries?
What is the imaging method of choice?
Pelvic/blunt trauma in males
Retrograde Urethrography RUG
What is the difference between luminal and mural?
Luminal- arise from mucosa, polyp/carcinoma
Mural- arise from wall, leiomyomas/lipoma
Loops of the GI system have to be in what position for accurate/effective examination?
Fully distended
What is the study of choice for diagnosing aspiration?
Esophagography
Lumen of the stomach is studied through what form of imaging?
Wall thickness and structures outside of the stomach are examined by?
Endoscopy
CT w/ contrast
What do the different colors on US mean?
Echogenic- bright/white
Sonolucent- dark/black
What is the direction orientation of images produced by US?
Sagital/longitudinal
Transverse
What effect is used by US to determine if fluids are heading towards or away from the transducer?
Doppler effect
US is the study of first choice for what abnormailities?
Biliary system
How can gallstones and biliary sludge be differentiated on an US?
Sludge doesn’t produce acoustical shadowing
PT undergoing HIDA exams will show what images if cystic duct is obstructed or if the common bile duct is obstructed?
Cystic- tracer won’t appear in gallbladder
Common bile- tracer won’t appear in small intestine
What is the screening study of choice for asymptomatic, pulsatile abdominal masses?
US
What is the imaging study of choice for evaluating pelvic masses and pelvic pain in female PTs?
US
What is the imaging study of choice for evaluating uterine fibroids?
US- transabdominal or transvaginal
When/why are MRIs used to evaluate uterine fibroids?
Complicated surgical case planning
What is the imaging study of choice for evaluating ovaries?
US
What form of imaging is used to identify the best location to perform a paracentesis to remove ascitic fluid?
US
What form of imaging is the best at identifying the presence of an intrauterine pregnancy?
US
Define Duplex Sonography
Vascular US with Doppler spectral waveform
What is the highest yielding sonographic exam for DVTs examination?
US of symptomatic PT w/ symptoms above the knee
MRI imaging is better than CT scans for what two purposes?
Soft tissue abnormalities
Changes in tissue over time
What form of imaging is preferred to study bone marrow components?
MRI
What is the prototype for generalized increase in bone density?
Carcinoma of prostate
Sclerotic lesions that affect the cortex usually cause what abnormality?
Sclerotic lesions that affect the medullary cause what abnormality?
Cortex- periosteal new bone formation
Medulla- punctate, amorphous lesions
What is the imaging method of choice for studying skeletal metastases?
Radionuclide bone scan
What is the imaging method of choice for detecting avascular necrosis?
MRI
How is Paget’s Disease diagnosed?
Conventional radiograph
What are the imaging hallmarks of Paget’s Disease?
Thickened cortex
Accentuation of trabecular pattern
Increased bone size
What is the imaging method of choice for bone mass density tests?
Dual energy x-ray absorptiometry DEXA
What is the most common primary malignancy of bone in adults?
Multiple myeloma
The most common cause of osteomyelitis is what microbe?
Staph Aureus
What is the difference between a simple and comminuted fracture?
SImple- two fragments
More than 2= comminuted
What is the most common form of arthritis and where does it show?
Primary OA
Hips, knees, hands
What is the imaging hallmark of Primary Osteoarthritis?
Marginal osteophyte formation, subchondral sclerosis, subchondral cysts, narrowing of the joint space
What bone abnormality is seen on all forms of arthritis?
Narrowing of joint space
Where is erosive osteoarthritis most common?
Proximal and distal interphalangeal joints
What is the most common cause of Charcot arthropathy and where is it seen?
Diabetes
Feet and ankles
Charcot arthropathy can mimic what other bone disorder?
Osteomyelitis
Where is CPPD commonly found in the body?
Wrists cartilage
Menisci of knee
Hook shaped formations at metacarpal heads
What are the 4 types of erosive arthritis?
RA
Gout
Psoriatic arthritis
Ankylosing spondylitis
What is the imaging hallmark for RA?
