The Inconvenient Truth

As I was recently preparing for a grand rounds presentation on multimodality imaging, I was reminded of the extraordinary technical features of echocardiography. As a part of my presentation, I often address the various components of imaging terminology such as spatial and temporal resolution. Doing so provides an opportunity to help clarify some of the fundamental reasons why someone might consider performing test A over test B.

For those of us who may read TTE, TEE, cardiac SPECT/PET, CCT, and/or CMR all in the same day, usually with trainees of various levels of skill (and interest) in multimodality cardiovascular (CV) imaging, the decision to perform one test versus another is frequently a point of discussion. The final decision is often determined based upon specific clinical questions needing to be addressed (e.g. pathologic mass assessment; valve physiology; ventricular size or function; etc). At other times, the test selection is determined based upon patient-specific comorbidities (e.g. patient age; clinical acuity; renal function; etc). For some institutions, it may be determined based solely upon which test or expertise is available to the patient (e.g. “To a man with a hammer, everything looks like a nail;” a quote most often credited to Mark Twain, but more accurately, credit should be given to Maslow's hammer or the law of the instrument).¹

There is another well-known quote that I have been known to revise slightly since I am a reader of multiple CV imaging modalities: “The grass may or may not be greener on the other side of the fence; but to know for certain, you must stand on top of the fence with an unobstructed view of both sides (a quote likely only credited to V. Sorrell).”

To help you understand these concepts, it is important to appreciate the following definitions.

• -Spatial resolution (SR): the proximity (closeness) of two objects that can still be defined as two, before they appear as one. A test with poor SR compared to a test with good SR will make two objects appear as a single object when they are much farther apart. However, the test with good SR will continue to demonstrate two objects even though they are in close proximity. Note: When specifically applying SR to ultrasound (in general) or echocardiography, this is further defined as axial (or longitudinal) resolution and lateral resolution depending on the location of the two objects relative to the ultrasound beam.
• -Temporal resolution (TR): the occurrence of two events that can be defined as two, before they appear as one. A test with poor TR compared to a test with good TR will make two events appear as a single event when they are less close in timing. However, the test with good TR will continue to demonstrate two separate events even though they occur within a very close time interval.
• -Contrast resolution: the ability to distinguish between two different image (echo) amplitudes adjacent to each other. The use of contrast material will commonly impact this characteristic.

If you are choosing your CV testing based upon specific clinical questions, then your appreciation of the imaging characteristics is essential in this approach. For example, ordering a test with poor SR for a very small mass may not identify the mass. Similarly, ordering a test with poor TR for something occurring at a rapid interval may not be adequate. The most common clinical scenario where both of these concepts play out in the same clinical arena is when selecting the best imaging study for the detection of endocarditis. Vegetative masses are often very small (maybe only a few millimeters) requiring a test with high spatial resolution. Similarly, these vegetations are located on rapidly moving thin objects (the cardiac valves) and are oscillating independently due to cardiac motion (and often tachycardia) requiring a test with high temporal resolution.

Recognizing our most common current approach to suspected endocarditis, I anticipate you will be able to answer the following questions correctly.

• Q1. In general, which of the following clinically applied CV imaging modalities has the best spatial resolution?

  • A.
    2D echocardiography
  • B.
    3D echocardiography
  • C.
    Cardiac CT
  • D.
    Cardiovascular MRI
  • E.
    Cardiac SPECT
• Q2. Which of the following clinically applied CV imaging modalities has the best temporal resolution?

  • A.
    2D echocardiography
  • B.
    3D echocardiography
  • C.
    Cardiac CT
  • D.
    Cardiovascular MRI
  • E.
    Cardiac SPECT

You may be surprised to learn that the single best answer to both questions is the same – the correct answer is A (Table 1). Of course, each imaging modality has the ability to make technical modifications to fine-tune SR and TR and echocardiography, more so than most, has highly variable resolution capabilities based upon those adjustments (e.g. 3D-echo has a much lower TR than M-mode; TEE has much higher SR than adult TTE; much lower than intracardiac echocardiography [ICE]).

Table 1.