Metacarpal radial deviation
Phalangeal ulnal deviation
What type of arthritis has atlantoaxial subluxation
RA
Proximal joints
Gout is a formation of what type of crystal?
Calcium urate
Define Tophi
Skin manifestation from gout
PT presents w/ olecranon bursitis, what is first diagnosis?
Gout
What form of arthritis presents with “pencil-cup” deformity
Psoriatic arthritis
Where is psoriatic arthritis found?
Distal interphalangeal joints
What are the 3 types of infectious arthritis?
Hematogenous- infection located elsewhere
Pyogenic
Non-pyogenic
What is the most common cause of pyogenic arthritis?
Staph Aureus
What is the most common cause of non-pyogenic arthritis??
TB in the thoracic spine
What are the two types of pyogenic arthritis and what type of damage does it cause?
Indirect- from blood
Direct- local infection outside of joint
Destroys articular cartilage and adjacent articular cortex
Where does non-pyogenic arthritis appear in kids and adults?
Kids- spine
Adults- knee
What is the most common skull fracture?
What is the most serious?
Linear
Basilar
What is the imaging study of choice for skull fractures?
Unenhanced CT
True AP and Lateral shoulder joint x-rays are AKA?
Y-view
Where are Bankheart Lesions located?
Where are Hillsack lesions found?
Glenoid
Humoral
“Sail Sign” in an elbow is indicative of what?
Hemarthrosis
What PT population is a slipped capital femoral epiphyses found in?
Sedentary obese children
Define a Mortise view?
Ankle series AP view w/ slight intentional rotation
What does the term “conventional radiograph” mean?
Plain film w/out contrast
CT numbers can be assigned within what range and means what?
-1000 to 1000
Measures how much x-ray bean is absorbed by tissues
What are two consequences of scattered radiation?
Degrades image quality
Radiates bystanders
Radiation can cause damage in what two ways?
Direct molecular damage
Free radical creation
Radiation can cause biological damage effects in what ways?
Deterministic- nonrandom when a threshold is met
Stochastic- random w/out meeting a threshold
What tissues have a low and high sensitivity for radiation?
Slowly dividing/mature= low
Rapidly dividing/differentiated- high
What are 5 uses of x-rays?
FAT CD Fx Abdomen air/obstruction Trauma Chest tube Debris
What are the risks of iodine contrast?
Lactic acidosis Asthma Diabetes Renal failure Dehydration
Define Contrast Induced Nephropathy
Serum Cr increases x 25% from baseline or, inc .5mg/dL 2-7 days post imaging
Define Spatial Lattice Relaxation
H+ proton realignment, relaxation process emits energy for image
Define Nephrogenic Systemic Fibrosis
MRI contrast agent side effect on renal system
What is the radioisotope used for cancer scans?
Fluorodeoxyglucose
What is the study of choice for dysphagia?
Barium esophagram
What is the imaging study of choice for examining the stomach lumen?
Endoscopy
What is the imaging preference for diagnosing gastric carcinomas?
Double contrast UGI and CT
What is the imaging preference for diagnosing duodenal ulcers?
Double contrast UGI
What is the imaging choice for examining the aorta?
CT w/ oral contrast
What is the imaging method of choice for diagnosing diverticulitis?
CT
What is the imaging preference for diagnosing appendicitis?
CT
What is the imaging method for diagnosing pancreatitis?
CT
What is the first form of imaging used for jaundice PTs?
US
Imaging method of choice for female pelvis or renal masses is?
US
Define Acoustical Impedance?
How much sound is transmitted vs how much is reflected back
What do a large or small acoustic impedance mean?
Large- greater sound reflection (gas/bone)
Small- greater transmission (fluid)
Define Echogenic
Hyperechoic
Tissue that reflects many echoes (white)
Define Sonoluscent
Hypoechoic/anechoeic
Tissues w/ few/no echoes (dark/black)
What is the provider/PT orientation for sonography?
PT head to left
Head up
What does US low and high frequency mean?
Low- object is moving away from transducer (Blue)
High- object is moving towards transducer (red)
What is the first image choice for biliary system abnormalities?
US
What form of imaging has a decreased sensitivity for detecting gallstones?
CT
What is the difference in imaging of biliary sludge and gallstones?
Sludge- shadows
Stones- no shadows
Define Murphy’s Sign
Positive sonogram w/ pain upon compression
What is the preferred imaging for acute T1titis?
HIDA
What imaging is used for examination of the intra/extrahepatic ducts and pancreatic duct?
US
What imaging is used to detect the presence of hydronephrosis?
US
How is renal disease progression seen on US?
Parechyma has increased echogenic than liver/kidney
When is a full bladder used as an acoustic window?
Transabdominal
Transvaginal= empty bladder
Any male PT w/ acute scrotal symptoms receives what imaging?
Color Doppler US
What is the image of choice for PTs with asymptomatic pulsatile abdominal masses?
US
What image is used for detecting uterine fibroids?
US
Transudate will have a ____ US appearance while exudate will have _____
Sonoluscent
Echo
What is the earliest sonographic finding in pregnancy?
Gestational sac
Embryo is the first structure seen in a gestational sac
What is used for noninvasive assessments of extracranial atherosclerosis?
Carotid US
When is US best for detecting DVTs?
Pt s/ pain above the knee
Where does osteromyelitis tend to occur in children?
At metaphysis
What are 2 examples of incomplete fractures in children?
Greenstick- part of cortex involvement
Torus- compression of cortex
There are almost always sesamoid bones in what locations?
Thumb
Posterolateral knee- fabella
Great toe
Where are accessory ossicles almost always found?
Foot
Fractures are described using what 4 major parameters?
Number of fragments
Direction of fracture
Relationship of fragments to each other
Open or closed
What are the 4 parameters for describing the relationship of fracture fragments?
Displacement
Angulation
Shortening
Rotation
What is the sequence of images for diagnosing stress fractures?
Radiograph
Bone scan
What is the most common fracture of the elbow in an adult?
What is the most common fracture of the elbow in a child?
Radial head
Distal humerus
Hip fractures in older adults are common and frequently related to what other disease?
Osteoporosis
What findings would be seen on imaging of primary osteoarthritis?
Marginal osteophytes
Subchondral sclerosis/cyst
Narrowed joint space
Radiologists report says PT is showing signs of juxtaarticular erosion and bony proliferation at tendon insertions, what does the PT have?
Psoriatic arthritis
What form of pyogenic arthritis is monoarticular?
Non-pyogenic
What is the study of first choice for studying the brain and spine?
What is the compromise?
MRI
Brain anatomy is easier w/ CT
What fissure separates the temporal from the frontal/parietal lobes?
Sylvian
What is the most superior portion of the ventricular system?
Bodies of lateral ventricles
What structure separates the cerebral hemispheres and can be seen as calcified in adults?
Falx Cerebri
What are some calcified structures that could be seen on CT but are non-pathologic?
Pineal gland
Basal ganglia
Choroid plexus
Falx and tentorium
What are some brain structures that can enhance after IV iodine contrast?
Venous sinuses
Choroid plexus
Pituitary gland and stalk
What causes streak artifacts on CT?
Dental fillings
Aneurysm clips
Bullets
What is the study of choice for intracranial and spinal cord abnormalities?
MRi
Why is MRI more sensitive than a CT for brain/neurological studies?
Increased contrast and soft tissue resolution
What is an MRI less sensitive at detecting when compared to a CT?
Lesion calcification
Cortical bone
What is the study of choice for acute head trauma?
What are the first two things to be evaluated?
Unenhanced CT to rule out mass effect and blood
Fractures of the cranial vault or most likely to occur at what skull bones?
Temporal
Parietal
Why are basilar skull fractures the most serious?
Tears of dura matter and CSF leaks
What is the imaging method of choice for facial fractures?
CT
What is the most common orbital fracture?
Blow out fracture
Define Diffuse Axonal Injury
Prolonged coma after head trauma
Head injury w/ poorest prognosis
What is the study of choice for identifying Diffuse Axonal Injuries?