Commonly cited values for resolution of common CV imaging modalities

CV imaging modality	Spatial resolution (mm)	Temporal resolution (ms)
CMR	1.0 – 2.0	20 – 50
CCT	0.5 - 0.6	83 – 135
SPECT	4.0 – 15.0	10 (sec) - 15 (min)
PET	4.0 – 10.0	5.0 (sec) – 5.0 (min)
Echo	0.5 – 2.0	< 5.0

Bold font: highest resolution; mm, millimeters; ms, milliseconds; CMR, cardiovascular magnetic resonance; CCT, cardiac computed tomography; SPECT, single photon emission computed tomography; PET, positron emission tomography; sec, seconds; min, minutes.

In this month's issue of CASE, there are a series of patient reports that highlight this inconvenient truth of how technologically advanced echocardiography is relative to other available CV imaging tools (#EchoFirst). Continuing our trend of receiving many manuscripts about adults with congenital heart diseases, Saldana et al. reported how valuable the role of echocardiography was in a patient with partial Shone complex. Roldan et al. demonstrated that cor triatriatum dexter is not always a benign finding and may be associated with various complications.

In the category of innovation, Bartkowiak et al. highlighted the phenomenal advances in TEE guidance of transcatheter mitral valve implantation with careful attention to maintenance of the neo-LVOT. Their graphical illustration is outstanding and a wonderful educational diagram for anyone working in the structural heart environment. Alnaimat et al. offer a clever approach to using conventional GLS bullseye display (focusing on the time to peak strain) to offer an amazing pattern in a patient with an apical-variant hypertrophic cardiomyopathy which they ingeniously termed the “blueberry-on-top” phenomenon. I anticipate that, like me, you'll be struck by this incredible image!

Finally, in the rare but deadly finding category, there are both human and veterinary reports that highlight the value of echocardiography in devastating examples of cardiac trauma. Deep et al. describe a patient with a ventricular septal rupture after a motor vehicle accident. It is very rare that these devastating cardiac complications are seen with echo since most of these individuals die prior to getting an echo performed – whether from their cardiac or their coexisting non-cardiac insults. In our animal friends, who offer us some novel insights unique to animals as well as many overlapping attributes also seen in humans, Maneval et al. reported a unique finding in a dog who suffered thoracic trauma. They do an admirable job of demonstrating a Gerbode defect (and more) using comprehensive echo/Doppler and have included highly correlative post-mortem pathologic findings which directly serve to improve our echo interpretation skills.

I hope after reading this editorial, you'll think about this when you perform a TEE for suspected endocarditis. It is absolutely the correct CV imaging modality for this indication, and it offers an unparalleled mix of spatial and temporal resolution which is required for that indication.

Remember, every echo you see today has a teaching point; and every teaching point is a potential new CASE report!

Suggested Reading

Cardiac CT imaging: Lin E, Alessio A. What are the basic concepts of temporal, contrast, and spatial resolution in cardiac CT? J Cardiovasc Comput Tomogr. 2009 Nov-Dec; 3(6):403-8. https://doi.org/10.1016/j.jcct.2009.07.003.

Cardiac MR imaging: Kellman P, Chefd'hotel C, Lorenz CH, Mancini C, Arai AE, McVeigh ER. High spatial and temporal resolution cardiac cine MRI from retrospective reconstruction of data acquired in real time using motion correction and resorting. Magn Reson Med. 2009 Dec; 62(6):1557-64. https://doi.org/10.1002/mrm.22153.

3D echo: Lang RM, Badano LP, Tsang W, Adams DH, Agricola E, Buck T, et al.; American Society of Echocardiography; European Association of Echocardiography. EAE/ASE recommendations for image acquisition and display using three-dimensional echocardiography. J Am Soc Echocardiogr. 2012 Jan; 25(1):3-46. https://doi.org/10.1016/j.echo.2011.11.010.

Echo artifacts: Le HT, Hangiandreou N, Timmerman R, Rice MJ, Smith WB, Deitte L, et al. Imaging Artifacts in Echocardiography. Anesth Analg. 2016 Mar; 122(3):633-646. https://doi.org/10.1213/ANE.0000000000001085.

Nuclear SPECT: Garcia EV, Faber TL. New Trends in Camera and Software Technology in Nuclear Cardiology. Cardiol Clin. 2009 May; 27(2):227-236. https://doi.org/10.1016/j.ccl.2008.12.002.