MRI
How are most acute stroked initially imaged?
Noncontrast enhance CT
How are most acute, nonhemorrhagic strokes found?
CT
How are aneurysms detected?
CT angiography
MR angiography
What is the imaging method of choice for identifying vestibular schwannomas?
Contrast enhanced MRi
What is the image of choice for identifying MS?
MRI
Unstable isotopes attempt to reach stability through what processes?
Fission
Emit particles- A/B
Energy- gamma
What is the difference between Gamma Ray and X-ray?
Gamma originate from nucleus
X-ray originates from outside of nucleus
What is the most widely used radioisotope?
Tc99m
What studies can be performed with a SPECT?
ANY
Especially myocardial perfusion, bone imaging, functional brain imaging
What is the downfall of nuclear medicine scanning capabilities?
Can not detect lesions less than 1cm
Resolution= 1cm
What is the screening method of choice for detection of osseous metastatic diseases and diagnosing fractures before they’re visible?
Bone scans
Metastatic bone diseases usually present with a pattern of multiple asymmetric focal areas/hot spots of increased uptake with what important exception?
Multiple myeloma
What imaging is done to differentiate between cellulitis and osteomyelitis?
Triple phase bone scan
What risk factor is often associated with PEs?
Immobilization after surgery
What is the modality of choice for diagnosing PEs?
CT-PA have replaced V/Q scans
VQ performed if PT is allergic to iodine or impaired renal function
What is seen on a V/Q scan if a PE is present?
Ventilation is maintained
Perfusion is absent
What is the most frequently used nuclear medicine liver study?
HIDA
What nuclear med test exposes a PT to ionizing radiation?
PET Scan
What are the hallmark imaging signs for avascular necrosis?
Crescent sign- femur
Snow capping- humerus
What form of imaging is most sensitive for osteoporosis?
Dexa scan
What form of imaging is most specific for pyogenic arthritis?
MRI
What form of arthritis shows up as a “Gull Sign” on images?
Erosive osteoarthritis
What bone disease has “rat bite” appearance?
Gout
What bone disease has a “Rocker Bottom Foot” appearance on imaging?
Charcot
What form of bone disease is not found on bone scan?
Multiple myeloma
X-ray
What are 3 scenarios that oral contrast is NOT given?
Trauma
Rock search study
Vascular study
What dictates surgical approaches to liver lesions?
Liver vasculature
Define Vicarious Excretion of contrast?
Kidneys can’t process/keep up with elimination so contrast is eliminated through alternate pathways
US computer measures what 3 pieces of info?
Time
Frequency
Magnitude/amplitude
Posterior acoustic shadow means ?
Posterior acoustic enhancement means?
Stone
Cyst
Increased frequency has what impact on the penetration?
Decreased
______ are echogenic and have an acoustic shadow
Gallstones
Where would osteoblastic lesions from osteoblastic metastases be found?
Vertebrae Ribs Pelvis Humerus Femur
What bone is most often involved w/ Paget Disease?
Pelvis
What is the difference between postmenopausal osteoporosis and age-related bone loss
Postmenopausal osteoporosis is characterized by increased bone resporption due to osteoclastic activity. Age-related bone loss begins around age 45 to 55 and is characterized by a loss of total bone mass
What are some of the findings of hyperparathyroidism on conventional radiographs?
Overall decrease in bone density, subperiosteal bone resorption (especially on the radial side of the middle phalanges of the index and middle finger), erosion of the distal clavicles, well-circumscribed lytic lesions in the long bones called Brown tumors
Osteolytic metastatic disease can produce focal destruction of bone. These lesions are most often produced by _______
Focal infiltration of bone by cells other than osteocytes
What is the image modality of choice for viewing the medullary canal?
MRI
On conventional radiographs, what are the classical findings of osteolytic metastases?
Irregularly shaped, lucent bone lesions (single or multiple. Geographic, mottled, or permeative, in order of decresing size of the most discrete lesion visible). Destruction of pedicles in the spine.
Findings of multiple myeloma on conventional radiographs includes:
The most common early manifestation is diffuse and usually severe osteoporosis. Plasmacytomas appear as expansile septated lesions frequently with associated soft-tissue masses. Later in its disseminated form, multiple small sharply circumscribed (described as punched-out) lytic lesions of approximately the same size are present, usually without any accompanying sclerotic reaction around them
What imaging modalities are frequently used for earlier diagnosis of osteomyelitis?
Conventional radiographs can take up to 10 days to display the first findings of osteomyelities so that other imaging modalities, such as MRI and nuclear medicine studies are frequently used for earlier diagnosis
Which three types of arthritis are usually diagnosed clinically instead of radiologically?
Septic (pyogenic) arthritis, psoriatic arthritis, GoutHemophilia
What is the study of first choice in evaluating for the presence of arthritis?
Conventional radiographs. They will demonstrate abnormalities of the articular cortex and the subchondral bone and will provide late, indirect evidence of the integrity of the articular cartilage. MRI is more sensitive in directly visualizing the soft tissues in and around a joint
Hypertrophic arthritis is characterized by bone formation, either _______ or _______
Subchondral sclerosis or osteophytes
Erosive osteoarthritis is a type of primary OA characterized by more severe _______ and by the development of erosive joint changes. It occurs most often in _______ women
Inflammation. Perimenopausal
Calcium pyrophosphate deposition disease results from the deposition of calcium pyrophosphate dihydrate crystals in and around joints, mostly in _______. This is especially common in the _______ and the menisci of the knee
Hyaline cartilage and fibrocartilage. Triangular fibrocartilage of the wrist.
Chondrocalcinosis can occur in CPPD, hyperparathyroidism, and _______
Hemochromatosis
What are the imaging hallmarks of psoriatic arthritis?
Juxtaarticular erosions, especially of the DIP joints of the hands. Bony proliferations at the sites of tendon insertions (enthesophytes). Periosteal reaction along the shafts of the bone. Resorption of the terminal phalanges (or the DIP joints with telescoping of one phalanx into another: pen-in-cup deformity)
Desiccation of the disk can lead to release of nitrogen from tissues surrounding the disk resulting in the appearance of air density in the disk space. What is this known as?
Vacuum-disk phenomenon
What is an uncovertebral joint (or joints of Luschka)?
Small joint-like strucutres at the lateral edges of C3 to T1
Compression fractures of the spine typically occur secondary to _______
Osteoporosis
Osteoporotic compression fractures usually involve the anterior and superior aspect of the vertebral body, leading to a _______ deformity (the so-called Dowager’s hump)
Wedge-shaped
In the spine, metastases may lead to _______. They tend to destroy the _______, including and especially the posterior aspect of the pedicles (differentiate this from osteoporotic compression fractures)
“Compression fractures. Vertebral body.
This is different because in osteoporotic compression fractures, the posterior vertebral body and pedicles usually remain in tact”
What is the prototype example of a primary malignancy that produces osteoblastic metastases, for men and for women?
Prostate cancer, breast cancer
What scan is used for spinal cord metastases?
T99m Bone scan
Infection of the disk (diskitis) is almost always associated with _______ of the adjacent vertebrae and it almost alsways spreads hematogenously from infection in another organ . The lumbar region is most commonly involved, and _______ is most often the pathogen
Osteomyelitis. Staph aureus
_______ should be considered any time there are abnormalities of the disk space and destruction of the adjacent vertebral body end plates
Diskitis
Osteomyelitis
How do internal endosteal healing and external periosteal healing differ?
Internal- indistinctness of the fracture line leading to eventual obliteration of the fracture line. External- external callus formation eventually leading to bridging of the fracture site
In what kind of bone do pathological fractures occur?
Bone with a preexisting abnormality
What is the gold standard for evaluating the structural intergrity of silicone breast implants?
MRI
For a cyst in the breast to meet the definition of benign by ultrasound, it must have what criteria?
Anechoic (echo-free)
Enhanced through transmission,
Thin back wall